Module 13 Panelboards and Switchboards

7/29/2019 Module 13 Panelboards and Switchboards

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Welcome

Welcome to Module 13, which is aboutPanelboard s and Switchboard s.

In Module 10, we discussed power distribution with a loadcenter. Loadcenters are usedmostly in residential applications. Distributing power in commercial and industrialfacilities is more complex than residential distribution. A typical industrialdistribution system is shown in Figure 1. In additional to distribution devices such asswitchgear, switchboards, transformers, and panelboards, such a system usually containsmetering equipment, main and branch disconnects, protective devices, power switchingdevices, and conductors.

We will look at switchboards and panelboards in detail in this module. Theseexpandable devices are used to supply electric service in industrial applications.

Figure 1. Typical Components of a Commercial and/or Industrial Distribution System

Like the other modules in this series, this one presents small, manageable sections of new

material followed by a series of questions about that material. Study the materialcarefully, then answer the questions without referring back to what youve just read.

You are the best judge of how well you grasp the material. Review the material as oftenas you think necessary. The most important thing is establishing a solid foundation tobuild on as you move from topic to topic and module to module.

A Note on Font Styles
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Power Supply Systems

Panelboards and switchboards get their power from a variety of sources. Both pieces ofequipment can serve asService Entrance Equipment, receiving power directly from theutility transformer. In addition, both can serve solely as distribution points, receivingtheir power from a panelboard or switchboard upstream.

In any case, power originates at the power company or local generator, and may bestepped down through transformers for distribution. There are three main powersupply systems in use today for panelboards and switchboards. These are:

Single-phase,

three-wire system

This system can deliver both 240-volt and 120-volt power. Making a

connection across both hot wires provides 240 volts. Connecting witheither hot and the third wire neutral provides 120 volts. Thissystem is predominantly found in residential applications.

Three-phase, four-wire, wye-connectedtransformer

In a 208Y/120 or 480Y/277 wye-connected system, a connectionacross any two of the three hot wires yields 208 or 480 volts,respectively. Connecting across any hot wire and the neutral provides120 or 277 volts.

Three-phase, four-wire, delta-connected

transformer

This system is a bit more complex. Connecting across any two of thethree hot wires yields 240 volts. A connection made from the neutralto either of the two adjacent hot wires (C or A) provides 120

volts.Finally, a connection across the neutral to the non-adjacent hotwire (B) provides 208 volts. This non-adjacent hot wire has to bespecially marked so the electrician does not accidentally connect to itwhen only 120 volts are desired.As a result, older delta-connectedsystems are being replaced by wye-connected systems.

Figure 2. Power Supply Systems
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Service Entrance Equipment

Sometimes panelboards and switchboards can be used as service entrance equipment. TheService Entranceis the single point at which electrical service enters a building. Serviceentrance equipment enables an operator to control and cut off the electrical supply to theentire building from one point.

To be classified as service entrance equipment, the panelboard or switchboard must meetthese requirements:

Must be approved and labeled Suitable for use as Service Equipment

Must have a means of disconnection and overcurrent protection

Must ground the neutral service conductor Must follow the six subdivisions of the main rule:

The service entrance conductors must have a readily accessible means of beingdisconnected from the power supply. This is why the NEC has aSix Subdivisions of theMain Rule. This ruling states that you must be able to throw no more than six handlesinto the off position to disconnect electrical service.

In other words, the service entrance panelboard or switchboard can contain up to sixovercurrent devices without the need for a singleMain Disconnect Device. If more thansix branch circuits are required, then a main device must be supplied upstream to

disconnect all the branch circuits at once. For this reason, there are two ways to configurea panelboard or a switchboard:

TheMain Breaker or main switch unit has a single mainDisconnect Device that

will disconnect power to all equipment being supplied by the service. It alsoprotects the system from short circuits and overloads (as well as Ground Fault s, ifequipped with Ground fault protection).

TheMain Lug Only (MLO) unit is equipped with up to six devices to disconnect

power to all equipment being supplied by the service. It does not have a singlemain device. The incoming supply cables are connected directly to the Bus Barlugs.
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Figure 3. Main Device (on left) Vs. Main Lug Only (on right)

Distribution Equipment

Both switchboards and panelboards can be used as Distribution Equipment. This is theterm given to a panelboard or switchboard used at a point downstream from theservice entrance equipment .

Electrically speaking, service entrance panels and distribution panels or switchboardsdiffer in only two respects:

Distribution panels or switchboards may or may not be protected by an

integral main breaker. This means the MLO is utilized for distribution. A feedercable from the service entrance equipment supplies the power to the distributionpanels. Therefore, it may be protected by the feeder cables Circuit Breaker in theservice panel.

The service entrance is the only point at which the neutral is connected to

ground. The neutral in any downstream equipment is isolated. The explanation

for this is in the next section on grounding.

A switchboard that serves both roles simply has a service entrance section and one ormore distribution sections, as shown below.
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Figure 4. Simplified Service Entrance/Distribution Switchboard

Grounding and Ground Fault

We have mentioned the concept of grounding a few times throughout this module but wehavent really defined it. The National Electrical Code defines ground as aconducting connection (intentional or accidental) between an electrical circuit orequipment and the earth, or to some conducting body that serves in place of theearth.

Proper grounding of any electrical system is vital, not just for personal safety, but also forequipment longevity. There are two objectives to the intentional grounding of electricalequipment. These are to:

Reduce the potential for electrical shock by minimizing the potential voltage

differentials between various parts of a system. Minimize the ground paths impedance. Lower impedance means higher current

when a fault occurs. That translates into faster-opening overcurrent protectiondevices.

Ground fault protection is required per the National Electrical Code for all serviceentrance equipment mains of solidly grounded wye systems where the voltage to groundexceeds 150 volts and the overcurrent main is rated 1000 ampere or greater and where aneutral is provided. This includes all 480Y/277 volt systems.

If the six disconnect rule is used, all overcurrent devices that exceed the requirementsabove will require ground fault protection. As an example, a 3000 ampere serviceentrance switchboard at 480Y/277 volts has six mains. Two mains are 1000 ampere, onemain is 800 ampere, and three mains are 200 ampere. The two 1000 ampere mains wouldrequire ground fault protection; the other four mains would not.


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As we have already stated, the neutral service conductor on service entrance equipmentmust be grounded. The neutral is connected to ground only at the voltage service. Inaddition to the service entrance, additional grounding is required at separately derivedservices, such as distribution transformers. Distribution panelboards and switchboardsbenefit from that upstream grounding in case of a short circuit or overcurrent problem. A

circuit is grounded only at the service entrance or separately derived services, never atany downstream equipment.

Figure 5. Grounding the Downstream Panel

For example, in Figure 6, the computer has a short circuit. If you trace the thick line back,you will see how fault current is returned to the source. This is why the downstreampanelboard contains a branch circuit breaker. It trips, disconnecting power from the load.


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Figure 6. A Short in a Downstream Load

Although properly grounded equipment is of vital importance, ground fault protection isequally important. This type of protection is designed to save lives and protectequipment.

A ground fault can occur when someone is washing down a countertop and wateraccidentally reaches an electrical appliance or outlet.

A special circuit breaker called a ground fault circuit interrupter breaker can provideprotection against this type of fault. It contains the normal thermal magnetic circuitprotection, along with a ground fault sensor.

A ground fault breaker can detect extremely low levels of current leakage to ground from four to six milliamps. Standard circuit breakers cant do that. The level of four to sixmilliamps was selected because, above this current level, it would be difficult for aperson to physically let go of a conductor.

When only equipment protection is needed, a level of 30 milliamps is used for detection.

Equipment Ratings

You must have two key pieces of information about an application before you can selectpanelboards, switchboards and overcurrent protection devices. These are:

Maximum continuous amps

Available fault current


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NEC article 110-9 states: Equipment intended to interrupt current at fault levels shallhave an interrupting rating sufficient for the nominal circuit voltage and the current that isavailable at the line terminals of the equipment. Equipment intended to break current atother than fault levels shall have an interrupting rating at nominal circuit voltagesufficient for the current that must be interrupted.

Section 110-9 was changed in the 1999 code by substituting the word interrupt for thework break in two places.

The interrupting rating of overcurrent protective devices is determined under standardtest conditions. It is important that the test conditions match the actual installation needs.Section 110-9 states that all fuses and circuit breakers intended to interrupt the circuit atfault levels must have an adequate interrupting rating wherever they are used in theelectrical system. Fuses or circuit breakers that do not have adequate interrupting ratingscould rupture while attempting to clear a short circuit.

There are two ways to meet this requirement: theFull Rating Method and the SeriesRated Method.

The full rating method selects circuit protection devices with ratings equal to orgreater than the available fault current.

Consider a building with 65,000 amps of fault current available at the service entrance.All downstream circuit protection device must be rated at 65,000Ampere InterruptCapacity (AIC). Although switchboards are available with short circuit Current Rating sup to 200,000 amps, anything over 100,000 AIC starts to get cost-prohibitive becauseadditional bus bracing is required. In Figure 7, the main circuit breaker and all branch

breakers are rated for 65,000 AIC.

Figure 7. Full Rating Method (on left) Vs. Series Rated Method (on right)

The series rated method states that the main upstream circuit protection device

must have an interrupting rating equal to or greater than the available fault currentof the system, but downstream devices connected in series can be rated at lowervalues.

Under fault conditions, both the main device and the downstream device would open toclear the fault.
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Consider a building with 42,000 amps of available fault current. Although the breaker atthe service entrance is rated at 42,000 amps, additional downstream breakers could berated at only 22,000 amps.

To receive UL listing, series-rated breaker combinations must first pass testing in series,

then pass tests installed in panelboards and/or switchboards.

There are additional rating terms that need to be understood when selecting panelboards,switchboards and appropriate circuit protection devices. These are:

Current Rating: This is the level of fault current a piece of equipment can withstandwithout sustaining damage.

Interrupting Rating: This is the current rating a protective device (such as a Fuseorcircuit breaker) can safely interrupt.

Ampere Rating: This is the current a protective device will carry continuously withoutdeteriorating or exceeding temperature rise limits.

Voltage Rating: The voltage rating of a switchboard or panelboard can be higher than thesystem voltage, but never lower. For example, a 480 VAC switchboard could be used ona 240 VAC system. A 240 VAC switchboard could not be used on a 480 VAC system.

Panelboards

Now that we have looked at the commonalities between the switchboard and thepanelboard, We will look at their specific features separately. Lets begin with thepanelboard.

Panelboard vs. Loadcenter

We looked at theLoadcenterin Module 10 of this series. The loadcenter and thepanelboard perform similar functions. They both serve to protect branch circuitsfrom overloads and short circuits.

A panelboard is for use in commercial and industrial applications, while aloadcenter is primarily for use in residential applications. This is because the bus barsin a loadcenter are typically rated at a maximum of 200 amps, while those of apanelboard can accommodate up to 1200 amps. Along the same lines, a loadcenter canhandle a maximum of 240 volts, but a panelboard can handle up to 600 volts.

Components of a Panelboard
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Now that you have a basic understanding of the functions a panelboard performs, letstake some time to consider the panelboards construction.

There are several components to a panelboard: a can, interior, circuit protection devices,label, dead front and trim, and filler plates.

Figure 8. Typical Panelboard Meeting NEC Definition

CanThe Can, also called the box, is the housing in which the other components reside.Typically, it is made of galvanized steel. The design of the can provides protection forboth personnel and the internal components. The cans end panels are removable,allowing the installer to locate and cut holes for conduit installation. Optionally, the endpanels can be provided with stampedKnockouts.

Interior

Inside the can, you will find overcurrent protection devices, bus bars, insulatedNeutralBars, and other components, depending on the application.

The centerpiece is the set of bus bars. A bus bar is a common conductor, used as aconnection point for multiple circuits. Bus bars are usually aluminum, but can also becopper. They provide a mechanical means of affixing branch circuit breakers. The busbars are mounted on the interior, which is mounted on studs in the box.

The neutral bar provides the termination point for the neutral wires from both theincoming service and the load circuits. The neutral bar can be mounted on studs in theback of the box, or on the panelboard interior.
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Figure 9. Bus Bars and Neutral Bar

Circuit Protection DevicesThe circuit protection devices are usually circuit breakers. They are mounted directlyto the bus bars. We will talk more about circuit breakers later.

Label

The panelboard label provides information regarding the units voltage rating andampacity.

Dead Front and TrimThese components cover the front surface of the panelboard. A hinged access door isprovided as part of the trim. The dead front provides access to the circuit breakersthemselves, while preventing contact with interior components such as the bus bars andinternal wiring.


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Figure 10. Components of a Panelboard

Filler PlatesFiller plates are used to cover any unusedPole spaces not used by a circuit breaker.

Circuit Identification

All circuits in the panelboard must be clearly identified with a number. This can be donein two different ways. The NEMA (National Electrical Manufacturers Association)numbering scheme assigns odd numbers to the poles on the panelboards right (your left),and even numbers to the poles on the panelboards left (your right). This is shown inFigure 11 on the left.

The other method simply used a vertical numbering sequence, shown on the right.

Figure 11. NEMA Numbering (on left) Vs. Vertical Numbering (on right)

Attached to the inside of the door, you will find a label containing a circuit directory.

Enclosures
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NEMA has established guidelines for electrical equipment enclosures. Most panelboardsare supplied as standard in a NEMA Type 1 enclosure. Descriptions of the enclosureoptions are listed below.

NEMA Type 1

GeneralPurpose

This enclosure type is for general purpose, indoor use. It is suitable for

most applications where unusual service conditions do not exist. Itprovides protection from accidental contact with enclosed equipment

NEMA Type3R Raintight

This enclosure type is intended for outdoor use. It provides protectionagainst falling rain and sleet, and damage from external ice formation. Ithas a gasketed cover.

NEMA Type4X CorrosionResistant

This enclosure type is intended for indoor or outdoor use, wherecorrosion resistance is required. It is constructed of stainless steel,polymeric, or fiberglass. It also provides protection from splashing orhose-directed water, wind-blown dust or rain, and damage from externalice formation.

NEMA Type 12Dusttight

This enclosure type is for indoor use. It provides protection fromdripping non-corrosive liquids, falling dirt, and dust.

Circuit Breakers

One circuit breaker is installed in a panelboard for each branch circuit the panelboardserves. In this section, we will consider the proper way to install circuit breakers in apanelboard.

Balancing the Load

In the case of a single phase system, there are two bus bars in the panelboard. Wheninstalling breakers, it is important to balance the load so that both bars are doingthe same amount of work. This prevents strain on the system.

Compare this to loading clothes into a washing machine. It is important to get an evendistribution of clothes all the way around the tub. And its not enough just to have anequal number of items on each side of the tub. Having a pair of jeans and a sweater onone side and two socks on the other side is NOT a balanced load.

The same is true for the panelboard. It is not enough to make sure there are an equalnumber of breakers on each bus bar. The total amperage ratings of the breakers oneach bus bar need to be equal.


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Figure 12. Balancing the Load

Types of Panelboards

There are two common types of panelboards:

Lighting and Appliance Panelboard

Power Panelboard

Lighting and Appliance Panelboard

The NEC definition for lighting and appliance panelboards has three primary elements:

A maximum of 42 overcurrent devices (poles) installed in one cabinet

At least 10% of the overcurrent devices must be rated at 30 amps or less

Neutral connections must be provided

Power Distribution Panelboard

The NEC defines power distribution panelboards as all panelboards that are not lightingand appliance. The only restriction is physical limitations. It is used to feed other panels,three-phase motors, and transformers.


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Figure 13. Types of Panelboards

Consider this panelboard as an example:

Number of Circuit Breakers Description Number of Overcurrent Devices

4 1-Pole, 30 A 4

22 1-Pole, 40 A 22

8 2-Pole, 40 A 16

Total 42

There are only four overcurrent devices which are rated at 30 amps or less. Thispanelboard does not qualify as a lighting and appliance panelboard, so it is a power and

distribution panelboard.

Consider what happens when a light switch is turned on. If the circuit takes 18 amps topower the lights, it will continue to take 18 amps until the lights are turned off.

Now consider a motor circuit. The start button is pressed and the motor experiences alarge inrush of current. This inrush could easily be 6 times the current needed to run themotor at full rated load at full speed. For example, a one-horsepower motor that requires21 amps to start will only need 3-1/2 amps when running at full speed.


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Because the power panel is designed to supply branch circuit loads such as this, it cannotbe regulated as closely as a lighting and appliance panelboard.

Installation and Mounting

A panelboard can beFlush-Mounted or Surface-Mountedagainst a wall. A flush-mounting is recessed into a hole in the wall. A surface-mounting is attached to (andprojects out from) the wall.

Flush Mount vs. Surface Mount

Many panelboards are flush mounted in commercial, office, school and publicbuildings. Flush-mounting offers some important benefits, such as:

Space savings Because the box is behind the wall, the room is not robbed of

space.

Appearance This option is more attractive, as the wiring and box are concealed. Safety Except for the trim, the panel does not jut out to possibly catch clothing

or other objects.
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Figure 14. Flush-Mount Vs. Surface Mount

Surface-mounted panelboards are generally used in industrial buildings andbasements of office and commercial buildings. Because most of these areas have walls

made of poured concrete, flush-mounting is impractical. Steel columns are often used forsurface mounting.

NEC Mounting Specifications

The NEC specifies clearances around panelboards. This is to provide access and workingspace. There are three basic rules:

Headroom in the location must be a minimum of 6-1/2 feet.

For systems up to 150 volts, the minimum distance from the panelboard to the

ground shall be 3 feet, 4 feet for 151-600 volt systems.

For access, there must be 30 inches minimum space in front of the panelboard,and sufficient space to allow the hinged door to open and rotate 90 degrees.


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Figure 15. Panelboard Clearance Requirements

Switchboards

Now lets turn our attention to switchboards. Switchboards are low voltage equipment,meaning 600 volts or less. The current range is 400 to 6000 amps.

Again, the purposes of a switchboard are to disconnect loads for safe maintenance, and toprotect conductors and equipment against excessive current due to overloads, shortcircuits, and ground faults provided ground fault protection is supplied.

The basis for a switchboard consists of a frame, bus,Overcurrent Protective Device s,instrumentation, and exterior covers.


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Figure 16. Typical Switchboard Meeting NEC Definition

Components of a Switchboard

Frame

The frame is the metal skeleton in which the other components reside.

Bus

Bus is mounted within the frame. Like the bus bars of the panelboard, bus is used todistribute power. Where bus bars move power from incoming utility power cables to thebranch circuits, bus move power between two or more components or circuits .

Horizontal Bus distribute power to each switchboard section. Vertical Bus distributepower through the circuit protection devices to the branch circuits.
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Figure 17. Switchboard Frame, Before and After Installing Bus

NEMA requires that the bus be installed with the phases in sequence when viewedfrom the front. In the case of the three-phase switchboard shown in Figure 17, thephases must be in the order A-B-C from left to right (for vertical bus) or from top tobottom (for horizontal bus).

If aNeutral Bus is required, it is placed at the end of the sequence. With vertical buses, itis placed at the left, yielding an N-A-B-C sequence. In the case of horizontal buses, it isplaced at the bottom, resulting in an A-B-C-N. (No vertical neutral bus is shown above.)

Protective DevicesProtective devices, such as circuit breakers and disconnect switches, are mounted to thevertical bus bars from the front of the unit.

Other devices installed at this time include meters, transient voltage surge protection(TVSS), utility compartments, panelboards, transformers, and other equipment.


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Figure 18. Installing Protective Devices

Types of Protective DevicesThere are four types of protective devices commonly used in switchboardapplications. These are:

Power Circuit Breaker

The term power is applied to a large circuit breaker with awide range of adjustable magnetic overcurrent and solidstate. trips. It does not have a thermal trip.It is built on anopen framework for easy servicing and adjustment.It can bemanually or electrically operated and is available with

ratings up to 4000 amperes.Power air circuit breakers can bestationary-mounted or draw-out type. Draw-out breakers canbe tested without removal from the switchboard.Low voltagepower circuit breakers are covered in Module 7.

Molded Case CircuitBreaker

This breaker is normally both a thermal and magnetic tripdevice. Ratings run 15 2,500 amps, with a variety of shortcircuit interrupting capacities.Some breakers featureinterchangeable trip elements. Others have solid state tripunits.Molded Case Circuit Breakers are covered in Module8.


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Fusible Switch

This is a hand-operated disconnect switch with a fuse oneach pole. When an overload condition arises, the fuse linkmelts and opens the pole, protecting the circuit.Fuses areavailable in two styles. The time delay fuse is suitable forload surges, such as motor start-ups. The instantaneous

fuse is designed to clear in a fraction of a second in theevent of a high current short circuit.They are typically inrated for 240- 600v and 15-1200 amps.The main differencebetween a circuit breaker and a fusible switch is that thecircuit breaker can be reclosed after an overload has beeninterrupted and corrected. The fuse needs to be replaced.

Bolted Pressure Switch

This switch type is primarily used for service entrance andfeeder circuits.The switch is quick-make/quick-break. Whenthe switch moved to the ON position, the line contacts aresqueezed together under pressure by the contact lockingmechanism. This stored up energy is released for quickmake. The same energy quickly breaks the contacts when themanual trip button is pushed.The protective elements areheavy duty, current-limiting high interrupting capacity fuses.Bolted pressure switches are rated 240-480 volts and 800 4,000 amps.Accessories are available to open the switchautomatically or remotely. It can be manual or an electrictrip

Instrumentation

Meters can be used in the service section to measure current, voltage, power usages, peakdemands just to name a few. This will be explored in Module 15, Metering andCommunication.

EnclosuresSwitchboards are usually only available in a NEMA Type 1 or NEMA Type 3Renclosure.


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Exterior Covers

Once the protective devices are installed, exterior cover panels are put in place on theframe. Like the dead front and trim of the panelboard, these covers allow access to theprotective devices while sealing off the buses and wiring from accidental contact. Thecover panels also serve as trim for the sake of product appearance.

An informational is attached to the cover panel, providing information on theswitchboard type, voltage and amperage ratings, and part number.

Figure 19. Installing Dead Front and Trim

Sections of a Switchboard

Now that you understand how a switchboard is put together, lets take some time toconsider the function of each section. We have made a few brief references varioussections of the switchboard without really describing them.

There are four main section types that a switchboard may have. Note that allswitchboards do not utilize all four of these section types. They are:

Service section

Pull section

Distribution section

IFS section

Service Section

This section always contains a main disconnect and usually also has utility and/orcustomer metering equipment.

The service section can be fed directly from the utility companys transformer to serve asa service entrance. Service from the utility can enter the service section in a variety of


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ways. Cable can be brought into the switchboard from the top or bottom, givingadditional application flexibility. You will need to work with the customer to determinewhat service entrance needs exist. This decision will affect the cost.

Figure 20. Bringing Service into the Service Section

Pull Section

The center example in Figure 20 shows the use of aPull Section. The pull section is acommon switchboard component. It is simply a blank enclosure containing emptyspace through which cables can be pulled , hence the name.

A pull section is used most often with service entrance switchboards where the utilityfeed comes up through the floor. This allows the service section to be fed from the topwithout any exposed conductors.

The fourth example shows the use of aPull Box, which is basically a top-mounted pullsection. This is useful if there are extra connections to be made because it permits morespace in which to work.

Distribution Section

Power moves from the service section to the distribution section. Here, it is divided andsent through branch circuit protection devices, then out to the branch circuits, to providepower to loads downstream.

In some applications, the size of the service sections main disconnect device andassociated bus requirements may necessitate the service section cabinet to be deeper thanthe distribution section cabinet. Because the rear of all cabinet sections are alwaysaligned (for installation against a wall), the front of the service section cabinet will

protrude. This is referred to as aRear Alignment.
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Figure 21. Front and Rear Alignment Vs. Rear Alignment

If the depth of the service section and distribution sections are the same, the switchboardinstallation hasFront and Rear Alignment.

Some switchboards may require a deeper distribution cabinet for the circuit protectiondevices and bus. Or, extra depth may be added as an option. Again, work with thecustomer to understand the need. This decision may affect the cost.

IFS Section

An IFS section, or Integrated Facility Switchboard, may include panelboards, dry typetransformers and blank back pans for mounting other equipment.

If panelboards and dry type distribution transformers are used within the same room asswitchboards, it may make sense to consider IFS sections. IFS sections can reduce theneed for lineal wall space required for equipment, thus reducing the area needed forequipment. Additionally, IFS can also significantly reduce the installation time whilereducing the number of pieces of equipment for handling.

Spare, Provision and Blank Space

Customer specifications for protective devices will also affect cost. We have alreadycovered the types of protection devices (switches and circuit breakers) commonly used inthe switchboard. But there are some extra terms used in this area pertaining to theprotective devices that you should know.
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For future expandability, the customer could request a Spare protection device. Thismeans that the protection device is bolted in place and connected to the bus. All thecustomer has to do is connect a branch conductor to it.

This is very different from ordering a provision. With this arrangement, a space is left in

the panel for the future insertion of a protection device. The bus connectors are in placeand ready to use when the customer is ready to expand.

Different still is the specification of a space. It is space on the panelboard/switchboardchassis that is simply covered with a blank panel. There is vertical bus in back of thepanel, but a connector kit would be required to install a breaker in the future. The spacecan be defined for a defined overprotective device, in which can an appropriately sizedfiller cover will be supplied.

Helping the Customer

Throughout our discussion of panelboards and switchboards, we have identified a numberof questions you will need to ask the customer to match a product to the application.

It may be helpful to spend a moment reviewing the information needed so that you aremore prepared to knowledgeably recommend a product to the customer.

Matching a Panelboard to an Application

As you talk with the panelboard customer, be sure to obtain the following information:

System type (including voltage, number of phases, and number of wires)

Required amperage rating

AIC rating or short circuit current rating

Service entrance equipment or distribution?

Main breaker or main lug only?

Top- or bottom-fed?

Flush- or surface-mount?

NEMA Type enclosure

Any desired special modifications

Required shipping timeframe

Shipped assembled or as components?

Armed with this information, you should be able to consult your product catalog andmake a good product recommendation.

Matching a Switchboard to an Application


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The questions you are required to ask of a switchboard customer are somewhat different.To save time, we have come up with a questionnaire-style form that should speed up theinterview process. It is reproduced here.

Figure 22. Switchboard Questionnaire

Help the customer to complete this form thoroughly. This information should provide agood start on matching a switchboard to the customers application.

With this information in hand, you might then try using the form below to sketch out theproposed switchboard.

Figure 23. Switchboard Layout Form

This should help you and the customer fine-tune the switchboards features to best servethe application.


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Glossary

AmpereInterruptCapacity (AIC)

Also Interrupting Rating. A rating of the amount of current that aprotective device, such as a fuse or circuit breaker, can safely interrupt.

Ampere Rating A rating of the amount of current a protective device will carrycontinuously without deteriorating or exceeding temperature riselimits.

Blank Space A space on a panel of a switchboard that will never be used forexpansion. It is covered with a blank panel and no bus runs behind it.

Branch Circuit A circuit that supplies power to the electrical loads in a building and isterminated at the panelboard or switchboard.

Bus Bar A component of a panelboard that serves as an extension of the mainservice conductors. Simplifies the connection of circuit protectivedevices to the main service conductors.

Bus Components of a switchboard that serves as an extension of the mainservice conductors. Simplifies the connection of circuit protectivedevices to the main service conductors. Utilized to move powerbetween two or more components or circuits.

Can Also Box. Component of a panelboard used to contain the other

components. It is designed to provide component and personnelprotection.

Circuit Breaker A reusable overcurrent protection device. After tripping to break thecircuit, it can be reset to protect the circuit again.

Current Rating A rating of the level of fault current a piece of equipment canwithstand without sustaining damage.

DisconnectDevice

A blanket term used to describe a number of circuit protection devices,such as fuses and circuit breakers.

DistributionEquipment

Also Distribution/ Power Only Panelboard. A panelboard orswitchboard used to feed lighting and appliance panelboards and three-phase motor loads. Also can power small transformers to provide othervoltage levels.

Flush-Mounted Recessed into a hole in the wall.

Front and RearAlignment

A type of switchboard arrangement where the front and the rear facesof the switchboard sections are aligned.

Full Rating A method of selecting circuit protection devices where all device


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Method ratings are equal to or greater than the available fault current.

Fuse A non-reusable overcurrent protection device. After tripping to breakthe circuit, it must be replaced to restore power to the circuit.

Ground A conducting connection (intentional or accidental) between an

electrical circuit or equipment and the earth, or to some conductingbody that serves in place of the earth.

Ground Fault An electrical fault where contact is accidentally made between anelectrical circuit or equipment and the earth, or to some conductingbody that serves in place of the earth.

Horizontal Bus Distributes power to each section of a switchboard.

InterruptingRating

Also Ampere Interrupting Capacity (AIC). A rating of the amount ofcurrent that a protective device, such as a fuse or circuit breaker, cansafely interrupt.

Knockouts Circular perforations in the top and bottom panels of a panelboard thatcan be removed to provide entrances and exits for feeder wiresenclosed in conduit.

Lighting andAppliancePanelboard

A panelboard that supplies electrical power to branch circuits that use aconstant amount of power.

Loadcenter A device that delivers electricity from a supply source to loads in lightcommercial or residential applications.

Main Breaker Also Main Circuit Breaker. A reusable overcurrent protection devicedesigned to protect an entire panelboard or switchboard. Power from

the mains is fed through a main breaker to the bus bars.

Main DisconnectDevice

A blanket term used to describe a number of panel protection devices,such as a main circuit breaker or main switch.

Main Lug Only(MLO)

A panelboard where power from the mains is fed directly to the busbars.

Neutral Bars Provides the termination point for the neutral wires from both theincoming service and the load circuits in a panelboard.

Neutral Bus Provides the termination point for the neutral wires from both theincoming service and the load circuits in a switchboard.

OvercurrentProtectiveDevice

A device such as a circuit breaker or fuse. In the event of an overloador short circuit, this device will quickly terminate power to the circuit.

Panelboard A wall-mounted electrical power distribution device for use incommercial and industrial applications. It provides circuit control andovercurrent protection for light, heat or power circuits.NEC defines itas a single panel or group of panel units designed for assembly in the


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form of a single panel; including buses, automatic overcurrent devices,and equipped with or without switches for the control of light, heat, orpower circuits; designed to be placed in a cabinet or cutout box placedin or against a wall or partition and accessible only from the front.

Pole A space in a panelboard (or switchboard) where a branch circuitprotective device can be attached to connect a branch circuit to the busbars (buses) and protect the branch circuit from overload.

PowerPanelboard

Any panelboard that does not qualify as a lighting and appliancepanelboard. It is used to feed lighting and appliance panelboards andthree-phase motor loads. Also can power small transformers to provideother voltage levels.

Pull Box A top-mounted pull section.

Pull Section A common switchboard component, a blank enclosure containingempty space through which cables can be pulled.

Rear Alignment A type of switchboard arrangement where only the rear faces of theswitchboard sections are aligned.

Series RatedMethod

A method of selecting circuit protection devices where the mainupstream circuit protection device must have an interrupting ratingequal to or greater than the available fault current of the system, butdownstream devices connected in series can be rated at lower values.

Service Entrance The single point at which electrical power enters a building.

Service EntranceEquipment

The term used to describe a panelboard or switchboard used as aservice entrance.

Service Section This section of the switchboard is where upstream power enters. Italways contains a main disconnect and usually also has utility and/orcustomer metering equipment.

Six Subdivisionsof the Main Rule

An NEC ruling which states that you must be able to throw no morethan six handles into the off position to disconnect electrical service. Ifthrowing more than six handles would be required, one upstreamdisconnect must be provided to disconnect all the branch circuits atonce.

Space A specification when ordering a switchboard. Room is purposely leftfor a protective device to be installed in a later expansion of service.

Bus bars are in place. The customer needs to buy and install aprotection device and connect a branch circuit to it.

Spare A specification when ordering a switchboard. An extra protectiondevice is bolted in place and connected to the bus. For future serviceexpansion, all the customer has to do is connect a branch circuit to it.

Surface-Mounted

Attached to (and projects out from) the wall.


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Switchboard A floor-standing electrical power distribution device for use incommercial and industrial applications. It divides large blocks ofelectrical current into smaller blocks of current used by electricalequipment.NEC defines it as a large single panel, frame, or assemblyof panels on which are mounted, on the face or back, or both, switches

overcurrent and other protective devices, buses and usuallyinstruments.

Vertical Bus Distributes power through the circuit protection devices of aswitchboard to the branch circuits.

Voltage Rating A rating of the voltage at which a piece of equipment is designed tooperate.

Module 13 Panelboards and Switchboards

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Transcript of Module 13 Panelboards and Switchboards