Adjusting Service Conductors 310.15

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C10

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Hello, sure would appreciate help with determining service conductors required considering adjustments per table 310.15. Or if another solution would be recommended.

The project is multifamily. Service load was calculated using 220.84. Load is 1276A with service voltage 240/120V single phase. The meter pack will have a 1400A breaker. The site also has 2 existing 4" schedule 80 pvc conduits. Ideally the 2 existing 4" conduits would be reused and not require trenching. Poco will not allow larger than 500kcmil. Conductors will be THHN copper.

Per table 310.15(B)(16), 4 sets of 500's THHN looks like an option with 2 sets in each of the 2 existing 4" conduits. Using 75C column for 500's is 380A. Each conduit would have 4 CCC (2 sets single phase and 2 neutrals). This would require adjusting at 80% per table 310.15. (380A)(4)(0.80) = 1216A unfortunately not large enough, poco will not allow larger than 500kcmil.

Per table 310.15(B)(16), 6 sets of 350's may work. Per table C.8(A) each 4" PVC conduit will allow the 3 sets (9-350's THHN). Using 3 sets in each conduit, we will have 6 CCC in each conduit. This will require using the 80% adjustment in 310.15 also. (310A)(6)(0.80) = 1488A.

So 6 sets of 350kcmil is adequate considering the 1400A breaker.

Thanks in advance!!!
 
Actually you can use THWN-2 and then you can use the 90? rating for derating purposes*.
Not relevant as it still won't bring your 500s up to 1400 amp, but worth noting
Your 350s appear compliant.
*You still can't exceed the 110.14 rating for terminating, normally 75?)
 
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C10 said:
Hello, sure would appreciate help with determining service conductors required considering adjustments per table 310.15. Or if another solution would be recommended.

The project is multifamily. Service load was calculated using 220.84. Load is 1276A with service voltage 240/120V single phase. The meter pack will have a 1400A breaker. The site also has 2 existing 4" schedule 80 pvc conduits. Ideally the 2 existing 4" conduits would be reused and not require trenching. Poco will not allow larger than 500kcmil. Conductors will be THHN copper.

Per table 310.15(B)(16), 4 sets of 500's THHN looks like an option with 2 sets in each of the 2 existing 4" conduits. Using 75C column for 500's is 380A. Each conduit would have 4 CCC (2 sets single phase and 2 neutrals). This would require adjusting at 80% per table 310.15. (380A)(4)(0.80) = 1216A unfortunately not large enough, poco will not allow larger than 500kcmil.

Per table 310.15(B)(16), 6 sets of 350's may work. Per table C.8(A) each 4" PVC conduit will allow the 3 sets (9-350's THHN). Using 3 sets in each conduit, we will have 6 CCC in each conduit. This will require using the 80% adjustment in 310.15 also. (310A)(6)(0.80) = 1488A.

So 6 sets of 350kcmil is adequate considering the 1400A breaker.

Thanks in advance!!!
Click to expand...

With 6 sets, you don't even need 350, 300 MCM is adequate, 250 is only 8 amps off....maybe the ahj will let you do a 310.15 (b)(2)(a) adjustment ;)
 
augie47 said:
Actually you can use THWN-2 and then you can use the 90? rating for derating purposes*.
Not relevant as it still won't bring your 500s up to 1400 amp, but worth noting
Your 350s appear compliant.
*You still can't exceed the 110.14 rating for terminating, normally 75?)
Click to expand...

If we have an existing set of 500's would we be allowed to run 2 sets of 250's in each 4" conduit to fit within the 4" conduit.

Thanks again.
 
C10 said:
If we have an existing set of 500's would we be allowed to run 2 sets of 250's in each 4" conduit to fit within the 4" conduit.

Thanks again.
Click to expand...

are you asking if you can parallel the 250s with the 500s ??
If so, No. 310.4
 
C10 said:
Yes, reading 310.4(B)3 says "Be the same size in circular mil area". Therefor the 3 sets within the same conduit must be same kcmil.
Click to expand...

Even if in separate conduits all paralleled conductors of the same phase, neutral or EGC must have the same characteristics. Size, insulation type etc.
 
The semantics might be improved, but the intent is EACH parallel conductor....
 
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