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Re: Super Neutral [Re: iwire] #173458 01/10/08 07:49 AM
Joined: Mar 2005
Posts: 1,213
S
SteveFehr Offline
Member
215.4(A) limits super neutrals to 3 sets of 3-wire feeders, or 2 sets of 4-wire or 5-wire feeders. 4x 120V circuits would be 2 sets and allowable, would it not?

The size of the neutral would be determined by the maximum current. If this circuit were balanced- EG, 2x one phase and 2x the other, the neutral must be rated for 40A and #8 THHN is acceptable... #10 is limited to 30A by that pesky 240.4(D). If all 4 of those were off 1-phase; well, #1, 215.4(A) would not allow it, and also, it would have to be rated for 80A, and would require #4 THHN.

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Re: Super Neutral [Re: SteveFehr] #173463 01/10/08 09:26 AM
Joined: May 2005
Posts: 984
G
ghost307 Offline
Member
215.4(A) is for feeders, not branch circuits. I don't think that there is a similar exception for branch circuits.


Ghost307
Re: Super Neutral [Re: iwire] #173474 01/10/08 03:47 PM
Joined: Dec 2000
Posts: 4,282
electure Offline
Member
Bob, does this bring any memories? laugh All the breakers are 20 Amp, with the exception of the main.

As you posted it:

From the 2002 NEC Handbook

[Linked Image]


and as I modified it:

[Linked Image]


Does anybody have any objection to this?


This subject has come up every couple of years since ECN's inception.

Re: Super Neutral [Re: electure] #173476 01/10/08 04:55 PM
Joined: Mar 2005
Posts: 212
G
Gregtaylor Offline
Member
" I don't think that there is a similar exception for branch circuits."
Exception to what? The practice is not prohibited in the first place. Why would it need to be excepted?
I find it to be poor practice, but when I think about it, that's only because someone else trained me that way.

Re: Super Neutral [Re: Gregtaylor] #173477 01/10/08 06:16 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Originally Posted by Gregtaylor
Exception to what? The practice is not prohibited in the first place. Why would it need to be excepted?


I agree 100%, it's not prohibited to start with.

Quote
I find it to be poor practice, but when I think about it, that's only because someone else trained me that way.


I like that open minded point of view. smile


This subject has come up before and I could not figure out a way that this could save me any money or time, terminations start to get expensive and boxes will need to be bigger.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Super Neutral [Re: electure] #173478 01/10/08 06:17 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Originally Posted by electure
Bob, does this bring any memories? :


Definitely, I still post it from time to time. smile


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Super Neutral [Re: iwire] #173528 01/11/08 08:04 PM
Joined: Dec 2007
Posts: 4
A
ALEXT Offline
New Member
i believe they call this a multiwire branch ciruit. 210.4 the biggest problem i see with that setup is that it's awfully risky. if that super nuetral ever got cut or unhooked there could end up being 240v on all 4 circuits. a lot of equipment would get fried.

Re: Super Neutral [Re: ALEXT] #173546 01/12/08 05:40 PM
Joined: May 2002
Posts: 1,716
R
Roger Offline
Member
Originally Posted by ALEXT
i believe they call this a multiwire branch ciruit. 210.4 the biggest problem i see with that setup is that it's awfully risky. if that super nuetral ever got cut or unhooked there could end up being 240v on all 4 circuits. a lot of equipment would get fried.


Alex, in reality there could not be 240 volts on any one of the circuits if the neutral were lost, the voltage on each load would be the determined by it's resistance and if they were all equal nothing would happen and the circuits would keep working like nothing was missing.

Below is an illustration of a multiwire circuit with a broke neutral, using the formula I x R = E calculate what the voltage dropped across each load would be, you will see that neither load will see 240 volts.

[Linked Image]

Roger

Re: Super Neutral [Re: Roger] #173550 01/12/08 10:40 PM
Joined: Dec 2007
Posts: 4
A
ALEXT Offline
New Member
Roger, that is true if the loads are equal. but chances are with four circuits the loads are not going to be the same. say you have a 1000w appliance on one leg and a 100w on another leg. if the nuetral is lost that 1000w is going have to low a voltage and the 100w is going to have to high a voltage. your right, neither would see 240 volts, but neither would have the correct voltage either.

Re: Super Neutral [Re: ALEXT] #173559 01/13/08 09:22 AM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Originally Posted by ALEXT

your right, neither would see 240 volts, but neither would have the correct voltage either.


In the real world the loads are rarely equal, but they are often close enough that damaging over voltage does not happen.

My own homes service lost the neutral at the utility, I put my recording meter on one leg of the service and had lows of 80 volts and highs of 140 volts, I did not lose any appliances or lamps.

The only way you can actually get a 240 volts on a 120 volt circuit is if the other side of the circuit has a direct short with no resitance.....a very unlikely scenario.

By the way....welcome to the forum. smile


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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