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Joined: Apr 2002
Posts: 257
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Question;
According to the 2002 NEC, what size copper conductors are required for a 3-Phase 120/240V Delta 400 Amp, 600 Amp and 800 Amp service? Figure 1 set of 4 conductors per conduit (PVC).
The answer may surprise you. It surprised me (if I read the code correctly).

Joined: Dec 2000
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I think I'd parallel some 4/0 or 250 in each of the apps (do your own math!).

Joined: Nov 2000
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How many service OCPDs are there and what is the calculated load on the services? It does make a difference in the code answer to this question.
Don(resqcapt19)


Don(resqcapt19)
Joined: Apr 2002
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resqcapt19,

Thanks for the reply.

There is one OCPD and assume the calculated load is the same (or slightly under) the OCPD rating.

Joined: Nov 2000
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master66,
I don't see any changes here. Any conductor or set of conductors with an ampacity exceeding 350 amps can be used with the 400 amp service, over 500 amps for the 600 amp service and over 700 amps for the 800 amp serivce is permitted by Exception #2 to 230.90(A).
Don

[This message has been edited by resqcapt19 (edited 06-09-2002).]


Don(resqcapt19)
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resqcapt19,

This is how I have done it for years but was told recently that the SE conductor could not be rated less that the OCPD and that the rating had to be calculated in accordance with 310.15. In my case, for a machine shop, I believe that the neutral should be counted.

I have also asked other inspectors this same question and was told the same as you said.

Who's right?

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master66
If derating is required because of more then 3 current carrying conductors or high ambient temperature, this changes the ampacity of the conductors. The ampacity after derating must be equal or greater then the numbers in my earlier post. This in not new. Table 310-16 ampacities are only for 3 or less conductors and an ambient of 30°C. If your grounded conductor is in fact a current carrying conductor, then you would have to use conductors with a table ampacity of 438 amps for a 400 amp service. This would require 600 kcmil or parallel 3/0s in seperate conduits, or parallel 4/0s in a common raceway. The wire sizes I have listed assume a 90°C insutlation.
Don


Don(resqcapt19)
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If the calculated load of your 400 amp service is 380 amps or less then 500 MCM copper is fine but if your load is 381 amps then you would need to use 600 MCM. The same goes for the 600 amp and 800 amp services. As long as the calculated load does not exceed the conductors rating your ok. What do you mean by "I believe the neutral should be counted"? If you are saying that the neutral should be counted for derating purposes I am not sure why. If this is a delta service the neutral is only carrying the unbalanced load of a single phase winding on the transformer so harmonics should not be a problem.


Curt Swartz
Joined: May 2002
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It is still classified as a current carrying conductor. didn't think you would have a neutral in an open delta. but you would in a "Y" or center tap (A-C Phase) high leg delta.


Be Fair, Be Safe
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Joined: Apr 2002
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Yes, it is current carrying conductor but it only carries the unbalanced load on the single phase winding of the transformer. There is no way all 4 conductors can be full loaded at the same time. 310-15(b)(4). If the conductor ampacity needs to be adjusted because of ambient temperatures then the neutral size would also need to be adjusted.


Curt Swartz
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