What changed, the wire or the breaker?

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).

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

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)

resqcapt19,

Thanks for the reply.

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

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).]

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?

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

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.

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.

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.

Sorry I was wrong again if the neutral is routed only with single phase circuit then you dont have to count it but if it is with 3 phase then you do as below now this is from "99"

Table 310-15(b)(2)(a). Adjustment Factors for More than Three Current-Carrying Conductors in a Raceway or Cable

(4) Neutral Conductor.
a. A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of Section 310-15(b)(2)(a).

(310-15(b)(2)(a)
.(2) Adjustment Factors.
a. More than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled longer than 24 in. (610 mm) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310-15(b)(2)(a).)

b. In a 3-wire circuit consisting of two phase wires and the neutral of a 4-wire, 3-phase wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of Section 310-15(b)(2)(a).
c. On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.

So... for a 3-Phase 240/120 Delta system. On a 400A, 600A & 800A service, what size conductors would be necessary. Each set paralleled in seperate conduits, using the largest size and the fewest parallels as practical.

I always used one set of 500 for 400A and two sets of 500 for 800A and the inspector passed it. Was this wrong?

I don't believe I have ever done a 600A service but I'm designing a job that someone else is intending to use paralleled 500's for a 600A service. I was told that the paralleled 500's would not give me 800A.

Who's right?

Thanks for all of the replies and good discussion.

Hi Master66

The 2 sets of 500's would give you a 760 amp service but it is permissible to use a 800 amp breaker to protect them. If your project requires a full 800 amp service or the calculated load is greater than 760 amps then you would need to use 600's. If you use 1 set of 500's for a 400 amp service then your load cannot exceed 380 amps but a 400 amp breaker is ok. The load on the conductors can not exceed the ratings shown in table 310-16 but if the ratings of the conductors do not correspond to a standard rating of fuse or circuit breaker the next higher rating can be used. This only applies if the rating is 800 amps or less. I would say that if the project calls for a 400 amps then you need to use 600's, 2 sets of 600's for 800 amps and 2 sets of 350's for 600 amps.

master66,
In a 3 phase 120/240 volt delta system, the grounded conductor is not current carrying and no derating would be required. If the system is 120/240 volt delta, I stand by the answer I posed on June 8. If the system was 208/120 wye, derating may be required if there is a lot of nonlinear loads.
Don

Thanks for the replies guys.

It seems that I have been right all along in my past practices. I just got some bad information recently.

Thanks again.

230.90.A states that each ungrounded service conductor shallhave protection NOT higher than the ampacity of the conductors....
if the service disconect is a single overcurrent device this does NOT allow you to use the next higher size if there is a single OCPD.

tom

Tom,
What about exception #2 to the section that you cited?
Don