Cable Pathway Use and Selection and the National Electrical Code (NEC)

Eric Sadler, BSME, MITSnake Tray Cable Management

Solutions

References:References to the NEC in this presentation are taken from a combination of the National Electrical Code Handbook, Twelfth Edition – With the complete text of the 2011 edition of the National Fire Protection Agency (NFPA) 70®, National Electrical Code® Copyright 2011, NFPA, and the National Electrical Code Handbook, Thirteenth Edition – With the complete text of the 2014 edition of the National Fire Protection Agency (NFPA) 70®, National Electrical Code® Copyright 2014

References:

At this time approximately half the states and municipalities have adopted the 2014 NEC. We will address a few of the differences here today.

• Common Terms versus code terms for types of circuits in pathways:

– Power = Conductors for General Wiring• Principally governed by Article 310

– Data and Telephony = BOTH – Class 2 and Class 3 Circuits

• Principally governed by Article 725, AND– Communications Circuits

• Principally governed by Article 800

• Article 725 versus Article 800:– Article 800 addresses Communications

circuits, defined as an extension of service from a utility

– Article 725 address Class 2 and Class 3 circuits, which explicitly includes Information Technology Circuits

– The ambiguity lies in whether an Ethernet Circuit with access to the utility service meets the definition of a Communications Circuit

• Article 725 versus Article 800:– According to a resource at the NFPA, 800

does apply if the network is extending a service from the utility such as a VOIP circuit with outside line connections

– There are many who believe otherwise– It is an interpretation, so consult your AHJ– There are a few differences between their

treatment, but none that contradict each other, so it is possible to comply with both

• We will consider aspects of some common pathways, including:

Metal Wireways – troughs

Raceways – conduits, tubing and the like

Devices to hang cables from the building structure –Hooks and Cable Routing Assemblies

Cable Tray

• Our goals in this hour:– Identify a few important rules,– Discuss the spirit of those rules, – Dispel a few myths

• As they pertain to Cable Pathways

The Spirit of the NEC

In the beginning . . .

There were 10 laws . . .

• And that was eight too many.

• All laws can be boiled down to two:– Do not hurt anyone– Do not touch his stuff

• To enforce these two laws, we used to have the feudal system

• The kindly lord would judge whether a person had hurt someone or touched his stuff, and dole out a sentence

Kindly Lord circa 1100 AD

• However, the decisions of the kindly (or not so kindly) lord were arbitrary (literally and figuratively)

• So we decided to enact laws to precisely define what people can and cannot do

• Fast forward 1000 years or so

• We now have the NEC and its Handbook,

• a 1500+ page document to essentially say:– Do not hurt anyone– Do not touch his stuff . . . – With electricity ;-)

Even with 1500+ pages, we still need the kindly lord:

• The Authority Having Jurisdiction (AHJ)

• We can do our best to interpret the NEC

• What he says, goes

• The NEC is written to protect people and property from the hazards of electricity.

• These hazards can be categorized by:– Fire– Electrocution– Physical Hazards

Part of fire prevention, is to prevent the spread of fire and

products of combustion.

• Don’t let the fire spread

• Both NEC 300.21 for electrical circuits and 800.26 for communication circuits call for firestop where pathways penetrate fire-resistant walls

• In no case shall wiring substantially increase the spread of fire and the products of combustion

Cable Routing Assembly through a wall with Firestop

• Don’t let the smoke spread• The NEC 300.22 (C)

– Defines plenum as “other spaces used for environmental air”

• This is a clarification from 2008 to disallow general and communications wiring in areas whose sole purpose is air distribution

– Requires wiring methods in plenum spaces to be totally enclosed

• Examples: conduit, MC cables, CMP (plenum cable)

– Specifically allows cable tray as a support system for wiring methods

– Further allows and qualifies cable trays with solid tops, sides, and covers as totally enclosed

Power Conductors in MC Cables in Cable Tray

• Better still, prevent the fire from starting.

• NEC 310.15 (A)(3) - Do not allow heat to build to unsafe temperatures that would ignite cables or surrounding materials

• NEC 250.4(A)(1) - Dissipate shorts to ground– Properly grounding wiring methods helps both to

prevent fires and to prevent electrocution

• Heat is generated by current in cables

• Safe currents for every common cable are well documented in the NEC

• Baseline currents under common conditions are NEC Tables 310.15(B)(16-21)

• The operating conditions for each table are as follows:– The NEC 310.15(B)(16) – 1-3 Insulated

Conductors Up to 2000V, 60-90 deg C, in EARTH, RACEWAY, CABLE JACKET, OR CONDUIT, based on 30 deg C ambient

– The NEC 310.15(B)(17) – Single Insulated Conductors Up to 2000V, 60-90 deg C, In FREE AIR, based on 30 deg C ambient

• Table operating conditions cont’d:– The NEC 310.15(B)(18) – 1-3 Insulated

Conductors Up to 2000V, 150-250 deg C, in EARTH, RACEWAY, CABLE JACKET, OR CONDUIT, based on 40 deg C ambient

– The NEC 310.15(B)(19) – Single Insulated Conductors Up to 2000V, 150-250 deg C, In FREE AIR, based on 40 deg C ambient

• Table operating conditions cont’d:– The NEC 310.15(B)(20) – 1-3 Insulated

Conductors Up to 2000V, 75-90 deg C, SUPPORTED ON A MESSENGER WIRE, based on 40 deg C ambient

– The NEC 310.15(B)(21) – Bare or Covered Conductors 80 deg C, In FREE AIR, based on 40 deg C ambient 2 ft/sec wind velocity

Of these tables, 310.15(B)(16) and (17) are referenced most

often, and used most commonly in commercial

applications

You choose the table based on the operating conditions. Then the percent of baseline current allowed, or “correction factor” is both ambient temperature

and pathway dependent

• Temperature correction factors for cables normally rated at 30 deg C (most common) are in 310.15(B)(2)(A) and vary from 1.29 to 0.29

• Temperature correction factors for cables normally rated at 40 deg C (less common) are in 310.15(B)(2)(b) and vary from 1.58 to 0.35

• For single conductors in raceways or in the jacket of a cable NEC Table 310.15(B)(3)(a) gives adjustment factors to table 310.15(B)(16).

• The factors are dependent only on the number of cables in the raceway or jacket

• As per NEC 392.80(A)(1) For multi-conductor cables in cable tray, you use the same chart:– You adjust for the cable, not for the tray– You use table (16) for cables in raceway or

jacket as these are in jacket.• Except where there is 1.8m (6ft) or more cover

on the tray, apply an additional correction factor of 0.95

• For power conductors, they must be cabled lined up in a row with one diameter spacing

• The NEC 392.80(A)(2) governs single conductors in a cable tray cabled together (bunched)– 0.75 correction factor applied to the FREE AIR

tables 310.15(B)(17 and 19) for cables 600kcmil or larger, 0.70 if the tray is covered.

– 0.65 correction for 1/0 thru 500kcmil, 0.60 where the tray is covered

– Where they are lined up in a row with one diameter spacing no correction factor is required.

• As per a special provision in article 690.31, for photovoltaic (PV) installations single conductor cables less than 1/0 are permitted in exposed wiring methods such as tray within the PV array.

• The NEC has not caught up to this with Tables, so correction factors must be estimated or calculated.

• Class 2 and 3 and Communications circuits do not carry sufficient current to cause a fire nor power to cause electrocution

• This did not stop them from writing entire sections of the NEC to govern their installation.

• An informational note from 725.1 states:

• Meaning we have fewer, and “less strict,” regulations for these circuits and their pathways

• As per NEC 725.24, Class 2 and 3 circuit cabling can be attached directly to building structure

• 800.24 states the same for Communications cabling

• This is usually accomplished with devices listed for that purpose, UL 2239 “Hardware for the Support of Conduit, Tubing, and Cable”

• Various hooks and cable routing assemblies are listed this way

UL 2239 listed Cable Hooks

UL 2239 listed Cable Routing Assemblies

• To prevent fire as well as physical hazards, in addition to ampacity, the NEC also governs allowable fill for most pathways

• The percentage of a pathway’s cross sectional area that can be physically occupied with cable

• Per 800.110(B) these requirements do not apply to Communications Cabling in raceways or wireways, but do apply to Class 2 and Class 3 circuits

• Cheat Sheet:

– Conduit and tubing (more than 2 conductors) NEC Chapter 9, Table 1, 40%, again, not Communications cabling

– Metal wireways (troughs) NEC 376.22(A) 20%, not Communication cabling

– Sheet metal gutters (open troughs) 366.22(A) 20%– Solid Bottom Cable Tray 392.22(A)(4) 40%– Ladder or Ventilated Cable Tray 392.22(A)(2) 50%

Ventilated Cable Tray Shown at 50% full

• The percent fill can be easily calculated using area formulas (πr2 and the like)

• Percent fills are valid on the presumption that ampacity requirements are met

The NEC has a section for each pathway that gives its installation requirements to

prevent physical hazards

• Cheat Sheet:– Securing and supporting raceways NEC 300.11

• “Securely fastened in place”• Where support wires are used they must

– Provide secure support – Be independent of ceiling grid support wires– Be secured at both ends

– Securing and supporting metal wireways NEC 376.30• Horizontally every 1.5m (5ft)• Vertically every 4.5m (15ft)

• Cheat Sheet cont’d:– Cable Tray Installation NEC 392.18(A)-(G)

• Maintain electrical continuity• Complete the tray before installing cables• Covers must be compatible• Going thru walls is permitted• Keep trays exposed and accessible• Attach conduits and boxes as per the rules for conduits and boxes• Mark high voltage where applicable• 392.30(A) Supported at intervals in accordance with the installation

instructions.

• Cheat Sheet cont’d:

– Cable Routing Assemblies• Support Horizontally every 3 feet, or as listed by the manufacturer• Vertically every 4, or as listed by the manufacturer• By 725.154 and 800.154(a) Cable Routing Assemblies cannot be

used in 300.24(C) other spaces used for environmental air.– Some Cable Routing Assemblies are also Cable Trays

– Building Structure NEC 725.24 and 800.24• Remember UL 2239

Myth #1:

If you choose to put your cables in a cable tray, you must use a tray that is UL listed

Fact:

Repeat after me:

There is no such thing asUL listed cable tray!

As per NEC 392.60(A) Tray can be (UL) Classified as an Equipment Grounding Conductor (EGC), if:

• It is identified as an EGC

• The cross sectional area of the conducting members of a cable tray meets the requirements of Table 392.60(A). (Greater than 0.2 in sq total cross section)

• That area is marked on the tray (for trays of channel or one-piece construction)

• Sections are connected in accordance with 250.96, using bolted connectors or bonding jumpers

• As per the table in NEC 392.60(A), the greater the cross-section, the greater the allowable ampacity

Myth #2:

Your cable tray must be a UL Classified EGC

Fact:

NEC 392.60(A) says that metallic cable trays, “Shall be permitted to be . . . ”

• Not:– Will be– Must be– Ought to be– Would be swell if– But:

• Shall be permitted to be used as equipment grounding conductors

• NEC 392.60(A) goes on to say that cable trays that support electrical conductors shall be “electrically continuous, and be effectively bonded and grounded as per 250.96 and Part IV of 250.”

• And trays containing only non-power conductors “shall be electrically continuous. . .”

• Therefore

• Cable trays containing only non-power circuits are NOT required to be grounded, only electrically continuous, i.e. bonded to each other.

• There are other reasons to ground your Class 2 and 3 and communications circuit pathways, such as cable performance

• And to prevent possible surges from other devices in contact with Class 2 and 3 and Communication cable pathways

• But it is not required by the NEC

Another Fact:• EGC are explicitly required for power circuits

• But nowhere in the code does it state that you can use metallic raceways as EGC, and for wireways and gutters it explicitly states that separate EGC are required.

• Why then do we want to use our cable trays as an EGC?

Myth #3:

You cannot put conductors for general wiring and class 2 or 3 circuits in the same tray unless separated by a UL listed divider

Fact:

NEC 725.136(B) states “barrier”

Underfloor Cable Tray with both Vertical and Horizontal Dividers

Another Fact:

The same is true of cables carrying 600V or less and more than 600V

NEC 392.20(B)(2) states “a solid fixed barrier of material compatible with the cable tray.”

Myth #4:

You cannot put conductors in cable tray of a size less than 1/0 AWG

Fact:

NEC 392.10(B) states “SINGLE CONDUCTOR shall be 1/0 AWG or larger . . .”

Another Fact:

You can use tray rated (TC) cable in cable trays and TC cable comes as small as 18 AWG for power.

• NEC 336.2 defines TC Cable as, “A factory assembly of two or more insulated conductors, with or without associated bare or covered grounding conductors, under a non-metallic sheath.”

• It is often qualified by UL 1227

For Class 2 and 3 and Communication circuits,

Multi-conductor cables meet the definition of Tray Cable

Myth #4a:

You cannot put powerconductors in cable tray of a size less than 1/0 AWG

Fact:

NEC 336.10 specifically permits(1) Power, lighting, control, and

signal circuits(2) In cable trays

• Myths propagate because we believe them and tell others.

• To be thorough about cable pathways, or any other aspect of the NEC we need to read the rules and study interpretation.

• This is of course true for any laws.

• Let’s bring this full circle . . .

• Recall the slide of the “10 Commandments”

Now count them

• “I am the Lord thy God” and “You shall have no Gods before me,” – Is that one or two?

• How about “Thou shalt not covet?”– So it is against the rules to even want

something that someone else has?– Anything?– Both impractical and hard to prove!

• Do not hurt anyone

• Do not touch his stuff . . .

• With electricity ;-)

Thank you for your time and attention