Planning the Farmstead Distribution System · Planning the Farmstead Distribution System con’t...

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Planning the Farmstead Distribution System

Notes taken from chapter 5 of Gustafson, Fundamentals in Electricity for Agriculture, 1988

Planning the Farmstead Distribution System

Electrical equipment and electronics on the farmstead are producing an ever-increasing demand for electricity. A well-designed farm wiring system must distribute electricity economically and efficiently both today and in the future.

Planning the Farmstead Distribution System con’t

A well-planned system must be safe, adequate to meet power demands, efficient, and expandable. Compliance with the National Electric Code (NEC) or prevailing code in the region is required to ensure safety.The wiring system must have properly selected electrical components, with a main service and building service entrances of sufficient capacities to supply the required power.

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Planning the Farmstead Distribution System con’t

The distribution system must have enough branch circuits and outlets of proper size and type, which are correctly located to meet the farmstead’s electrical needs.Three types of costs are considered important: initial cost, maintenance costs, and the cost of energy lost in delivery. Minimizing initial cost could sacrifice operating efficiency and thus increase overall costs.

Demand Load for Farm Buildings

The first step is to determine the demand load for each building or service area. The term service area refers to loads, such as buildings, grain drying and handling systems or pumping stations.

Demand Load for Farm Buildings con’t

A demand load must be determined for each building individually.This system presents a methodology for combining known and anticipated loads within a building to determine a demand load by which to size the service for the building. The method is taken from NEC section 220-40.

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Since it is very unlikely that all loads in a service area will operate at one time, the NEC suggests a method for determining the maximum demand load. This demand load is then used to select the correct ampacity of service for the building.


List known and anticipated loads:1. Large or permanently connected

machines or appliances. The full-load ratings of all equipment rated at or above 1500 W (1 hp) are included in this category. The full-load current of the larger motor should be multiplied by 125% to allow for a 25% overload of the motor.


2. A load of 1.5 A at 120 V (0.75 A at 240 V) should be allotted for each convenience outlet. This accounts for portable tools and appliances not listed in #1.

3. A load of 1.5 A at 120 V (0.75 A at 240 V) should also be allotted for each lighting outlet.

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Complete list by calculating the amperage at 240 V required to service each load. Amperage at 120 V is converted to amperage at 240 V by dividing amperage at 120 V by 2. The amperages converted to 240V will be summed to determine total load. The service equipment selected is then rated for amperage at 240 V.


Single loads, such as an irrigation pump or crop dryer would not use the demand system.The largest combinations of loads which are likely to operate on the circuit at a given time make up the load without diversity. Determining load without diversity requires both a knowledge of farm operation and good judgment.

Gustafson, Fundamentals in Electricity for Agriculture, 1988

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Service Entrance EquipmentService entrance equipment has a limited to number of standard packages to select from, which are rated by their amperage at 240 V. The most common sizes are 30, 60, 100, 150, 200, 300, and 400 A.The ampacity selected must be equal to or larger then the calculated demand load. For the above milkhouse example, we would select the 200 A service entrance.

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Service Entrance Equipment

It is usually much less expensive to allow extra capacity when first installing a system, than to later replace the system with a larger system.The NEC demand system requires a minimum 60 A service for buildings. However, a 30 A service may be used for small or single load systems such as a well.

Central Metering and Distribution

The most common type of distribution system on a farmstead has a centrally located distribution center as shown.


Central Metering and Distribution con’t

The power suppliers meter is usually located at the central or main service location. A service drop or feeder runs from the central point to each building or service area.A central distribution service equipment may consist of up to 6 circuit breakers or fused switches. One switch is commonly used as the main disconnect, which then controls power to all the buildings and may be used as a transfer switch for a standby power unit.

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Central distribution centers have a number of advantages:

1. Safety- loss of one building will bring down whole the system. A separate service drop connected ahead of the main disconnect can run a well to ensure a water supply in case of a fire which might require service interruption on the farm.


2. Expandability- feeders to the other buildings are unaffected, when the loads within a building change or a new building is added.

3. Minimizes main service size- diversity of load between buildings minimizes the capacity of the main service needed.

4. Least investment in wire- the technique minimizes the cost of wire for the system.


5. Convenience- the meter is located outside the building, which means the reader does not require entry.The optimum location for the central distribution point and main service equipment is the load center, which is the geographic center of the loads. The load center for a farmstead is found using a scaled map of the farmstead.

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The location and demand load for each building should be noted.The demand loads are used as weighing factors for the distances.




A number of extraneous factors such as; the topography of the farmstead, location of driveways, trees, buildings, and other obstacles may require the distribution center to be located some where other than the load center. Consult the power supplier before locating the distribution center.

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Capacity of Main ServiceWhen using a centralized distribution system, take advantage of the diversity between buildings in sizing the main service. Not all buildings will operate at their full demand at the same time. The next table outlines a demand system, based on NEC Table 220-41, for calculating the necessary capacity of the main service equipment for the farmstead. The minimum capacity for the main service is the sum of the load times their appropriate demand factors.

Capacity of Main Service


Sizing a Main Service


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Selecting Service Conductors

Service conductors from the main service to each service entrance can be run overhead, called service drops, or underground, called service laterals.Three major factors to consider:

1. Type of wire and insulation needed to meet requirements of its surroundings

2. Size of wire and insulation type necessary to safely carry the current

3. Size of wire necessary to prevent excessive voltage drop in the lines.

Selecting Service Conductors con’t

The type of wire and insulation are needed. A wide range of insulation types with different temperature and physical environment capabilities are available.If overhead service conductors are used, they will be exposed to the weather and must approved for wet conditions.


The types of service-entrance conductor insulation often used in farm applications are THW, THWN, and XHHW. Underground service laterals (without conduit for protection) must be acceptable for direct burial. Types USE and UF meet these requirements.

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Selecting Service Conductors con’tThe NEC has established maximum safe values of current (ampacities) for various wire sizes and insulation types. Operation within the limits of desing tables insures that the insulation and wire will not be damaged by excessive heat build-up or high temperatures. Tables A5 and A6 give the allowable ampacities for various copper and aluminum conductors.Tables A5 and A6 list allowable ampacities. The first set of columns in each table deals with copper conductors and the second set with aluminum conductors.


Table A5 deals with conductors in raceways and cables, which covers most internal building wiring situations.Table A6 Part 1 covers insulated conductors in air supported by a messenger. This is the most common situation for service drops (above ground) where the bare messenger is used for support and as the neutral conductor.


Table A6 Part 2 covers the configuration for service laterals (underground).If other configurations are being considered, the NEC Article 310 should be consulted for allowable ampacity.The wire size needed for small overhead services may be limited by the mechanical strength of the wire. According to NEC, at least No. 10 may be used for spans up to 50 ft. and No. 8 for longer spans.

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Table A5


Table A5 con’t


Table A6 Part 1


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Table A6 Part 1 con’t


Table A6 Part 2


Table A6 Part 2 con’t


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Using Allowable Ampacity Tables

When used for a service drop or lateral, what is the allowable ampacity of:#2 aluminum, UF insulation, direct burial

119 A (Table A6, Part 2)

#6 copper, THW, conductor in free air

76 A (Table A6, Part 1)


Voltage drop in the conductors, is controlled by the resistance of the wires. Resistance of the wires is a function of the cross-sectional area and the length of the wire.When the length of the wire increases, the resistance goes up. When its cross-section increases, the resistance goes down.


Table A2 lists the resistance values by wire size for 1000 ft and 1000 m segments of copper and aluminum wires.

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Line loss is the power lost due to resistance in the wires. It is a function of current flow through the conductor and voltage drop along the conductor.


Selecting Service Conductors con’tLine loss is dissipated as heat in the wires and represents a energy cost which can be controlled by proper wiring sizing. The NEC specifies that the voltage drop in a branch circuit (within a building) shall not exceed 3% and that the maximum total voltage drop for service and branch circuits shall not exceed 5%. Commonly used design percentages are 3% for feeders and 2% for branch circuits.

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The wire size needed by a specific application to meet the allowable voltage drop criterion can be calculated using the ampacity of loading and the allowable voltage drop to calculate an allowable resistance value.

Selecting Service Conductors con’tThe allowable resistance can be calculated usingOhm’s law in the form,



After the allowable resistance is known, division by the length of the conductor can be used to convert to a number which can be compared to tables for standard wire sizes.A wire of equal or less resistance would need to be selected.


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Selecting Service Conductors con’tThe total length of a conductor carrying the

current will usually be twice the distance from the source to the load to have a complete circuit.



Proportions can be used to change the value to a value which can be compared to Table A2 (conductor properties).The wire selected must have a resistance equal to or lower than the R value calculated.



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Service conductors to a building will consist of both of the hot conductors and in generaly a neutral conductor (120V). In a single-phase building service, the two-phase conductors are sized based on the building service entrance ampacity. When a rated wire ampacity equal to the service rating is available, that size is the minimum permitted.


The neutral wire carries only the imbalance current, the neutral wire of a service will almost always carry a smaller load than the hot wires. Since it is difficult to accurately control the balance of 120 V loads, it is recommended that the neutral wire in farm buildings not be more then two sizes smaller than the hot conductors.


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Motor ProtectionElectric motors will continue to try an provide the power required by the load, even if it eventually results in excessive currents and temperatures that lead to self destruction due to excessive winding temperatures.Overload protection specifically matched to the motor must be provided. Starting currents of up to 6 times the running current occur normally and must be allowed.

Motor Protection con’t

Control contacts and wires must be capable of carrying these larger currents without causing damage or excessive voltage drop.There are two main types of motor protective equipment available; fuses and thermal-overload devices.Fuses are commonly included with manually operated switches.


Thermal-overload devices are used on both manual and electromagnetic controllers. Time-delay fuses afford both short-circuit and overload protection. Time-delay fuses can tolerate an overload for a brief period, thereby allowing for starting current surges.

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Thermal overload protection devices can be either bimetallic elements (like circuit breakers), or eutectic elements (like normal time-delay fuses).A thermal-overload switch is often

built into the motor.


Thermal-overload devices built into motors or controllers can be either manual-reset or automatic-reset.Manual-reset means a button must be pressed to reset the tripped mechanism.Automatic-reset mechanisms automatically attempt to restart the motor after the thermal device cools.


The heater/overcurrent protection device is typically a removable item which is selected based on the nameplate current rating of the motor to be controlled.The following table gives the recommended and maximum ratings for overcurrent protection by percentage of full-load current rating.

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Wiring for Motor Branch Circuits

For safety purposes the frame of each motor should be connected to the grounding system, equipment grounded.Motors perform best at rated voltages, therefore wires must be sized to avoid excessive voltage drop.Branch circuit conductors to individual motors should be selected to carry 125% of full-load motor current with 2% or less voltage drop. The 125% factor allows for occasional current overload.

Wiring for Motor Branch Circuits con’tWhen conductors supply more than one motor on a branch circuit, the wire is sized for a current value of 125% of the largest motor, plus 100% of any additional motor currents.Tables A3 and A4 give full-load currents for single- and three-phase motors. Current values from those tables can be used in wire selection process unless the motor nameplate current is larger. In which case the nameplate current should be used.

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Wiring for Motor Branch Circuits con’t

The wire size required to meet the voltage drop and current carrying limitations is determined in the same manner as described for feeder wires.



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