Neutral as a current carring conductor

[bob]

I received an email with this question and got his ok to post it to the forum for your opinion.
Q. On a 1,600A 277/480V service, is the neutral conductor from the utility transformer to the service disconnect considered a current carrying conductor if the only line-to-neutral loads is a 200A lighting panel supplying non-linear loads?

A. The neutral (grounded) service conductor for this installation is considered a current carrying conductor for conductor ampacity adjustment purposes because a major portion of the load consists of nonlinear loads [310.15(B)(2)(a) and 310.15(B)(4)(c)].

My Response to Mike was:
You say that the neutral is a current carring conductor because a major portion of the load consists of nonlinear loads [310.15(B)(2)(a) and 310.15(B)(4)(c)].
I would, based on the information supplied, disagree . I would say that since the main is 1600 amps, the load shown in the question below is not the only load on the main. Therefore the 200 amp load shown may be very small and not a major portion of the load on the main switch.

Mike's Response :
If the 200A is 100% of the total neutral load, then it's a major portion. This conductor is 100% loaded with nonlinear loads. It's not the % of load based on the ungrounded conductors. The issue is heat and a 100% loaded conductor (sized 4x smaller) is still a heat generating conductor

 

[charlie b]

Re: Neutral as a current carring conductor

I tend to agree with you, but we need to be clear about a couple of points.

First, let us agree that the feeder to the panel has 200 amps (or so) worth of load, all of which is non-linear. That means that the neutral conductor counts as current-carrying, and that the feeder?s ungrounded conductors must be de-rated. That said, your question relates to the service, not to a feeder.

Next, me speak in terms of design decisions (I?ll get to code issues last). My opinion will depend on your selection of a size for the service neutral.
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• <font size="2" face="Verdana, Helvetica, sans-serif">Suppose you sized the service neutral the same as the service ungrounded conductors. In this case, 200 amps worth of neutral current going through a conductor sized for 1600 amps will not cause much heating. This neutral would not count as current-carrying.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">Now suppose you did a service calculation that showed a total neutral current of under 200 amps. Suppose further that you sized the service neutral for the 200 amps only. (ASIDE: I am not certain that reducing the neutral this far is even allowed. But this is an academic discussion, not a code discussion) In this case, the 200 amps worth of neutral current going through a conductor sized for 200 amps will cause a great deal of heating. This neutral would count as current-carrying.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">Looking, for example, of a set of 4 parallel 600 MCM cables in the first case, and at a single 3/0 in the second case, the difference in the heating effect is a 14 to 1 ratio. That is, the heating effect of 200 amps flowing through a set of 4 parallel 600?s is only about 7% of the heating effect of the same 200 amps flowing through a single 3/0. I base these figures on the resistance values in Table 9.</font>

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Finally, here is my take on the cited code sections. Please note that they speak of the majority of the ?load.? They do not mention the majority of the ?neutral current.?
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• <font size="2" face="Verdana, Helvetica, sans-serif">The load is 1600 amps. The portion that is non-linear (i.e., 200 amps) is not the majority of the load, for it is only 12.5% of the load.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">The total neutral load is 200 amps. The portion that is non-linear (i.e., all 200 amps) is certainly the majority of the neutral load, for it constitutes 100% of the neutral. But that is not relevant.</font>

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[don_resqcapt19]

Re: Neutral as a current carring conductor

Charlie,
For all practical purposes the effect of the current from this neutral can be ignored as you have pointed out. If there are 4 sets of 600 kcmil for this service the code minimum neutral would be a 1/0 in each raceway. However, I'm not sure about the code rule. The section [ 310.15(B)(4) ] is titled "Neutral Conductor" and you can imply that the non-linear loads in 310.15(B)(4)(c) only apply to the neutral and not to the total service load.
Don

 

[bob]

Re: Neutral as a current carring conductor

Charlie and Don
Thanks for your reply. I think that if the main switch had been a 600 amp or a 400 amp this example would have been easier to understand. In this case if the designer had called for a full neutral or even one with a capacity of 800 amps, the 200 amp load would have been insignificant.