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Tray Cable - Interstitial Ground Wire Terminations - Best Practice?

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NWGAsparky

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Calhoun, GA
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What is best practice for terminating the ground wires within tray cable? Especially when you have a parallel tray cable feeder?

For example: A parallel tray cable feeder is installed in cable tray to a 400 amp distribution panel. At the panel, the cable is installed in conduit(s) for the vertical drop.

- 2 x #3/0-3C THHN-PVC Tray Cables with Ground are used. (The cable contains 4x interstitial ground wires, with a combined equivalent of 4 AWG)

- The cable(s)'s interstitial ground wires do not meet the ampacity requirement (Table 250.122) for the parallel installation, so a bare copper ground is installed in the cable tray sized to the largest OCPD that will be present in the tray (250.122(c)). Let's say 1200 amps, so #3/0 bare copper. At the vertical drops, a #3 insulated wire is split bolted onto the #3/0 and run with the tray cable in each separate conduit down to the panel.

In the panel: you now have 2x #3 ground wires, plus the 8x interstitial ground wires from the tray cable. This appears to be what is permitted under 250.122(f).

My questions is: What is the best practice for complying with 250.122(f)(2)(c)?

"If multiconductor cables are paralleled in the same raceway, auxiliary gutter, or cable tray, a single equipment grounding conductor that is sized in accordance with 250.122 shall be permitted in combination with the equipment grounding conductors provided within the multiconductor cables and shall all be connected together."
(2023 NFPA 70)

- Split bolts only allow 2-3 conductors, and most ground terminals in the panel allow 1 conductor.
- Following these rules, for the 10 ground wires, would you use split bolts (or similar lug/connector) to combine 2-3 ground wires until they are reduced to the number of terminals you have available? (This is my general method, just wondering if anyone has encountered a better solution.)
 
infinity

infinity

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NWGAsparky said:
But that would require a custom made cable... Are you saying that standard configuration tray cable cannot be used in parallel installations?
Click to expand...
I'll preface this by saying that we don't use tray cable but the general rule from Article 250 is that the EGC in each parallel cable must be full sized based on 250.122. Sounds like you options are custom cable or another wiring method.
 
don_resqcapt19

don_resqcapt19

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NWGAsparky said:
But that would require a custom made cable... Are you saying that standard configuration tray cable cannot be used in parallel installations?
Click to expand...
That is correct in may cases. One option would be to use a 5 conductor cable where only a 4 wire cable is required, and just not use the EGC in the cable.
 
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Hondaman

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AHJ and Engineer
From another forum; https://electrical-engineering-port...grounding-and-bonding-of-cable-trays#comments

Paul Abernathy
MAR 10, 2019
Greetings Edvard
Less we not also forget the beneficial provisions of 250.122(F)(2) as well
when the installer of traditional multiconductor cable assemblies are used,
such as MC Cable or TC Cable to name a few. Traditionally the EGC in
these cables are designed via their associated UL/ANSI Standard for use
based on the OCPD protecting the single cable assembly so they aren’t
intended for parallel use. However, where used as such the benefits of using
250.122(F)(2) just may save the day. So if you are using standard cable
assemblies in a cable tray, in parallel, be sure to check out 250.122(F)(2) as
it just may keep you from having to buy specially made cables.
 
don_resqcapt19

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
Hondaman said:
From another forum; https://electrical-engineering-port...grounding-and-bonding-of-cable-trays#comments

Paul Abernathy
MAR 10, 2019
Greetings Edvard
Less we not also forget the beneficial provisions of 250.122(F)(2) as well
when the installer of traditional multiconductor cable assemblies are used,
such as MC Cable or TC Cable to name a few. Traditionally the EGC in
these cables are designed via their associated UL/ANSI Standard for use
based on the OCPD protecting the single cable assembly so they aren’t
intended for parallel use. However, where used as such the benefits of using
250.122(F)(2) just may save the day. So if you are using standard cable
assemblies in a cable tray, in parallel, be sure to check out 250.122(F)(2) as
it just may keep you from having to buy specially made cables.
Click to expand...
That is a great out for where the cables are in the tray for the total run, however, per the OP, the run is partially in conduit, so the cable itself will be required to have the full size EGC.
 
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wwhitney

Senior Member
Location
Berkeley, CA
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Retired
don_resqcapt19 said:
That is a great out for where the cables are in the tray for the total run, however, per the OP, the run is partially in conduit, so the cable itself will be required to have the full size EGC.
Click to expand...
250.122(F)(2)(c) would still apply if only a single conduit is used for the conduit portion of the run. Of course, that might cause other problems, such as with derating.

Cheers, Wayne
 
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