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Subject - Down sizing Feeder Nuetral
LordDrago I've always been able to downsize a 200 amp service nuetral to a 2/0 when using a 4/0 feeder. I've seen the code many years ago before that allows this, but for the life of me, I can't seem to find the code anywhere to back up my position. I often train a lot of helpers, and its rough when your helper ask how you are allowed to do something and you can't provide the code. I keep telling him that I've done this for 16 years, and never have I had a red tG FROM ANY INSPECTOR. Please help.
I may have been missleading in the way I presented my question. It is a residential service. 200 amp, single phase, 4/0 XHHW (Line), 2/0 XHHW (Nuetral). Again I just had an inspection 10-21-04 with a real hard @$$ inspector. I figured if I was to be rejected, it would be by him. He's a real stickler.
KSsparky Have a look at 250.24(B) for the minimum requirement. Other than that, the grounded conductor must be capable of safely carrying the load as calculated in Article 220.
David Hyatt On your residential feeder calculation, 220.22 states that a neutral on a dryer, ranges and ovens can be counted at 70%. This is how your grounded conductor can be smaller.
As far as commercial goes, someone chime in is there a derate on the neutral?
Ryan_J Yes David, you can reduce the nuetral in commercial under certain conditions.

The one area where you can never reduce the nuetral is on a 4 wire wye system where using a 3 wire circuit (2 hots+nuetral).
lctrc789 This is a little confusing to many folks. I say that you are right on the 70% demand factor etc. residential loads can vary for many reasons, they may add range or dryer or central air etc.
I have always used 2/0 copper on all three wires to avoid any confusion in residential We always use the same conductor for the neutral in commercial as well, unless it is a paralleled and we use the 12 1/2 % rule. Article 250-24 (b) is confusing to many. It states you can use a conductor the size of the grounding wire for a grounded conductor but you have to read all the FPN and exceptions. It has been confusing for a long time. Maybe this should be addressed some time in the future.
RS377
quote:
Originally posted by David Hyatt

On your residential feeder calculation, 220.22 states that a neutral on a dryer, ranges and ovens can be counted at 70%. This is how your grounded conductor can be smaller.
As far as commercial goes, someone chime in is there a derate on the neutral?


Yes you derate the nuetral, as it is a curent carrying conductor. You don't have to derate the grounding conductor, as it never sees current (hopefully).
Ryan_J RS: I think you are mixing up applying demand factors and ampacity adjustment.
lctrc789 RS377, the only article I can think of in a larger job would be ARTICLE 250-24 a (b)1-2.
The grounded conductor can be used in the 12 1/2 % rule.
RS377
quote:
Originally posted by Ryan_J

RS: I think you are mixing up applying demand factors and ampacity adjustment.


You are 100% correct, I saw "derate" and jumped the gun
Ryan_J Thats an easy mistake to make. Lord knows I have jumped the gun too many times as well! :)
Pierre Belarge

I would just like to add that when sizing a grounded (neutral) conductor, some caution should be advised. I agree that most electricians seem to be confused as to the permitted size(s) that are allowed as per 250.24(B)(1). Remember that 250.24(B) deals with the grounded conductor brought to the service equipment. This conductor serves two purposes.
1. As an 'incoming' conductor it serves as the 'neutral'.
2. When a ground fault occurs, it completes the path to the source for an effective ground fault current path.

Because of number 2, sizing is very important as it is the link from the premises wiring to the source and therefore a very important link so the OCPD will open during a ground fault.

The less resistance (larger size) permits more current to flow, therefore assuring the OCPD to open in a fashion that helps to protect people and equipment.

Pierre
journeyman491us ...Any commercial guys out there with "oversized" or "super-neutrals". Think this would be a good topic for discussion.
lctrc789 It seems to be that in the past few years that have have wanted a larger neutral conductos for derating etc. I think you should follow the maufactures guidelines for transformers as well as any specs, in comm/Ind. jobs.
I know in residential in certain applications you can down size a neutral for the sake of argument why would you?
You never know what future demands may be put on the service.
luckyshadow What we have been seeing more of in the commercial field is 200% neutral. With the increased use of electronic ballast's the load on the neutral is much higher then years before. I have even heard talk of AHJ wanting increased neutral size on the branch circuits if wired as a multiwire. 3 # 12 phase conductors and 1 # 8 neutral. Haven't seen it yet but have heard the talk.