GEC size

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hhsting

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Attached sketch shows 75kva transformer two secondary conductors:

1. Is 4#3/0

2. Is 4#4/0

The two panelboards are 200 MCB and 225 MCB 208/120V three phase


Question:

1. Its confusing NEC 2017 section 250.66 how does one size the grounding electrode conductor sum of 3/0 and 4/0 or based on largest which is 4/0 or something else? What code section as well?


f6a4cada8442cfab5e6d365437cb1685.jpg
 
I think you may have another problem. 75kVA is only a little over 200 Amps at 208 Volts. I don't see how two panelboards in parallel like this provides adequate protection for the transformer secondary.
 
petersonra said:
I think you may have another problem. 75kVA is only a little over 200 Amps at 208 Volts. I don't see how two panelboards in parallel like this provides adequate protection for the transformer secondary.
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Primary breaker is 100A at 480V three phase. Table 450.3(b) if primary is 125% then secondary no overcurrent protection require.

So would it still be a problem?
 
hhsting said:
Attached sketch shows 75kva transformer two secondary conductors:

1. Is 4#3/0

2. Is 4#4/0

The two panelboards are 200 MCB and 225 MCB 208/120V three phase


Question:

1. Its confusing NEC 2017 section 250.66 how does one size the grounding electrode conductor sum of 3/0 and 4/0 or based on largest which is 4/0 or something else? What code section as well?


f6a4cada8442cfab5e6d365437cb1685.jpg
Click to expand...
See Note 1 to Table 250.66
 
petersonra said:
Primary only protection for secondary conductors is allowed for delta and 2 wire systems only. 240.4(F)
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Sure, so the secondary conductors have to be protected in accordance with 240.21(C). But the transformer itself can be protected by primary only per 450.3(B). Those are two separate issues.

Cheers, Wayne
 
wwhitney said:
Sure, so the secondary conductors have to be protected in accordance with 240.21(C). But the transformer itself can be protected by primary only per 450.3(B). Those are two separate issues.

Cheers, Wayne
Click to expand...
yes.

why not make both of them 225 A PBs. make one of them MLO the other with a MCB. Feed to the MCB unit and feed thru to the MLO unit.
 
hhsting said:
That note is about Service entrance conductors. Transformer secondary conductors are Not service entrance conductors. How do you figure that note applies?
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See 250.30(A)(5). Also read the parent text before Table 250.66. You can never just look at one small part of a code rule.
 
don_resqcapt19 said:
See 250.30(A)(5). Also read the parent text before Table 250.66. You can never just look at one small part of a code rule.
Click to expand...

Ok I am guessing because of 250.30(A)(5) one can replace service entrance conductors in notes and table 250.66 with secondary derived conductors. If thats the case then I wonder when Table 250.66 note #2 would come into play for transformer grounding electrode conductor size? When their is no secondary derived conductors according to note #2 table 250.66
 
hhsting said:
Ok I am guessing because of 250.30(A)(5) one can replace service entrance conductors in notes and table 250.66 with secondary derived conductors. If thats the case then I wonder when Table 250.66 note #2 would come into play for transformer grounding electrode conductor size? When their is no secondary derived conductors according to note #2 table 250.66
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Everything related to the size of the grounding electrode conductors found in 250.66 applies to the grounding electrode conductor for a SDS. It is a catch all rule for grounding electrode conductors, and in older codes it was also for supply side and other bonding conductors, but the words always said "service". The bonding conductors sizing was moved to 250.102 a few cycles ago.
 
hhsting said:
Attached sketch shows 75kva transformer two secondary conductors:

1. Is 4#3/0

2. Is 4#4/0

The two panelboards are 200 MCB and 225 MCB 208/120V three phase

Question:

1. Its confusing NEC 2017 section 250.66 how does one size the grounding electrode conductor sum of 3/0 and 4/0 or based on largest which is 4/0 or something else? What code section as well?
Click to expand...
Read 250.64. Also 250.64(D)(1) specifically addresses your question. Largest ungrounded conductor or sum of largest ungrounded conductors (in parallel (referenced table 250.66 left columns description/instructions). Not really any different than sizing of EGC branch circuit run together in a conduit that we sized to largest conductors.
The reference to 250.30(A)(5) also says "sized in accordance with 250.66"
 
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