Derate Common Conductor, 220-22 2002

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tryinghard

tryinghard

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California
1) Can I derate the common conductor in a 3PH 4W delta feeder?

2) I understand this to mean I cannot derate the common conductor in a 3PH 4W wye feeder is that correct?

3) Am I referring to the correct article to qualify this issue?

If I am left alone I am in the habit of not derating it but this feeder is existing.
 
First, Article 220 has nothing to do with ?derating.? That word has to do with limitations on the ability of a wire to carry current. Article 220 is calculating the amount of load the wire is going to be called upon to carry.

Secondly, I don?t know what you mean by a 3 phase, 4 wire delta. Are you talking about grounding the center point of one of the phases?

If your question has to do with selecting a size for the neutral wire, and having that size be smaller than the phase conductors, then 220.20 is the right article to be using.

On a 3 phase, 4 wire WYE system, you are allowed to have the neutral be smaller than the phase conductors. It only has to be sized for the calculated value of unbalanced current. That is, it is based on having an unbalanced load in the phase conductors. In addition, if the calculated value is above 200 amps, then you can apply a demand factor of 70% to the part that exceeds 200. For example, if you calculate a neutral current of 300 amps, you only need to size the neutral wire for 270 amps (200 plus 70% of the remaining 100).
 
charlie b said:
Secondly, I don?t know what you mean by a 3 phase, 4 wire delta. Are you talking about grounding the center point of one of the phases?
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Well, yes currently I have an existing feeder to a panelboard as: 120/240 3PH 4W, 3-1/0?s & 1-#4 cu
charlie b said:
If your question has to do with selecting a size for the neutral wire, and having that size be smaller than the phase conductors, then 220.20 is the right article to be using.
Click to expand...
Yes this is the issue at hand with the #4 cu common conductor (neutral)
charlie b said:
On a 3 phase, 4 wire WYE system, you are allowed to have the neutral be smaller than the phase conductors. It only has to be sized for the calculated value of unbalanced current. That is, it is based on having an unbalanced load in the phase conductors. In addition, if the calculated value is above 200 amps, then you can apply a demand factor of 70% to the part that exceeds 200. For example, if you calculate a neutral current of 300 amps, you only need to size the neutral wire for 270 amps (200 plus 70% of the remaining 100).
Click to expand...
In this light how does the end of 220-22 apply (in 2005 this is located in 220-61(C)?

This takes me back to my question, can I [reduce the size] of the common conductor in a 3PH 4W delta feeder, in other words can I make it smaller than the phase conductors?
 
220.61(C) does not apply to a 3 phase high leg system. You treat the grounded conductor just like on a single phase 3 wire system.
Don
 
tryinghard said:
How do I size this grounded conductor?
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Simply speaking, first as Don mentioned, you can ignore the fact that this is a high-leg system, and look only at the two 120v phases and the neutral. Any load that uses only the ungrounded conductors, meaning 240v with no 120v load, will have no neutral current.

Your neutral's greatest possible load would be if you turned off every breaker on one phase, and turned on everything connected to the other phase. If your load was 100a at 240v and 75a at 120v (per phase), the neutral need only be sized for the 75a portion.

What I find interesting is that the high-leg conductor is as large as the other two phases. I say that because the usual high-leg Delta service, often but not always an open-Delta system, has a majority of single-phase loading, and lesser 3-phase.

My most recent service has two 3/0 hots, one 1/0 neutral, and a #4 high-leg. In fact, the neutral could have been as small as a #3, but I went larger because the only grounding electrode was the driven rods, as the entire plumbing system was plastic.
 
Well I?m going to calc it! This application gets even more bizarre in that this is a tap. Any more I rarely see neutrals down sized. I do know most electricians do not consider the neutral load when they add future loads especially on design build projects. I have been in the habit for quite a few years of sizing the grounded conductor to match the phase conductors this does cost more but eliminates neutral overloading.
Thank guy?s I appreciate your input!
 
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