Electrical Code (NEC) Snake Tray Cable Management Solutions

Transcription

1 Cable Pathway Use and Selection and the National Electrical Code (NEC) By Eric J. Sadler, BSME MIT Snake Tray Cable Management Solutions

2 References: All references to the NEC in this presentation are taken from 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

3 Common Terms versus code terms for types of circuits in pathways: Power = Conductors for General Wiring Principally i governed by Article 310 Premise Data = Class 2 and Class 3 Circuits Principally governed by Article 725 Utility/Service Provider Data = Communications Circuits Principally governed by Article 800

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

5 Raceways conduits, tubing and the like Metal Wireways troughs

6 Cable Tray Devices to hang cables from the building structure Hooks and Continuous Cable Routing Assemblies

7 Our goals in this hour: Identify a few important rules, Discuss the spirit it of those rules, Dispel of few myths As they pertain to Cable Pathways

8 The Spirit of the NEC

9 In the beginning... There were 10 laws...

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11 And that was eight too many. All laws can be boiled down to two: Do not hurt anyone Do not touch their stuff

12 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 their stuff, and dole out a sentence

13 Kindly Lord circa 1100 AD

14 However, the decisions of the kindly (or not so kindly) lord were arbitrary (literally and figuratively) So we decided to enact laws to precisely So we decided to enact laws to precisely define what people can and cannot do

15 Fast forward 1000 years or so We now have the NEC and its Handbook, a page document to essentially say: Do not hurt anyone Do not touch their stuff... With electricity ;)

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17 Even with pages, we still need the kindly lord:

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19 The Authority Having Jurisdiction (AHJ) We can do our best to interpret the NEC What he says, goes

20 The NEC is written to protect people and property from the hazards of electricity. These hazards can be categorized by: Fire Electrocution Physical Hazards

21 Part of fire prevention, is to prevent the spread of fire and products of combustion.

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23 Don t let the fire spread The NEC calls for firestop where pathways penetrate t fireresistant walls

24 Cable Routing Assembly through a wall with Firestop

25 Don t let the smoke spread The NEC Redefines plenum (in 2011) as other spaces used for environmental air Discouraging wiring in areas whose sole purpose is air distribution Requires wiring methods in plenum spaces to be totally enclosed Examples: conduit, MC cables Specifically allows cable tray as a support system for wiring methods Further allows and qualifies cable trays with solid tops, p, sides, and covers as completely enclosed

26 Power Conductors in MC Cables in Cable Tray

27 Better still, prevent the fire from starting. NEC (A)(3) - Do not allow heat to build to unsafe temperatures that would ignite cables or surrounding materials NEC 250.4(A)(1) - Dissipate i shorts to ground Properly grounding wiring methods helps both to prevent fires and to prevent electrocution

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30 Heat is generated by current in cables Safe currents for every er common cable are well documented in the NEC Baseline currents for cables in free air are in NEC (B)

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32 The percent of baseline current allowed, or correction factor is both temperature and pathway dependentd

33 Temperature correction factors for cables normally rated at 30 deg C (most common) are in (B)(2)(A) 15(B)(2)(A) and vary from 1.29 to 0.29 Temperature correction factors for cables normally rated at 40 deg C are in (B)(2)(b) and vary from 1.58 to 0.35

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36 For single conductors in raceways or in the jacket of a cable NEC Table (B)(3)(a) gives adjustment factors. The factors are dependent only on the number of cables in the raceway ay or jacket

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38 As per NEC (A) For multiconductor cables in cable tray, you use the same chart: You adjust for the cable, not for the tray Except where there is 1.8m (6ft) or more cover on the tray, apply an additional correction factor of 0.95

39 The NEC (A)(2) governs single conductors in a cable tray cabled together (bunched) 0.75 correction factor for cables 600kcmil or larger 0.65 correction for 1/0 thru 500kcmil

40 As per a special provision in article , 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.

41 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 It does mean fewer, and less strict, regulations for these circuits and their pathways

42 As per NEC , 24 Class 2 and 3 circuits i can be attached directly to building structure This is usually accomplished with devices listed for that purpose, UL 2239 Hardware for the Support of Conduit, Tubing, and Cable Various hooks and continuous cable routing assemblies are listed this way

43 UL 2239 listed Cable Hooks UL 2239 listed Cable Routing Assemblies

44 Myth #1: If you choose to put your cables in a cable tray, you must use a tray that t is UL listed

45 Fact: Repeat after me: There is no such thing as UL listed cable tray!

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

47 It is identified as an EGC The cross sectional area of the conducting members of a cable tray meets the requirements of Table (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 , using bolted connectors or bonding jumpers

48 As per the table in NEC (A), the greater the crossp ( ), g section, the greater the allowable ampacity

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50 Myth #2: Your cable tray must be a UL Classified EGC

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52 Fact: NEC (A) says that metallic cable trays, Shall be permitted to be...

53 Not: Will be Must be Ought to be Would be swell if But: Sh ll b itt d t b d Shall be permitted to be used as equipment grounding conductors

54 NEC (A) goes on to say that cable trays that support electrical conductors shall be electrically continuous, and be effectively bonded and grounded as per and Part IV of 250. And trays containing only non-power conductors shall be electrically continuous...

55 Therefore Cable trays containing only non-power circuits are NOT required to be grounded and bonded, only electrically continuous.

56 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 ihclass 2 and d3 cable pathways But it is not required by the NEC

57 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 is required. Why then do we want to use our cable trays as an EGC?

58 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

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60 Fact: NEC (B) states barrier

61 Underfloor Cable Tray with both Vertical and Horizontal Dividers

62 Another Fact: The same is true of cables carrying 600V or less and more than 600V NEC (B)(2) 20(B)(2) states t a solid fixed barrier of material compatible with the cable tray.

63 Myth #4: You cannot put conductors in cable tray of a size less than 1/0 AWG

64 Fact: NEC (B) states SINGLE CONDUCTOR shall be 1/0 AWG or larger...

65 Another Fact: You can use tray rated (TC) cable in cable trays and TC cable comes as small as 18 AWG.

66 NEC 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

67 For Class 2 and 3 circuits, Multi-conductor cables are permitted in cable tray

68 Myth #4a: You cannot put power conductors in cable tray of a size less than 1/0 AWG

69 Fact: NEC specifically permits (1) Power, lighting, control, and signal circuits (2) In cable trays

70 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 ay cross sectional area that can be physically occupied with cable

71 Cheat Sheet: Conduit and tubing (more than 2 conductors) NEC Chapter 9, Table 1, 40% Metal wireways (troughs) NEC (A) 20% Sheet metal gutters (open troughs) (A) 20% Solid Bottom Cable Tray (A)(4) 40% Ladder or Ventilated Cable Tray (A)(2) 50%

72 Solid Bottom Cable Tray Shown at 50% full

73 The percent fill can be easily calculated using area formulas (πr 2 and the like) Percent fills are valid on the presumption that ampacity requirements are met

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75 The NEC has a section for each pathway that gives its installation requirements to prevent physical hazards

76 Cheat Sheet: Securing and supporting raceways NEC 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 Horizontally every 1.5m (5ft) Vertically every 4.5m (15ft)

77 Cheat Sheet cont d: Cable Tray Installation NEC (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 Building Structure NEC Remember UL 2239

78 Myths propagate because we believe them and tell others. To be through 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.

79 Let s bring this full circle... Recall the slide of the 10 Commandments

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81 Now count them

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83 I am the Lord thy God and You shall have no Gods before me, Is that t 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!

84 Do not hurt anyone Do not touch their stuff... With electricity ;)

85 Thank you for your time and attention