Derate for 320 amp service calculation

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Nomad776

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Houston, TX
I am currently designing a 320 amp three phase 120/208 service for a commercial interior buildout in Houston. I plan on paralleling using 6-2/0 hots, 2-1/0 nuetral, 2-#6 for my cable schedule and I will be using EMT conduit. But Im exactly clear on the calculations when it comes to derating using the tables 310.16 and 310.15 (B)(2)(a). Can someone be so kind in showing me how to calculate the adjustment factors?
 
What size breaker are you planing to use?

Will you have one or more service disconnects?

What is the calculated load?

At best two sets of 2/0 CU will provide 350 amps of capacity at worst (In the same conduit) will only provide 312 amps of capacity.
 
I plan on using 400 amp fused at 320; load calculation came out at exactly 300 amp and I will have only one disconnect. I plan on using one conduit 3" using THHN insulation
 
With all the conductors in a single conduit and assuming there is not a significant amount of harmonic current you need to derate 80%

Assuming your using a 90 C insulation 2/0 Copper is rated 195 amps.

195 amps * 2 * .8 = 312 amps.

240.4(B) will allow up to 350 amps of overcurrent protection.
 
chris kennedy said:
Why not 4 400's (335 amps) and a 3 in 3 inch?
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It's a 3 phase service, your few conductors short.

In a perfect world with a 400 (320) amp meter and a 400 switch I would want a single set of 600s (420 amps) or two sets of 4/0s in a single raceway (416 amps) to provide a true 400 amp service.

But it's not my money. :)
 
chris kennedy said:
What are we fusing at here?
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He said the calculated load is 300 and he plans on 320 amp fuses.

He can do this with two sets of 2/0 copper in a single raceway but the calculated load can not exceed the 312 amp capacity of the conductors.
 
Bob, your 600's and a 3 will fit in a 3 1/2. 500's will give you 380 amps in a 3 inch. I'm just wondering why derate when one set of conductors will do the job?
 
chris kennedy said:
Bob, your 600's and a 3 will fit in a 3 1/2. 500's will give you 380 amps in a 3 inch.
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Not sure he needs the '3' as he said this is a service.

chris kennedy said:
I'm just wondering why derate when one set of conductors will do the job?
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Left up to me I would not, but some people do not have the tools to work with 600 copper or are just not used to it.

We install a lot of 400 amp feeders and our typical would method would be four - 600 Kcmil coppers in a single 4"
 
320 amp service

320 amp service

I would have to agree on 600s. By the timne you wrestle those in some of today's meter sockets, I'd definitely parallel conductors 1st. In this case, unless theres considerable expense involved, I'd probably go 500's, not much better that 600s..but a bit easier IMHO,. I'd be more comfortable with a 380 amp capacity for future growth and the availabilty of 400 amp fuses vs 320 or 350. I'd bet good odds, if as fuse ever did blow, a 400 would end up back in its place.
 
Parellel feeders

Parellel feeders

Our Local utility companys are adopting the the EC's here not feed overhead services with parallel runs anymore, The last 400 amp service i installed i had to run three 500's, it was only single phase.Jersey Central Power and Light was the Utility.
 
iwire said:
If this is a service like the title implies you don't need (and should not include) EGCs.
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That's why I asked. The EGC's in the service raceway are not needed and would create a parallel path for the neutral current.
 
iwire said:
If this is a service like the title implies you don't need (and should not include) EGCs.
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I understand the "don't need" part(358.60)
I don't understand about the "should not" part? Line side of the disco(230.82)?
 
celtic said:
I understand the "don't need" part(358.60)
I don't understand about the "should not" part? Line side of the disco(230.82)?
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If we are talking about service conductors the 'grounded' conductor provides the bonding.

If you add an EGC to the service conductors you just added the dreaded parallel grounded conductor.

If these are not service condutors but are feeders then I would say run the EGC. :)
 
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