Hi, I was wondering if the NEC permits downsizing the neutral in a feeder on a 1 phase 120/240 system when it primarily feeds 240 v motors? I have ~35 amps @ 240v in motors going on a feeder of #6 copper on a 70A breaker. Does this mean that I could downsize the neutral (to a #8) to carry a load of the remaining 35 amps for 120v loads? Or is this not allowed?
I don't normally try to skimp on this sort of thing, but I am forced to pull a feeder through 1" PVC sch 80 for 125' that probably has 360 degrees of bends in it. Every little bit counts. I need to fit 3#6, 1#8 (EGC) and 4#12 (control wiring) in to this thing. Am I crazy? The calcs work out to less than 40% fill, but I am still worried that this will be tough. Any comments? This is for a pool, original installers 15 years ago pulled in 12 #12 instead of a feeder, ignoring derating, worked for a while, but now a subpanel is needed near pool equipment.
Thanks for any help, Mike
[This message has been edited by mhulbert (edited 01-05-2005).]
I think that if you only have 2 120v breakers in the sub, on opposite phases your max neutral load is 20a, 140% of that is less than 30 so you could use a #10
220.22 Feeder or Service Neutral Load. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net computed load between the neutral and any one ungrounded conductor, except that the load thus obtained shall be multiplied by 140 percent for 3-wire ...
Re: Downsizing Neutral in Feeder#91212 01/05/0509:35 AM01/05/0509:35 AM
mhulbert- You might want to check with your inspector and see if he has a problem with running what I think you discribed as motor control wires in the same raceway with a feeder. If he is a purist he may want the wires separate from feeder conductors. The code permits it for branch circuits to functionally associated motors but that's it. Just me being picky.
Re: Downsizing Neutral in Feeder#91213 01/05/0511:36 AM01/05/0511:36 AM
George, those control wires are for lighting contactors. The timer was going to be by the main panel, and then I was planning on putting 2 contactors w/ 120v coils next to my subpanel to control outdoor lighting. Is this legal? I am having a brain fart, but I thought the code allowed exceptions for control wires, can't remember what section.
Re: Downsizing Neutral in Feeder#91214 01/05/0504:00 PM01/05/0504:00 PM
These are 120v (coil) lighting contactors? If so the signal wires are class 1. 725.26(B)(1) says they can share the same raceway with your power supply conductors. Since these contactors draw fairly low currents (~.5a) 725.28(B)(2) says you only have to look at the power supply conductors for derating and since there are only 3 it is moot. If your lighting controller provides supplimental O/C protection you can probably go with 16ga or 18 ga signal wires, 725.23. Just be sure they have normal 600v insulation.
Re: Downsizing Neutral in Feeder#91215 01/05/0507:06 PM01/05/0507:06 PM
hi, gfretwell, if we interpret 250.122 to mean that a 70A circuit needs a #8 EGC, then would it be dangerous for the neutral to be sized smaller? i thought the EGC was sized for current to open the breaker on a short, wouldn't that reasoning suggest at least that wire size for a short to neutral ?
Re: Downsizing Neutral in Feeder#91219 01/07/0502:28 PM01/07/0502:28 PM
The neutral is sized to 140% of the unbalanced load. You are describing a bolted fault in the feeder itself (the sub-panel O/C device protects the branch circuits) and if the code addressed that you could never put small conductors in a pipe with larger ones. You are right that the EGC must be sized by 250.122 but I don't think that applies to the neutral if the computed unbalanced load is significantly smaller than the total load. I am certainly open to be shown wrong tho.