EGC and SSBJ

B

bwat

EE
Location
NC
Occupation
EE
This is partially a real scenario and partially hypothetical.

On the secondary of customer owned 12kV-480V transformer, it goes to 4000A switchboard.

Let's say I'm doing 11 parallel sets of 500kcmil CU for ungrounded conductors between transformer and swbd, each set in own pvc conduit

This is separately derived system, and System Bonding Jumper (SBJ) is at transformer. I must then have Supply Side Bonding Jumper (SSBJ) in each conduit between transformer and swbd. 250.102 (C)(2) dictates that I only have to size the SSBJ according to the ungrounded conductors in each raceway or cable. So my SSBJ is then 2/0 CU.

Now let's take the same exact scenario, but let me add a 4000A fused disconnect between transformer and switchboard. Still going to do the 11 parallel sets on both sides of this disco, and on the transformer side of the disconnect, everything will be exactly the same as before. But between the disconnect and switchboard this is no longer SSBJ but is now an EGC and needs to be sized per 250.122 which would require 500kcmil CU for this EGC.

Two questions:

1. Is my interpretation and application of the code correct here?

2. If #1 is "yes", how in the world does this make sense? Not that I have a naive view that the NEC always makes perfect sense, but this one seems more off than usual. If I have a fused disconnect, I have to make the ground wire 500kcmil. If I don't have fused disconnect, I can get away with 2/0. Adding protection makes the ground wire size go up...
 
1) Yes
2) EGC'S cannot be paralleled but for the SSBJ for all intents and purposes they are.
 
LarryFine said:
Oh, okay. Now I grok.
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I was saying that you can connect multiple EGC's together but that cannot be paralleled as you would for ungrounded or neutral conductors, making a larger conductor out of more than one smaller conductor.
 
infinity said:
I was saying that you can connect multiple EGC's together but that cannot be paralleled as you would for ungrounded or neutral conductors, making a larger conductor out of more than one smaller conductor.
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They can be paralleled, but they would not qualify as a larger conductor.

How's that?
 
Thank you for all the responses!

I will say that #2 didn't really get answered though. Saying it makes sense because that's what the code allows/prohibits doesn't really answer the how this makes sense in the first place. Adding protection makes the ground wire size go up in this case. Very possible that it just doesn't make sense and this is a fringe case that isn't really considered that often!
 
bwat said:
Saying it makes sense because that's what the code allows/prohibits doesn't really answer the how this makes sense in the first place.
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The question should be for the parallel raceways with an EGC in each raceway why is it that you cannot parallel several smaller conductors to make a single larger one? For example if you had a 2000 amp feeder with 5 raceways each with 600 kcmil conductors you would need a 250 kcmil copper EGC in each raceway or 5 total EGC's connected in parallel. If you had those same 5 sets of conductors in a wireway you would only need one 250 kcmil. Doesn't make any sense either.
 
infinity said:
The question should be for the parallel raceways with an EGC in each raceway why is it that you cannot parallel several smaller conductors to make a single larger one? For example if you had a 2000 feeder with 5 raceways each with 600 kcmil conductors you would need a 250 kcmil copper EGC in each raceway or 5 total EGC's connected in parallel. If you had those same 5 sets of conductors in a wireway you would only need one 250 kcmil. Doesn't make any sense either.
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That's fair. So you're saying it's really more of overall "issue" that the CMPs have had with considering paralleling of EGCs, that that they don't seem to have with SSBJs for whatever reason. The scenario I presented is then just a specific demonstration of how that could manifest itself in strange (or dare I say "technically unsubstantiated") ways.
 
bwat said:
That's fair. So you're saying it's really more of overall "issue" that the CMPs have had with considering paralleling of EGCs, that that they don't seem to have with SSBJs for whatever reason. The scenario I presented is then just a specific demonstration of how that could manifest itself in strange (or dare I say "technically unsubstantiated") ways.
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Unfortunately the stuff in the NEC doesn't always make perfect sense. To me the additive use of several SSBJ's (like for a transformer secondary) makes more sense than putting a full size EGC in each raceway.
 
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