Shared Neutrals


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Old 10-07-07, 07:25 PM
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Arrow Shared Neutrals

We're adding an addition, and I have a few questions about shared neutral wires: (Everything must be up to code since everything will be inspected.)

(1) Are there any exotic rules about what circuit cans share a neutral wire? I plan to run 12/3 (shared neutrals) for the two 20A appliance circuits in my kitchen and 14/3 for the disposal and dishwasher circuits. But can I do the same for say two different lighting circuit in different rooms? Or say the washing machine circuit and the appliance circuit in my laundry room?

(2) Am I required to use double-pole breakers on circuits with shared neutrals, or can I get away with just making sure two single-pole breakers are on different phases at the service panel.

Thanks,
Mitch
 
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Old 10-07-07, 08:14 PM
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Originally Posted by mopo999
We're adding an addition, and I have a few questions about shared neutral wires: (Everything must be up to code since everything will be inspected.)

(1) Are there any exotic rules about what circuit cans share a neutral wire? I plan to run 12/3 (shared neutrals) for the two 20A appliance circuits in my kitchen and 14/3 for the disposal and dishwasher circuits. But can I do the same for say two different lighting circuit in different rooms? Or say the washing machine circuit and the appliance circuit in my laundry room?

(2) Am I required to use double-pole breakers on circuits with shared neutrals, or can I get away with just making sure two single-pole breakers are on different phases at the service panel.

Thanks,
Mitch

A1) you can do that but you will have some issue with the GFCI most GFCI dont work very well with MWBC so one option you can do is when you run the MWBC to the first box you can split into 2 indepent small appliance circuits but normally i heard most just run 2 separed circuits to advoid some issue with MWBC in kitchen circuits and normally most kitchen i useally work it will have more than 2 circuits in the kitchen and by the way do not have any light run off from this circuit.

for garbage dispoal and dishwasher you can run in MWBC also but there is a fine line printed in the NEC and also maybe have Local code will trumped on this also so check it out first before you start pullthe wires it will change alot if not carefull here

normally for garbage dispoal and dishwasher most used standard duplex repectale and have switch on the wall to turn them on / off but break off the tab so it dont get mixed up with this and with this setup you must use the two pole breaker you can not use two single breaker on this one

the lighting circuit it will be a fine line to cross over because there is upcomming code will deal with that if you are in the area that requried the AFCI in bedroom circuit currentaly you can not run the MWBC on that format

for landury room i strongly recomoned that circuit have to be it own just like bathroom is set up and also no light tied off from this circuit also and i think they will ask for GFCI [ depending on the code is check that one also ]

2A] for MWBC circuits in resdentail area yes you must have two pole breaker for MWBC
[ the code will change in 08 code cycle to fix in commercal/ industrail side as well ]

Merci , Marc
 
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Old 10-08-07, 05:41 AM
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You can use a multi-wire circuit anywhere. It does not make sense to use two circuits that are no near each other.

For the kitchen counter it does not make sense in the US unless you use a GFCI breaker because you would need to pout a GFCI receptacle at each location unless you split the circuit.

Do not put your dishwasher and disposal on two 15 amp circuits (or a multi-wire as you propose), put them on 20 amp circuits.

You need a double pole breaker on a multi-wire circuit if the two sides of the circuit ever share a common device. However, I recommend a double pole breaker for ALL multi-wire circuits. This guarantees proper legs (phase is the wrong word) of the incoming 240 volts.

If you do go with separate breakers, make note in the panel of this so that in the future someone does not inadvertently and incorrectly move the breakers.
 
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Old 10-08-07, 08:32 AM
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Cool Thanks, new questions

Thanks! So now I have to ask about a couple more issued you raised (none of which are very important).
1) Why do you suggest 20A for disposal and diswasher? My dishwasher manual asks for 15A. Do disposals really pull a lot of current? Were older machines more power-hungry?
2) Why is "phase" the wrong term? I'm an electrical engineer, this seems like the obvious term. What's the term "phase" used for?
3) Am I supposed to wire the disposal and dishwasher into the same duplex outlets (tabs broken) or are two outlets allowed (in other words, will I get in trouble for having an extra outlet on each of these circuits? Actually, on second thought, it must be allowed because I'd rather use GFCIs for these, and then there's no tab to break. Right?


Originally Posted by racraft
You can use a multi-wire circuit anywhere. It does not make sense to use two circuits that are no near each other.

For the kitchen counter it does not make sense in the US unless you use a GFCI breaker because you would need to pout a GFCI receptacle at each location unless you split the circuit.

Do not put your dishwasher and disposal on two 15 amp circuits (or a multi-wire as you propose), put them on 20 amp circuits.

You need a double pole breaker on a multi-wire circuit if the two sides of the circuit ever share a common device. However, I recommend a double pole breaker for ALL multi-wire circuits. This guarantees proper legs (phase is the wrong word) of the incoming 240 volts.

If you do go with separate breakers, make note in the panel of this so that in the future someone does not inadvertently and incorrectly move the breakers.
 
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Old 10-08-07, 08:44 AM
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1. I suggest 20 amp circuits for the disposal and dishwasher because the cost difference is minimal and because in the future you or someone else may want a larger dishwasher, a larger disposal, or it may be desired to add something like an instant hot water appliance. Two 20 amp circuits under the sink gives you the most flexibility.

2. The power entering your home is single phase. Both legs of the incoming 240 volts are derived from a single phase of power. Phase, when referring to utility power, refers to three phase power that is supplied to some businesses. While it is true that your two legs (each 120 volts) are out of phase with each other, they are in fact from the same phase of utility power.

3. Do not use GFCI receptacles or provide GFCI protection for your dishwasher or disposal. It is not required and will only complicate things if or when the GFCI trips. No, you do not have to use a single receptacle. But why wouldn't you, if convenient.
 
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Old 10-08-07, 08:49 AM
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Originally Posted by mopo999
Are there any exotic rules about what circuit cans share a neutral wire?
Bedroom circuits should not be on an MWC, because bedrooms require AFCI protection and only one manufacturer makes a 2-pole AFCI breaker. Also GFCI receptacles cannot be used for downstream protection with a shared neutral.

Am I required to use double-pole breakers on circuits with shared neutrals
The upcoming 2008 code will require double-pole breakers for all multi-wire circuits. Previous versions only require double-pole breakers if the two legs terminate on the same yoke; e.g. each leg feeds a different half of a duplex receptacle. I think a double-pole breaker is a good idea for all MWBC.

Why do you suggest 20A for disposal and diswasher? My dishwasher manual asks for 15A. Do disposals really pull a lot of current?
Because the cost of putting in #12 instead of #14 wire now is trivial compared to the cost of upgrading later if you get a bigger dishwasher or disposal in the future. You can certainly use #14 if you wish which will be fine for typical 1/2 HP disposal.

Why is "phase" the wrong term? I'm an electrical engineer, this seems like the obvious term. What's the term "phase" used for?
Residential services are fed from a single 240V phase which is split into two 120V legs (which 180° out-of-phase with each other) via a center-tapped transformer. For clarity, it is common to refer to "phases" only in the commercial three-phase environment and "leg" in the single-phase residential environment.

Am I supposed to wire the disposal and dishwasher into the same duplex outlets (tabs broken) or are two outlets allowed
Either is allowed.

I'd rather use GFCIs for these, and then there's no tab to break. Right?
You can use GFCI protection if you wish, but it is not required for either disposal or dishwasher. EMI from the disposal motor can cause nuisance tripping of the GFCI, but modern GFCI are much better at filtering that out. Correct, GFCI receptacles have no tabs to break.
 
 

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