Reduced Neutral Size

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steve66

steve66

Senior Member
Location
Illinois
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Engineer
I want to double check my sanity:

Singel phase Line-Line loads don't cause any current to flow on the neutral, right?? And if I have 6 L-L loads, all connected to different combinations of phases, there still won't be any neutral current, no matter what combination of loads are on or off at any time, right again??

I'm still trying to fit a 400A feeder into an existing 3" conduit (208v, 3 phase service). The total calculated load is 137 KVA, but only 35KVA is for lighting and receptacles. The other 102KVA is all 208V Line-Line HVAC loads (most of that is 10KW electric heaters).

I am thinking I can reduce the neutral size to about 250A. So I would have (3) x 500KCM, a 250KCM neutral, and a #3 ground from the service disonnect to the main panel.

Steve
 
Re: Reduced Neutral Size

I will wait for others to post but it sounds to me that you are correct.
 
Re: Reduced Neutral Size

Originally posted by steve66: I want to double check my sanity:
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I can?t help you with that! :D
Single phase Line-Line loads don't cause any current to flow on the neutral, right??
Click to expand...
Yes.
I am thinking I can reduce the neutral size to about . . . .
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I think you are free to do that. I am presuming that you have ?calculated? the neutral load, in the same fashion that you calculated the phase load. Annex D has several examples in which the calculated neutral load turns out to be less than that of the phase conductors. I believe that you only have to size the neutral for the calculated load.
 
Re: Reduced Neutral Size

If you want to get an exact load for the grounded (neutral) conductor try this little formula. Find the square root of A squared plus B squared plus C squared minus A times B minus A times C minus B times C.

Example phase A 30, B 40, C50 amps each. The grounded (neutral) would have a load of 17.32 amps.
 
Re: Reduced Neutral Size

Watch out for a new rule in the 2005 NEC that requires a feeder grounded neutral conductor to be sized according to 250.122, not Art 220.
See 215.2(A)(1).
 
Re: Reduced Neutral Size

Thanks for the replies. I guess I'm still sane.

And please vote for me at the next election:
George Washington
 
Re: Reduced Neutral Size

steve66 - A reduced neutral in a feeder (or service) is very realistic & commonly used for an application like this.
You may even find that # 3/0 (or smaller) is to your liking.

I believe the formula that jwelectric shows is for where neutral connections are involved for all loads.
That is not your case.
 
Re: Reduced Neutral Size

I should have been more careful with the wording, since there are so many sharp people here. I plan to fuse the main disconnect at 400 amps, but the largest calculated phase current is 378 amps.

Steve
 
Re: Reduced Neutral Size

Steve, I would size the neutral to what is necessary too, but you could put (4)500's and a #3 (THHN/THWN) in 3" EMT, IMC, or RMC and be code compiant.


Roger
 
Re: Reduced Neutral Size

Thanks Roger. It is also very close to fitting in a 3" schedule 40. I calculate that at a 40.3% fill. It is times like this I am tempted to round off the decimal, and call it close enough. Even if I rounded 43% down to 40%, is it really going to make the installation unsafe?

But I don't know if the conduit that is existing is Schedule 40 or Schedule 80. There isn't much exposed, and the labeling seems to have worn off.

Steve
 
Re: Reduced Neutral Size

Originally posted by tom baker:
Watch out for a new rule in the 2005 NEC that requires a feeder grounded neutral conductor to be sized according to 250.122, not Art 220.
See 215.2(A)(1).
Click to expand...
I'd heard that we couldn't "derate a neutral." I can live with a real table to go from, that's all right. :D

Edit--just read it. Actually the rules haven't changed, we just have 250.122 as a minimum size, now. Careful how you say things, Instructor. ;)

Hey Tom, nice train. Do you have to be a moderator to get an avatar around here?

[ April 27, 2005, 08:06 AM: Message edited by: georgestolz ]
 
Re: Reduced Neutral Size

Originally posted by georgestolz:Hey Tom, nice train. Do you have to be a moderator to get an avatar around here?
Click to expand...
Yes. It's one of the "perks." :D It's also one way to tell quickly when a comment or question is posted by a Moderator. But I agree: Nice train.
 
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