Shared Neutral

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washingtonw

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In a commercial application, can the neutral be shared between two or more receptacles fed from the same phase? If so, does the neutral have to be sized to accommodate the return for both circuits at the same time? For instance, if the(2) circuits are rated for 20 amps does the neutral have to sized for 40 amps?

Thank you.
 
Re: Shared Neutral

I am not sure if it is legal, but it seems to me to be ill advised. You are right in suspecting that all current flowing through the two hot conductors will return to the source via the single neutral. So you would need 40 amps worth of neutral conductor. You can get there with a pair of #12s, but with a single neutral conductor it would have to be a #8. So I can?t see a good reason for trying it.
 
Re: Shared Neutral

Go to 210.4. You may indeed have more than one circuit on a single neutral. However, it must be a multiwire branch circuit. :D
 
Re: Shared Neutral

Steve, don't waste too much time looking for something to prove it wrong because if you go to 225.7(B) you will find one application that specificaly allows it, and no where else in the NEC will you find it prohibited.

Roger
 
Re: Shared Neutral

To be clear about what we are talking about here is an example.

Three 15 amp breakers all on phase A.

Each breaker has a 14 AWG ungrounded conductor.

The grounded conductor for all three circuits is one 6 AWG CU.

NEC compliant or not?

What code section addresses this?

:D
 
Re: Shared Neutral

Steve please do not die! :D

The handbook even gives some examples of this.

Handbook Commentary following 225.7(B)
Multiwire branch circuits consisting of a neutral and two or more ungrounded conductors are permitted, provided the neutral capacity is not less than the total load of all ungrounded conductors connected to any one phase of the circuit.

Exhibit 225.3 illustrates a 120/240-volt, single-phase, 3-wire system. In Exhibit 225.3, all branch circuits are rated at 20 amperes. The maximum unbalanced current that can occur is four times 20 amperes, or 80 amperes.

;)
 
Re: Shared Neutral

Thanks,

But it just seems so WRONG.I have NEVER wired anything like that. Am I wrong for thinking this way? Somebody save ME! LOL

STEVE
 
Re: Shared Neutral

Originally posted by jimwalker:
Why would you even want to do this ? What could you gain ? harder to work with and harder to pull.And maybe even cost more
Actually for feeding site lighting this might work out very well. :)

However we usually do not use neutrals at all for site lighting, given the option we will run 480 volt circuits.
 
Re: Shared Neutral

Originally posted by 612278:
But it just seems so WRONG.I have NEVER wired anything like that. Am I wrong for thinking this way? Somebody save ME! LOL

STEVE
Steve I would not say it is wrong to think that way. I have never done this either and probably will not in the future. :) :p

Bob
 
Re: Shared Neutral

iwire-

It just seems if that neutral (60amp) ever let loose or broke or what ever it would do more damage that smaller individual conductors.

I don't know this just rub's me wrong. :confused:

STEVE
 
Re: Shared Neutral

Steve, it might do damage to more equipment, and this would need to be considered when using this method, but on the other hand we shouldn't plan on this happening.

A more common problem is the service neutral being lost, which would take care of the same amount and even more equipment damaged.

I have personally seen many more neutrals lost ahead of the meter than on the load side.

Roger
 
Re: Shared Neutral

Originally posted by iwire:
...
That said if done correctly what is the problem? :) :p

Bob
I would have asked ?That said if done safely what is the problem?"

I looked at Bob's illustration to see if I could spot a specific problem. Technically, I don?t believe this is a true multi-wire branch circuit; at least, if only one phase were considered:
Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.
With a sufficiently long neutral a substantial back-feed voltage to ground could appear on a single opened CB in the panel. You would only be serving line to neutral loads in this scenario and not require all ungrounded conductors to be opened per 210.4(C) Ex2. .

So to me the question is, ?Is it unsafe?? And if it is, what mitigates it?
 
Re: Shared Neutral

Originally posted by rbalex:With a sufficiently long neutral a substantial back-feed voltage to ground could appear on a single opened CB in the panel.
Quite true. But (as you say) is it unsafe? I think not. Here is why.

Let us presume we are working with a pair 20 amp circuits. Let us presume that the hot conductors are #12. To be conservative, and only for the purposes of this calculation, let us presume the neutral is also #12 (clearly undersized, but ignore that fact for the moment).

Open one breaker, and load the other to the full 20 amps. The impedance of a #12 (using for illustration purposes Table 9, effective Z at .85pf, uncoated copper, PVC conduit) is 1.7 ohms per 1000 foot. Therefore, the voltage drop along the neutral is 20 amps times 1.7 ohms, or 34 volts per 1000 feet. I would say that 34 volts is high enough to create a risk of a shock hazard. However, nobody runs a 1000 foot circuit, without upsizing for voltage drop.
 
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