Parallel feeder conductor question

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electro7

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Northern CA, US
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Electrician, Solar and Electrical Contractor
I think I did this wrong and was wondering if I could get some input?

I have a 600A MCB in a switchgear cabinet, 120/240V single phase. We are coming off the load side of breaker to an ATS (600A) and back out of the ATS to the distribution. We ran 2 sets of 350 kcmil parallel conductors in the same conduit. My original thought was 2 350 kcmil would have an ampacity of 620A at 75C- (not thinking about circular mils).

It dawned on me the other day that I may have calculated this wrong and need to look at circular mils. When I do that calc 2 350 kcmil comes to 354 circular mils which is equal to just under 700 kcmil (355 circular mils). 700 kcmil ampacity is only 460A at 75C.

I was wondering if somebody can help me to know if the 2 350 kcmil conductors paralleled is okay or not okay?

If not okay, I was thinking about downsizing the main breaker to 500A to save thousands $ in conductors. I thought I could go to NEC310.15(A)(7) to leave the 350 kcmil conductors in and downsize the main breaker to 500A, but saw the 83% rule is only for 100A-400A.

Needing some help on this one. Thanks ahead of time!
 
BTW the lug configuration at the ATS is a two lug that accepts 500 kcmil max. That would mean I can land 2 500 kcmil conductors per phase max.
 
Assuming 75° termination and Cu, parallel 350s would be adequate for 620 amps (310 X 2)
If you have 4-6 current carrying conductors in one conduit longer than 24", you need to apply the 80% derating factor but using a 90° conductor that would apply to the 90° raing of 350amps and result in a 560 amp rating
If your load is 560 amps or less then Art 240 would allow you to use the 600 amp breaker.So, IMO, it depends on the load and IF the 80% derating id applicable.
 
Okay! So how do I figure the load? Take a load calculation of all the appliances, lights, etc?

So I may be okay. Yes on the derating 80%. I installed all parallel conductors in the same conduit.
 
electro7 said:
It dawned on me the other day that I may have calculated this wrong and need to look at circular mils. When I do that calc 2 350 kcmil comes to 354 circular mils which is equal to just under 700 kcmil (355 circular mils). 700 kcmil ampacity is only 460A at 75C.
Click to expand...

Not sure how you got some of these numbers, 2-350's equals 700 kcmil but for the purpose of ampacity you take the ampacity of one 350 and multiple by 2. So for example a 350 kcmil THHN conductor at 75 degrees C would have an ampacity of 310 amps, 2 sets would give you 620 amps. Augie gave you the adjustments required for using only one raceway.
 
electro7 said:
Okay! So how do I figure the load? Take a load calculation of all the appliances, lights, etc?

So I may be okay. Yes on the derating 80%. I installed all parallel conductors in the same conduit.
Click to expand...

As someone eluded to, the easiest way is to find the "As-Built" plans and use the calculations done on them by an Engineer.
 
Look closer to bottom of table to see how much larger a single conductor would need to be to get 600 ampacity.

Parallel conductors you add individual ampacity together - but do need to use adjustment factors if you put them all in same raceway.
 
Thanks for all the feedback! I think I am good and had it right in the first place. It is a new construction home and I will double check the load calcs.
 
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