current carrying neutral

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sameguy said:
What everyone has told you is true.

But, let me put his out there for you guys,
He pulls all "full boats" lands the last one, not needed, on a duplex off some place (side of panel) now he has reduced neut. like the rest of the mwbc?
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I think I understand what you are saying, but that would not work after the side of the panel receptacle, after that the neutral will be current carrying because of the 120 degree separation of the two phase conductors.
 
Besoeker3 said:
Yes.
Sometimes, on this forum, I get a bit surprised by some of the questions.
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It's obvious this is a learning time for the OP. I hope his correction is not too expensive. I hope he learns from this and gets an understanding of how this works (the theory of it).

I've gone behind a "seasoned" (aka old, been doing this for decades) electrician back in the 70's that just couldn't understand why he couldn't put two A-phase conductors on one neutral (same size wire as the phase conductors).
 
oldsparky52 said:
It's obvious this is a learning time for the OP. I hope his correction is not too expensive. I hope he learns from this and gets an understanding of how this works (the theory of it).

I've gone behind a "seasoned" (aka old, been doing this for decades) electrician back in the 70's that just couldn't understand why he couldn't put two A-phase conductors on one neutral (same size wire as the phase conductors).
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Yes, I agree. I didn't mean to cause offence to the OP or anyone else but, if I did, I apologise unreservedly.

I've been around myself for decades and came across quite a few problems that surprised me that the site electrician couldn't diagnose.
 
Besoeker3 said:
Yes, I agree. I didn't mean to cause offence to the OP or anyone else but, if I did, I apologise unreservedly.

I've been around myself for decades and came across quite a few problems that surprised me that the site electrician couldn't diagnose.
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I don't think you were offense, and I agree with your comment that sometimes the questions seen are eyebrow raiser's. I learned to do electrical work by OJT. I was lucky enough that I had basic electrical theory in electronics school prior to starting as an electrician helper.

I know I've asked some pretty dumb questions in here in the past (I'm hardworkingstiff reborn), and everyone always tried to help me learn.

If the OP was offended by anything on this thread, then he really needs to toughen up. :)
 
To explain why a shared neutral for two lines of a 3ph system carries current:

Start with a balanced 3ph MWBC, where the neutral current is zero. Now, reduce the current on one line by one amp; the neutral current rises to that same one amp. Reduce the current by ten amps, and the neutral rises by the same ten amps.
 
hhsting said:
How does the last 2 phase and neutral partial MWBC is counted as 3 current carraying? Neutral is unbalanced from lets say Phase A and Phase B currents.

And on full MWBC neutral if all loads same would be balanced so counted as current carrying?
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Every neutral conductor of your branch circuits is carrying same current as it's associated phase conductor, there is no way to balance any currents in this arrangement to reduce heating effects caused in any of the conductors involved. The feeder to the panel your circuits originate from however would only carry unbalanced current on it's neutral, so presuming all 11 circuits had identical loads and are as balanced as possible across three phases - only two circuits would be contributing to unbalance on the feeder neutral.
 
oldsparky52 said:
I don't think you were offense, and I agree with your comment that sometimes the questions seen are eyebrow raiser's. I learned to do electrical work by OJT. I was lucky enough that I had basic electrical theory in electronics school prior to starting as an electrician helper.

I know I've asked some pretty dumb questions in here in the past (I'm hardworkingstiff reborn), and everyone always tried to help me learn.

If the OP was offended by anything on this thread, then he really needs to toughen up. :)
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Well, I hope he wasn't.
A bit of an aside - mods be kind but I think it somewhat relevant in the context of dumb questions raised.
A little eyebrow raised for me was on DC drives for winches. Quite an interesting project destined for tin mining dredges in Malaysia. We made the drives and the company who made the winches was located not far from where I lived.

Fairly big at 275kW and they were tested back to back, one winching in and the other out or rendering as is was known. This was a repeated cycle over a week or so as I recall.

Anyway, I got a call at home one Saturday morning from the mechanical designer..
"The drives are making a thumping noise."
Tootled over there and, sure enough, there was a thumping noise. And sounded very much one of the SCRs misfiring. Put the scope on the current feedback - perfect balance on both drives.

The motors were coupled by an internally toothed belt and toothed pulleys. Morse belts they were called. It was too loose and jumped a cog now and again hence the thump.

My point is that the mechanical designer, a clever chap, assumed it to be electrical. I've found that to be the case on a number of projects. Sometimes people, who should maybe know better, see the electrics as a bit of a black art.
 
Besoeker3 said:
My point is that the mechanical designer, a clever chap, assumed it to be electrical.
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Don't you just hate that? I took a job with a pump and tank company. They hired me because they were having problems with the electrical contractors in wiring up the fuel control systems (1981). For the 1st 6 months every time a service tech was having problems with the start up his response was it's a wiring problem.

After over and over proving the wiring was fine and helping to find "the" problem, the service startup techs quit saying it was a wiring problem and started actually troubleshooting.
 
hhsting said:
NEC 2014 Article 310.15(B)(5)(a): neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a).
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310.15(B)(5)(a) seems to be talking about a shared neutral circuit. A common residential example would be the garbage disposal on the black line and the dishwasher on the red line, both sharing the same neutral. In this case, the unbalanced current between the disposal and dishwasher is carried by the neutral so it is not counted. I think 310.15(B)(5)(a) should include the term "shared neutral circuit" for clarification.
 
captaincutback said:
310.15(B)(5)(a) seems to be talking about a shared neutral circuit. A common residential example would be the garbage disposal on the black line and the dishwasher on the red line, both sharing the same neutral. In this case, the unbalanced current between the disposal and dishwasher is carried by the neutral so it is not counted. I think 310.15(B)(5)(a) should include the term "shared neutral circuit" for clarification.
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What is written "neutral conductor that carries only the unbalanced current from other conductors of the same circuit" says that but is very specific to what is intended. Simply saying "shared neutral" is more vague.
 
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