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#210452 - 06/24/13 12:46 PM 250.122 Parallel circuits  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 826
Michigan
Question :
3 sets of (480V 3 phase )600 MCM off a 1200 amp feeder OCP. to a Mcc. in Tray cable.
Ground wire in each not correctly sized for 1200 amps (3/0)
from factory stock comes (#3) Can 3/0 ground run with tray
cover it??
Yoopersup


2017 / 2014 NEC & Related Books and Study Guides

#210453 - 06/24/13 01:51 PM Re: 250.122 Parallel circuits [Re: Yoopersup]  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
IMO, no. All equipment ground conductors in each cable must be full sized. Fault currents may not divide equally depending on where the fault is and this could exceed the rating of the equipment ground if not full sized. AFAIK, there is no exception to the requirement.

Last edited by Tom; 06/24/13 01:52 PM. Reason: spelling

Few things are harder to put up with than the annoyance of a good example.

#210454 - 06/24/13 03:44 PM Re: 250.122 Parallel circuits [Re: Yoopersup]  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
2011 NEC 250.122(F) seems to allow a single EGC in a tray...

or if under certain conditions, the tray can be the EGC...


#210455 - 06/24/13 05:29 PM Re: 250.122 Parallel circuits [Re: Yoopersup]  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 826
Michigan
292.60 2011 NEC
Cable trays SHALL be permitted to be used as Equipment grounding conductors.
Then refers you to 250.96 70-118.
Other grounds then are supplemental.
Supplemental grounds are additional grounds & are then they still required to be full size???
2011 McGraw Hills handbook page 517 states:
The last sentance of 250.122 (A) points out that metal raceways and cable armor recognized as an equipment grounding conductors: Table 122 does not apply to them.
Yoopersup


#210456 - 06/24/13 06:32 PM Re: 250.122 Parallel circuits [Re: Yoopersup]  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
Where did you get the information that "Other Grounds are Supplemental?"


#210457 - 06/24/13 06:32 PM Re: 250.122 Parallel circuits [Re: Yoopersup]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,118
Estero,Fl,usa
If the tray meets the cross section requirement in table 392.60(A), you agree that this is "where continuous maintenance and supervision" and they tray is connected to the equipment at both ends with a 3/0, it is legal.
I agree supplemental grounding is just that, more than you require.

The handbook warns about assemblies of conductors may have insufficient grounding conductors when used as parallel sets.

Quote
Cable assemblies are manufactured in standard conductor size configurations, and the equipment grounding conductor in a cable is properly sized for some circuit arrangements but not necessarily for all parallel circuit arrangements. Where the cable is used in large-capacity parallel circuits, it is necessary to ensure that the equipment grounding conductor in each cable is sized as required by Table 250.122, based on the size of the overcurrent device protecting the circuit.



Each parallel set (in a raceway) shall be 250.122.
The cable tray gives you more latitude the way I read it.



Greg Fretwell

#210458 - 06/24/13 09:04 PM Re: 250.122 Parallel circuits [Re: Yoopersup]  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 826
Michigan
Section 392.60(A)
Grounding and Bonding
(A) Metallic Cable Trays
. Metallic cable trays shall be
permitted to be used as equipment grounding conductors
where continuous maintenance and supervision ensure
that qualified persons service the installed cable tray
system and the cable tray complies with provisions of this section. Metallic cable trays
that support electrical conductors shall be grounded as required for conductor enclosures
in accordance with 250.96 and Part IV of Article 250. Metal cable tray containing only
non-power conductors shall be electrically continuous, through approved connections or
the use of a bonding jumper not smaller than a 10 AWG.
Informational Note: Examples of non-power conductors include nonconductive optical fiber
cables and Class 2 and Class 3 Remote Control Signaling and Power Limited Circuits.
Analysis of Change:
Section 392.60 (392.7(A), 2011 NEC was revised to require the metal cable tray systems
to be bonded when used with non-power conductors.

250.96 (A) Bonding other Enclosures.
Metal raceways ,Cable trays, Cable Armor, Cable sheath ,Enclosers, Frames, Fittings and other metal non-current carrying parts that serve as Equipment grounding conductors with or without the use of Supplementary grounding conductors Shall be bonded where necessary .
Yoopersup

The (Tray or RMC) or anything approved by 250.118 by itself
that can carry the fault current well do the trick.
So is anything else Supplementay.


#210459 - 06/24/13 09:57 PM Re: 250.122 Parallel circuits [Re: Yoopersup]  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
So...are you going to run a separate equipment grounding conductor or use the tray?


#210460 - 06/24/13 10:32 PM Re: 250.122 Parallel circuits [Re: Yoopersup]  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 826
Michigan
Both / Ground wire attached to side of tray probably 4/0 with listed & labeled ground clamps & all conduits coming off the tray attached to it. All non contentious tray sections bonded together via 4/0 also.
Always interested in other electricians take on things! Not everything in the Code is clear to me anyway!
Yoopersup



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