Transformer GEC vs EGC 250.122, 250.66

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Npstewart

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I received a building department comment that exactly reads:

It appears that an equipment grounding conductor from the transformer to panel ?K1? is sized per 250.122. The transformers are separately derived AC systems and the transformers have a grounding electrode system installed per NEC 250.66 & 250.30.

Situation:
I have a 112.5 kva transformer feeding a 400A MCB panel. I am using a parallel feeder of (4) #3/0 and (1) #3 EGC to feed my 208/3 panel from the transformer. I have a detail that shows a ground bus in the transformer, the ground bus has a GEC sized per 250.66 ( I used a 2/0) and coming out of the ground bus I have a EGC which is the #3 which goes to feed my panel. My detail also shows bonding jumper etc etc.

I think he wants me to run a 2/0 GEC to the panel if I am reading him correctly, am I missing something here? Is this correct??
 
david luchini

david luchini

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I don't quite understand if your parallel sets are in one conduit or two.

If you have two conduits, you would need a supply side bonding jumper in each. It would have to be #4 minimum per 250.66.

If the parallel sets are in one conduit, the supply side bonding jumper would need to be #2 minimum per 250.66.

250.122 does not come into play between the transformer and the panel.
 
infinity

infinity

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As mentioned your SSBJ's need to be sized according to the phase conductors in each parallel raceway according to table 250.66 which is #3 Cu.

Your system bonding jumper and GEC are sized according to the parallel sets by the equivalent size of a single conductor in Table 250.66.

#3/0=167800 circular mils
#3/0 * 2 sets = 335600 circular mils

Table 250.66 over #3/0 to 350kcmil = #2 Cu GEC and system bonding jumper
 
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