Grounded (Neutral) Conductor Sizing NEC 220.61

[cdcengineer]

What's the consensus. If ungrounded conductors are increased in size due to voltage drop concerns, is the neutral or grounded conductor also increased?

I know the equipment ground must be upsized per NEC 250.122(B). I'm thinking the answer is yeas based on 240.23. Although the term "shall be permitted" rather than must be, or is required has got me thinking.

Thoughts and input are greatly appreciated.

 

[Dennis Alwon]

What's the consensus. If ungrounded conductors are increased in size due to voltage drop concerns, is the neutral or grounded conductor also increased?

I know the equipment ground must be upsized per NEC 250.122(B). I'm thinking the answer is yeas based on 240.23. Although the term "shall be permitted" rather than must be, or is required has got me thinking.

Thoughts and input are greatly appreciated.

The grounded conductor would only need to be upsized if it is required for VD or other reasons. The grounded conductor, however, cannot be smaller than the egc.

 

[cdcengineer]

Voltage Drop shouldn't be an issue with the neutral unless it's a current carrying conductor (as might be the case with certain types of lighting, etc.). No?

So the neutral is not automatically upsized to match the feeders when the feeders are upsized due to voltage drop?

 

[don_resqcapt19]

If this is a feeder, take a look at 215.2(A)(1) in the 2008 code.

215.2(A)(1) ... The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel. ...

 

[Dennis Alwon]

Voltage Drop shouldn't be an issue with the neutral unless it's a current carrying conductor (as might be the case with certain types of lighting, etc.). No?

So the neutral is not automatically upsized to match the feeders when the feeders are upsized due to voltage drop?

I would agree in that scenario. In some cases the neutral is only needed for a 120v control circuit. In that case the neutral still cannot be smaller than the egc. Don gave the article I neglected to cite.

 

[cdcengineer]

Thanks to you both..

Our scenario is this:

We have a 400Amp, 208Y/120V handle in the MDS which is serving a branch circuit panelboard (84 circuit, rated 400Amps) with 350kcmil AL THW feeders with a 250kcmil neutral. The EGC has been upsized with the ungrounded feeders. The actual length is roughly 275', and the calculated load is roughly 320Amps.

My voltage drop calculation (@3%) at the max panel rating (400Amps) allows a run of roughly 290' with 350kcmil AL, at 325Amps (the calculated load), the 250kcmil AL would be good for roughly 260'. So I'm not sure why the neutral was ever down-sized in the 1st place?

Now bear in mind that these are existing conditions... If new, I wouldn't size a neutral smaller than the ungrounded conductors unless I could do so based on 220.61, however, the ownership wants to keep these feeders unless it's absolutely necessary to change them.

I think we're good with the 250kcmil neutral.

Thanks

 

[cdcengineer]

Bump

So to ask again, if I upsize feeders due to voltage drop concerns, do I have to upsize the neutral? I know we upsize the equipment grounding conductor.

 

[Dennis Alwon]

I am still not sure what your calculated neutral load is?

 

[cdcengineer]

It's hard to say. There are a ton of lights, general use receptacles and small pumps and motors.

If I had to estimate, the total load on the panel is 350 Amps (125kVA). Of that, I would estimate that 90kVA is 120V load. Hard to say what the neutral might see.

Am I looking at this right?

 

[Smart $]

It's hard to say. There are a ton of lights, general use receptacles and small pumps and motors.

If I had to estimate, the total load on the panel is 350 Amps (125kVA). Of that, I would estimate that 90kVA is 120V load. Hard to say what the neutral might see.

Am I looking at this right?

Yes... but your neutral load is dependent on balance. If balanced, it would be 90kVA ? 3 = 30kVA estimated. If unbalanced, you'll have to separate the calculated load by line and go with the greatest line value.

 

[Dennis Alwon]

So if your neutral load is balanced then as Smart said you have 30KVA on the neutral which is about 250 amps. So now you do a VD calculation based on the distance and the load-- Thus if your distance is 200 feet one way then at 250 amps and a 3% drop you would need a 400kcm copper conductor.

 

[cdcengineer]

Thanks to all for the input.

For some reason years ago when a contractor upsized the feeders, they failed to upsize the neutral. Now they'll have to pull em' all out and repull with larger neutral.

 

[Smart $]

Thanks to all for the input.

For some reason years ago when a contractor upsized the feeders, they failed to upsize the neutral. Now they'll have to pull em' all out and repull with larger neutral.

They'd have to do that regardless. Aluminum 350kcmil THW is only rated for 250A max.

 

[cdcengineer]

Parallel sets..

 

[Smart $]

Parallel sets..

That's a significant piece of info that you left out earlier...

The voltage drop on parallel neutrals, for stated conditions, is at a practical level IMO. In making your own evaluation, consider the likelihood the neutral will ever see the max neutral current. The easiest part of that evaluation is considering what the neutral current will be when the panel is operating at maximum (i.e. calculated) load.

 

[cdcengineer]

Whoops, I thought I stated parallel sets early on, but I did forget.

Damn, I was hoping to avoid calculating the neutral current. And what worries me, is the amount of space left in the panel and what they might fill it with in the future.

The real question though is, does the neutral always need to be upsized with the feeders? I know the equipment grounding conductors are increased by the same percentage in diameter as the feeders. I couldn't find anywhere in the code that forces the neutral to be increased. However I do see where 220.61(B)(2) might allow a decrease if the loads are unbalanced.

Thanks.

 

[Dennis Alwon]

The real question though is, does the neutral always need to be upsized with the feeders?

I thought we answered that. NO, if the load does not call for it then there is no need however, again, the neutral cannot be smaller then the egc. If calculation permit I could have a 100 amp feeder with a number 6 neutral. Even if the calculated neural load is 10 amps I still would need a #6 because of T. 250.122. Now when I upsize the egc would upsize so the neutral must also. If, however I already had a #2 neutral and upsize the ungrounded for VD then I would not need to upsize the neutral.

 

[cdcengineer]

Thanks Dennis. I completely understand this part of the concept, but what I wasn't so sure of is that if we had a 100A feeder w/ a number 6 neutral and I upsized the 100A feeder (or #1's) to a #2/0 (assuming all CU). Than would the #6 be bumped to a #4 - re:
Table 8 properties applied as follows:
133100(#2/0)/83690(#1) = 1.59, 1.59*26240(#6 neutral) = 41732 - which pushes it to a #4 AWG CU.

I don't think I represented this question clear at all, my apologies and thanks to all for the input.

 

[cdcengineer]

Base the neutral on the neutral load (with voltage drop considered). There's no need to automatically upsize the neutral with the line feeders unless the neutral size is based on a minimum per 250.122 and than, when the EGC grows, so too does the neutral.

This confusion is why I've never reduced a neutral in the first place. Unless you have a fixed load, it appears like it could be a risk because as load grows, so too can the unbalanced portion which might push the neutral up.
Thanks.

 

[Dennis Alwon]

Base the neutral on the neutral load (with voltage drop considered). There's no need to automatically upsize the neutral with the line feeders unless the neutral size is based on a minimum per 250.122 and than, when the EGC grows, so too does the neutral.

This is exactly correct. That is why I asked what the neutral load is. Generally, esp. in commercial application with non linear loads you general want the neutral the same size and at times I have seen it speced larger than the ungrounded conductors.