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 electro 10-29-2009 07:54 PM

Sharing Neutrals on the same phase

At work we are having a debate on sharing neutrals .
I believe that , " branch circuits on different phases ( one from each phase ) can share the same neutrals ". If you share with the same phase you will have a unbalanced load and a higher current on the neutral .
I can't find anything on the NEC . Any lead on the electrical Theory proof covering this topic and NEC would be appreciated .

 Scuba_Dave 10-29-2009 08:13 PM

You can't share neutrals from the same buss, power used across the netral will add & can exceed the wire rating
From opposite sides they cancel out
In addition under NEC 2008 you have to use a 240v breaker for a MWBC, so sharing across the same buss is not possible

 electro 10-29-2009 08:32 PM

I agree , but I'm having trouble proving it .
I'm not familiarized with the acronym " MWBC " What does that stand for .

 Plumbvoltage 10-29-2009 08:34 PM

Multi Wire Branch Circuit

 220/221 10-29-2009 08:35 PM

Multi wire branch circuit.

If you have 20 amps from one hot and 20 amps from another hot, you have 40 amps on the neutral if they are on the same phase.

If they are on different phases, they use the neutral at different intervals.

 Scuba_Dave 10-29-2009 08:50 PM

You don't have to prove it, they do
Out of phase - they do not add
If they were in phase they would be adding
Anybody that thinks they can draw 20a across 2 hots off the same buss & use the same neutral does not know enough about electricity to bother trying to convince them of anything

 williswires 10-29-2009 10:02 PM

Quote:
 Originally Posted by electro (Post 347123) I believe that , " branch circuits on different phases ( one from each phase ) can share the same neutrals ".
You are correct in this belief. As already mentioned, this is a Multi-Wire Branch Circuit, covered in section 210.4 of the NEC.

Quote:
 Originally Posted by electro (Post 347123) If you share with the same phase you will have a unbalanced load and a higher current on the neutral
A better way to say it is that the neutral will carry the total load of the two circuits. "Unbalanced" isn't the correct term - an unbalanced load would be the difference between the two circuits of a MWBC.

Say you have (2) circuits, each rated at 15A (having 15A circuit breakers, and run with 14AWG wire).

Circuit 1 has a 10A load, and
Citcuit 2 has a 12A load.

WRONG WAY: The neutral would carry 24A of total current, at the same time. This is too much for the single neutral that is sized the same as the hots (14AWG in our case). The permanent circuit wiring must be properly designed based on circuit rating, overcurrent protection, wire ampacity, etc.

RIGHT WAY: If this was a properly wired MWBC, the circuits would be supplied from opposite phases. If the two circuit currents were 12A and 12A, the neutral would be carrying the two 12A currents at different times. If the two currents are the same amount, the neutral is carrying a balanced load.

Example: Picture a set of kitchen countertop receptacles. Since the NEC requires (2) 20A receptacles circuits for kitchen countertops, they are frequently run as one MWBC.

 electro 10-30-2009 04:57 AM

" "Unbalanced" isn't the correct term " I thought about that a little while after I posted , That term would refer to the current carrying conductors .

Thanks guys .
It's good to have conformation that I'm on the right page .

 HouseHelper 10-30-2009 05:35 AM

The better way to consider the neutral in a single phase, three wire circuit is that it carries the difference in current between the two hot legs.

 AllanJ 10-30-2009 06:30 AM

In addition, if the feeds for different branch circuits should enter the same box, their neutrals are not tied together. Each neutral serves all of and only the loads served by its accompanying hot wire.

The same goes for a GFCI receptacle unit; the neutral on the load side may not be directly or indirectly connected to the neutral on the line side. The load side of a GFCI is treated as "a different circuit". When a multiwire branch circuit aka 120/240 volt circuit (which shares a neutral) feeds a GFCI receptacle unit, anything on the load side of the GFCI is not part of the MWBC and a separate neutral must accompany any continuation of the load side hot.

I'll try my luck at rewording:

RIGHT WAY: If this was a properly wired MWBC, the hot conductors would be supplied from opposite sides of the 120/240 volt system (or two phases of a 3 phase system). If the two circuit currents were 12A and 12A, the neutral is carrying the return currents (12A) for both of them but the return currents cancel each other out with a net current flow in the neutral conductor equal to any difference. If the two currents (three taken from all three phases respectively in the case of a 3 phase system) are the same amount, then the circuit is carrying a balanced load.

 Billy_Bob 10-30-2009 12:01 PM

Its the same thing as the main electric service wires going to each home...

Two hots (opposing phases) and one neutral. This is the same way a MWBC works.

Place equal 120 volt loads on opposing phases, then measure with a clamp amp meter the current flowing through the neutral!

Then place the same load on just one leg and no load on the other, then measure the current flowing through the neutral!

 Stubbie 10-30-2009 12:52 PM

2 Attachment(s)
Quote:
 Originally Posted by electro (Post 347123) At work we are having a debate on sharing neutrals . I believe that , " branch circuits on different phases ( one from each phase ) can share the same neutrals ". If you share with the same phase you will have a unbalanced load and a higher current on the neutral . I can't find anything on the NEC . Any lead on the electrical Theory proof covering this topic and NEC would be appreciated .
Quote:
 I believe that , " branch circuits on different phases ( one from each phase ) can share the same neutrals ".
Looking in today at the forum... this is the kind of thread I cannot resist replying to even if I am on extended break....:)....get ready for my usual long winded reply but hopefully it will help.

It would be better to say two 120 volt branch circuits on opposite hot legs in a single phase residential 3 wire 120/240 volt system can share the same neutral..... not neutrals. Notice Househelper and I said single phase meaning there are not 'two phases' or opposite phases. Now having said that I think everyone knows what is intended when someone says different phases in a single phase system. But for clarification what you really have is a single phase that has two hot wires one of which has been offset by 180 degrees from the other. See scuba daves graphic.

It's also good to have the correct terminology. If I have a NM-b (romex) 2 wire with ground 120 volt branch circuit I have one hot wire (black), one grounded wire (white) and one equipment ground (bare). Notice I said 'grounded' wire for the white. You will also hear this wire called the grounded leg. This wire is not technically a 'neutral' because it carries all the current of the branch circuit. Which makes sense ... it serves only one hot wire.

This is a grounded leg..... your typical branch circuit ... even though it is common to call it a neutral..

http://media8.dropshots.com/photos/4...030/102539.jpg
A neutral wire in the correct sense of the word serves two or more hot wires and carries the difference in current between those hot wires. And those hot wires cannot originate from the same point of the transformer winding ...ie... what we have been calling the 'same phase' in this thread.

Again though we all know what is intended using the term 'neutral' in a 2 wire with ground single phase branch circuit. Just remember it is really just a grounded leg intentionally connected to the mid point of the transformer by bonding it with the service neutral on the neutral bus of the main panel or service equipment. It is grounded by the fact there is a connection to earth both at the main panel and the transformer midpoint tap where the service neutral connects.

But I think it is important to understand the difference between a grounded leg and a neutral if we are to understand why we cannot share a grounded leg and we can share a neutral.

So I think we should start with what creates a neutral wire. Best example is your service neutral in a residential system. It is a single shared neutral that serves both hot wires of your service. These hot wires terminate at the service equipment on different buses and different points on the transformer winding. You will also see that all the grounded legs (or neutrals if you prefer) terminate on the same neutral bus or buses that join with the service neutral. The current from all these branch circuits has only one path to the service transformer and that is over the service neutral. So both hot legs (phases if you must) coming from the transformer use a shared wire called the service neutral to have a completed circuit for 120 volt loads with the transformer.

At first thought you might expect that all the current from both hot wires is returning to the transformer over that service neutral. However this is not the case. The physics of offsetting the single phase by 180 degress creates a cancelling effect between the neutral current generated by the two hot wires. Meaning if one transformer hot wire is supplying 50 amps to the house and the other is supplying 30 amps then the service neutral will only be carrying 50 - 30 = 20 amps. So it is a true neutral in that it serves two hot wires and carries the difference in current between those two hot wires.

This can only happen if the shared neutral is serving these two hot wires originating at opposite ends of the transformer winding. So it makes sense that if I take away one hot wires load the canceling effect ends and the service neutral will carry all the load of the single hot wire. In the above example if I take away the load on the hot wire supplying 30 amps to the house the service neutral will now start carrying 50 amps not 20 .. or .. 50 - 0 = 50 amps.

Looking at it in terms of branch circuits. If I take the hot wires of two branch circuits and connect them to two 15 amp circuit breakers connected to the same bus in the panel and therefore the same service hot wire of the transformer, there is no canceling effect if I share a grounded leg (neutral) between those hot wires. If one of those hot wires is serving a space heater that uses 12 amps and the other is supplying a hair dryer that needs 12 amps and both are operating at the same time then there will be 24 amps on the shared grounded leg. Neither circuit breaker will trip because they only see 12 amps flowing thru them but the grounded leg (neutral) is overloaded and overtime will get hot and hotter yet as you add loads to either branch circuit. If I add 5 amps to either branch circuit the neutral is now carrying 29 amps. If this load is carried long enough the eventual result will be the grounded wire (neutral if you must) being 14 awg getting so hot that it will cause a fire and neither breaker will trip. Circuit breakers do not monitor the grounded leg or neutral.

EDIT: One thing I should add is that we never combine the grounded legs of different branch circuits once they leave the panel. We either share a common neutral as when running a multiwire branch circuit or we keep the grounded legs isolated to the branch circuit and loads they serve. Never tie the grounded leg of one branch circuit with another this will create a parallel path for current and may overload any grounded leg or neutral you try to share.

 HouseHelper 10-30-2009 01:05 PM

Long winded, yes, but as usual, an excellent explanation. When does the textbook come out.:thumbup:

And I thought you were supposed to be fishing?

 busman 10-30-2009 01:52 PM

Yeah, but who can find the code violation. It's a killer to find.

Mark

 Stubbie 10-30-2009 02:11 PM

Quote:
 Originally Posted by HouseHelper (Post 347357) Long winded, yes, but as usual, an excellent explanation. When does the textbook come out.:thumbup: And I thought you were supposed to be fishing?

Yeah I know but weather has me 'grounded'....:) Habits are hard to break but I'm finding it less taxing of my time to look in on the forum and respond to something that I feel I may add some drawings that will help what all you guys explain so well. Other wise I am just being redundant...:thumbsup: I will be replying but it will not be that 6 or 7 hours a day more like once or twice a month.

Most of you guys have more residential experience than me anyway so Ill just drop in now and then as long as it doesn't offend anyone. The reality is it beats doing the laundry....:wink:

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