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 ...

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.

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.

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.

Be careful, the Class 1 conductors need to be "functionally associated" and if the Class 1 conductors are in a raceway with a feeder that is supplying a panel it may be an issue. Look at 725.26(B)(1).

Hmmm, I wonder what "functionally associated" means? The contactors are fed from the panel that these control, in fact, both the panel and the contactor are adjacent, inside of a nema 4 cabinet.

Will have to ask the AHJ on this one...

mike

That's the way I was looking at it too Mike.
The feeder is the "power supply conductors" for the contactors.

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 ?

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.

I cannot fine any reference in the 2003 NEC but in the 2005 they seem to address this issue 215.2(A)(1)"The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122".I would like to know how you are fusing #6 conductors at 70 amps article 110.14(C) (1) (4) allows the use of the 75ºC column provided the ampacity of such conductors does not exceed the 75ºC ampacity which is 65A.If I am missing something Please let me know

There seems to be a major change in sizing the grounded conductor for feeders in the '05 NEC. The other issue of fusing could be a case of rounding up based on 240.4(B).

He should really be looking at voltage drop if he is really using any appreciable part of that 70a. I assumed he was just planning on a lot of inrush on his motors.
I do agree you can hold your nose and "round up". It might not even be a terrible design decision if the FLA of the motors is 15-20a, plus whatever he would have on a pair of 20a 120v circuits. The cruising amps may really be in the 25-30a range.

If you really have 35 amps for the motors them selves, there is only 17 amps left for the 120v loads. (210.19)

Just to answer a few of the above posts, I'm fusing at 70 A based on rounding up according to 240.4(B), using 75 deg terminations. Did not know about the new code change in 2005 regarding following 250.122 with the neutral. I will size it up to a #8(50A), and stay with #6 for my hots.

With all three motors on, it is 35A (@240V), that's using table 430.148, actual FLA is a little bit less due to high eff motors, however the biggest (3 HP) is just for jets and thus a non-continuous load. I am a little worried about inrush, however the pool controller will not turn all 3 on at once, so some stagger should alleviate that, they are running from #12's right now, and seem to work. This could be a heavily loaded feeder with everything on (hopefully rare).

I was thinking of upsizing to 2 #4, 1 #6, 1 #8, 2 #14 but I think the pull would be just too much (1" Sch 80, ~360 deg of bends). The math says it just barely fits in the pipe...it would be a beastly pull to be sure!

'd build it a lot bigger if I could start from scratch, but all of this 1" PVC is under a pool apron that's not getting ripped up. I am look in to reducing the HP on 2 of the pumps, or a higher efficiency. What else can I do?

Mhulbert
You are probably going to have a very difficult time pulling these conductors when all is said and done.

The 140% that is being mentioned does not apply for this installation. 220.22 mentions the 140% for "3-wire, 2-phase or 5-wire, 2-phase systems." These type of systems are very rare today, and the pool system may be 3-wire, but it is not 2-phase. It is a 3-wire single phase installation. If the load calculation for the feeders is less than the phase conductors, you can reduce it up to 70%.

Also the mention of Class 1 Remote-Control and Signaling Circuits for the "loadside" of these contactors may be a misinterpretation of the said conductors. I do not believe they are Class 1.
If in fact I do not understand your post, and these are Class 1 Remote Control and Signaling Circuits, than I believe they will not be permitted in the same raceway as the new feeders for the pool.


Pierre

Pierre, sorry about the 140%, you are right. I am thinking that throws me back to "The maximum unbalanced load shall be the maximum net computed load between the neutral and any one ungrounded conductor..."
If he puts one or two 20a breakers in there that means it could be a #12 (before the 2005 change)
As for the original question, has he actually looked at the load on a meter? If the 240v load is 35a and he can get by with two 15a 120v circuits he can use a 60a breaker and the neutral could be #10 under the 05 code.
He is still looking at voltage drop as being the deal killer.
If he is bumping up to #4 maybe he is looking at a 240/120v transformer and skipping the neutral.