Service Conductor Size

[Electriman]

Greetings,

I need to have a 120/240V , 1000A, three phase, 4 wire service for a building. My questions are
1- Do I need to upsize the conductor since there are four current carrying conductors? I am thinking three sets of 600 MCM Al wire.
2- Is there anywhere in the NEC that allows me to down size the high leg and Neutral?

Thank you in advance.

 

[LarryFine]

1. No; neutral carries only unbalance current.
2. Yes; both may be sized per calculations

 

[Electriman]

1. No; neutral carries only unbalance current.
2. Yes; both may be sized per calculations

Where in the code says that?

 

[charlie b]

Regarding your question 1, 310.15(B)(5) says we don't count the neutral if it only carries the unbalanced current from the phase conductors. But I am not certain that is the case for a high-leg delta. I don't have the time at present to look into this. Perhaps someone else can offer an opinion. Regarding your question 2, we size all conductors to carry the calculated load. That is not "downsizing." It is "sizing."

 

[Electriman]

Regarding your question 1, 310.15(B)(5) says we don't count the neutral if it only carries the unbalanced current from the phase conductors. But I am not certain that is the case for a high-leg delta. I don't have the time at present to look into this. Perhaps someone else can offer an opinion. Regarding your question 2, we size all conductors to carry the calculated load. That is not "downsizing." It is "sizing."

I size it to carry the requested service in this case 1000 A. The load could be 400A or 500A and it changes due to future expansion.

 

[Dennis Alwon]

I have never seen anybody downsize just one leg of a 3 phase system. You cannot do this because the wire size must match the overcurrent protective device.

 

[Electriman]

I have never seen anybody downsize just one leg of a 3 phase system. You cannot do this because the wire size must match the overcurrent protective device.

What size of Al cable would you select for 1000A service?

 

[charlie b]

For a 1000 amp feeder, I would use four sets of 350 MCM aluminum conductors or three sets of copper conductors. You can do the math to see which would be cheaper.

 

[charlie b]

Dennis, the question I raised, the thing I don't know, is whether the neutral conductor of a high leg delta would carry only the unbalanced current from the three phases. To me, it is not obviously a yes or a no. On one hand, whatever current leaves on Phase A and doesn't return on either Phase B or C must necessarily return on the neutral. So that tends to make the answer "yes." But the math is not as simple as it is on a 4-wire from a wye secondary. I once proved, mathematically, that the total heat generated by balanced current in three phases will be higher than the total heat generated by lower currents in the phase conductors and the remaining unbalanced current in the neutral, assuming the same power is fed in both circumstances. But I don't have that proof handy anymore, and I don't know whether it would work the same for a high leg delta.

To that I will add that310.15(B)(5) gives us two cases in which the neutral would count as a CCC. The high leg delta is not one of those cases. So it is left up to us to decide whether the neutral only carries unbalanced current from the phases.

 

[Electriman]

For a 1000 amp feeder, I would use four sets of 350 MCM aluminum conductors or three sets of copper conductors. You can do the math to see which would be cheaper.

If we pick 4 sets 350 MCM, then you would have 4*250=1000A ampacity at 75. But once you derate it since you have 4 current carrying conductors then the maximum amapcity would be 280A*4*.8=896A. Would you in this case bump it up to a larger size wire?

 

[Dennis Alwon]

I don't know either Charlie but for commercial work the norm is to either upsize the neutral or keep it the same as the phase conductors. The code section in 310 does not allow the neutral to NOT be counted for a delta. The only exceptions are for a wye connection so that may tell us something

 

[Dennis Alwon]

If we pick 4 sets 350 MCM, then you would have 4*250=1000A ampacity at 75. But once you derate it since you have 4 current carrying conductors then the maximum amapcity would be 280A*4*.8=896A. Would you in this case bump it up to a larger size wire?

Yes, you would have to go larger if you are using a delta system. You don't see them very often. May I ask why you have a delta system

 

[charlie b]

Sorry Dennis. My point was not clear. I am not discussing whether we can reduce the size of the neutral. Rather, I am discussing whether the neutral should be counted as a current-carrying conductor for a high leg delta. Given the absence of harmonics, it would not count in a 4-wire wye. Would it count for a 4-wire high leg delta?

 

[Electriman]

Yes, you would have to go larger if you are using a delta system. You don't see them very often. May I ask why you have a delta system

Delta system is very common in TX for 120-240 three phase.

 

[charlie b]

The code section in 310 does not allow the neutral to NOT be counted for a delta. The only exceptions are for a wye connection so that may tell us something

I am not sure what "exception" you mean. 310.15(B)(3)(a) gives us a circumstance that does not require counting the neutral. The next two paragraphs (b) and (c) give us two circumstances that do require counting the neutral. I just don't know whether the high leg delta falls under the paragraph (a).

 

[Dennis Alwon]

I was looking at (b) and (c) as exceptions -- not an exception but it reads as one to me...hahaha

The fact that they mention wyes and not deltas I always assumed that a delta didn't apply but you would know more than I if (a) would apply.

 

[don_resqcapt19]

I have never seen anybody downsize just one leg of a 3 phase system. You cannot do this because the wire size must match the overcurrent protective device.

I have seen 4 wire delta systems with 3 conductor sizes many times where the service OCPD was provided by fuses. The two ungrounded conductors that serve the single phase loads are the largest, then the neutral, and the high leg the smallest.

 

[don_resqcapt19]

I treat the grounded conductor of a 4 wire high leg delta just like I treat the grounded conductor of a 3 wire 120/240 volt single phase system. For all practical purposes, it is the same electrically.

 

[LarryFine]

Regarding your question 1, 310.15(B)(5) says we don't count the neutral if it only carries the unbalanced current from the phase conductors. But I am not certain that is the case for a high-leg delta.

It is. A high-leg delta is (exactly the same as) a 120/240v 1ph source superimposed on a 240v 3ph source. The neutral behaves as expected on the 1ph portion of the load, and is irrelevant to the 3ph portion.

Regarding your question 2, we size all conductors to carry the calculated load. That is not "downsizing." It is "sizing."

Correct.

 

[LarryFine]

I have never seen anybody downsize just one leg of a 3 phase system. You cannot do this because the wire size must match the overcurrent protective device.

What I said above applies here, too. For example, you can feed a 200a 1ph MB panel and a 100a 3ph panel, the high leg only needs to be sized for 100a, and the neutral only needs to be sized for the 1ph neutral load.