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Old 09-20-2011, 12:41 PM   #1
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Shared Neutral


Recently demolished the kitchen that formerly had an electric oven and an electric cooktop. Each was supplied with a whip containing two blue wires and one white and controlled by a double-pole 30A breaker. I assume it was two hot wires handling up to 15A each on the same phase with a shared neutral (It seems like this isn't right because if the wires are the same phase the neutral could easily be overwhelmed, but shouldn't the wires have been blue and red, or blue and black?). Now that I switched to gas appliances those circuits aren't needed for cooking. Instead, the kitchen needed to be updated to current code to have at least two 20A circuits for the counter (which only had one outlet). I believe the electrician utilized those wires to create 4 separate 20A circuits (the max for the 12AWG wire), but with only two neutrals. I believe two circuits each share a neutral. From what I understand, this is OK with code as long as the breaker for those circuits is double pole.

Please forgive my limited understanding of the mechanics of circuit breakers, but will a double pole breaker trip any time one of the circuits draws more than 20A, but allow both circuits to draw 40A simultaneously?


Last edited by BobSmitt; 09-20-2011 at 12:52 PM.
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Old 09-20-2011, 01:32 PM   #2
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Another related question: Is there a reliable way to test if a neutral wire is shared by more than one circuit? I know how to test if I already know it's shared by turning on a load on one of the shared circuits and measuring the current on the other circuit, but I'm wondering how I might detect a shared neutral without knowing if it's shared. I can't imagine it's practical to turn on a device on every other circuit in the house and then test the neutral for the circuit in question, but maybe that's the way to go.

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Old 09-20-2011, 01:46 PM   #3
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Were there any ground wires inside the whip, or the box?
The neutral in a mwbc only carries the unbalanced load from the 2 circuits.
If you use 10 amps on each circuit, the neutral current will be zero.

If you excede 20 amps on either leg, the breaker will trip, but the most current you will see on the neutral will be 20 amp.
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Old 09-20-2011, 02:04 PM   #4
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Originally Posted by jbfan View Post
Were there any ground wires inside the whip, or the box?
Not that I could see. This was a 1960's wiring job. Just a 1-inch conduit with three wires (two blue and one white).

Quote:
Originally Posted by jbfan View Post
The neutral in a mwbc only carries the unbalanced load from the 2 circuits.
If you use 10 amps on each circuit, the neutral current will be zero.

If you excede 20 amps on either leg, the breaker will trip, but the most current you will see on the neutral will be 20 amp.
That's my main concern. I just want to make sure I can run each circuit at its rated capacity without tripping the breaker. I guess I just don't understand the mechanics of how the double-pole breakers "knows" when either leg has reached 20A. I assume that any one of three conditions will trip the breaker:

  1. 20+A on leg A
  2. 20+A on leg B
  3. 20+A on neutral
It has to monitor all three conditions, otherwise there would be scenarios where you could exceed a circuit's capacity without tripping the breaker. Am I correct?

Does that mean I need a 40A double-pole (20A each circuit), or 20A double-pole?

Last edited by BobSmitt; 09-20-2011 at 02:08 PM.
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Old 09-20-2011, 02:10 PM   #5
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Nothing forbids the two hot wires from being on opposite side of the line and being the same color (here, blue).

The circuit breaker only monitors current through the single conductor going through it.

It is not necessary to monitor the current through the neutral because on a properly wired multiwire branch circuit the neutral current will never exceed the greater of the two hot line currents.

Each half of the double breaker works independently. Common methods of sensing overcurrent are magnetic fields and heat generated.

If one breaker trips, the other breaker, if still on, will continue to handle up to its maximum (here, 20 amps) at 120 volts using the neutral. Any 240 volt usage will stop. (2011 National Electric Code requires that if one breaker trips, the other is flipped off using a handle linkage.)

For 20 amps on each side use a 20 amp double breaker.
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Last edited by AllanJ; 09-20-2011 at 02:21 PM.
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Old 09-20-2011, 02:12 PM   #6
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A breaker never measures or care about the neutral current.
It is only concerned with the current on the wires attached.

A 20 amp 2 pole breaker allows 20 amps per leg.
The neutral will never see more than 20 amps, because the breaker will trip over that amount.
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Old 09-20-2011, 02:25 PM   #7
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OK. I get it. Since the panel already has 30A breakers, and assuming the wiring is legitimate, is there some reason I should switch them out for 20A breakers just because it's been converted from one 220V to a 2-120V MWBC (other than that the outlets are only rated for 20A)?
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Old 09-20-2011, 02:35 PM   #8
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Yes, it is against code to have genaral use recrptacles on 30 amp breakers.
The breakers need to be changed to 20 amp.
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Old 09-20-2011, 02:37 PM   #9
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I was just thinking in terms of wiring. The wiring can handle it, but the receptacles cannot.
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Old 09-20-2011, 02:43 PM   #10
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Quote:
Originally Posted by BobSmitt
OK. I get it. Since the panel already has 30A breakers, and assuming the wiring is legitimate, is there some reason I should switch them out for 20A breakers just because it's been converted from one 220V to a 2-120V MWBC (other than that the outlets are only rated for 20A)?
If you were to use 30 amp breakers, the breakers isn't gonna trip if something is pulling 25 amps. Which is dangerous when your wire, and recepticals are only rated for 20 amps. This will cause a fire so don't attempt it
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Old 09-20-2011, 02:55 PM   #11
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Reviewing your original post:
Quote:
Each was supplied with a whip containing two blue wires and one white and controlled by a double-pole 30A breaker.
Did these two whips originate at the same junction box?
The one 30 amp breaker supported both the oven and cooktop?

Quote:
I believe the electrician utilized those wires to create 4 separate 20A circuits (the max for the 12AWG wire), but with only two neutrals. I believe two circuits each share a neutral. From what I understand, this is OK with code as long as the breaker for those circuits is double pole.
Four separate circuits would mean you have four new 20 amp breakers in the panel. What happened to the 30 amp breaker that was in the panel to support the old appliances?
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Old 09-20-2011, 03:43 PM   #12
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Quote:
Originally Posted by BobSmitt View Post
Another related question: Is there a reliable way to test if a neutral wire is shared by more than one circuit? I know how to test if I already know it's shared by turning on a load on one of the shared circuits and measuring the current on the other circuit, but I'm wondering how I might detect a shared neutral without knowing if it's shared. I can't imagine it's practical to turn on a device on every other circuit in the house and then test the neutral for the circuit in question, but maybe that's the way to go.
Usually you find a shared neutral just by looking. You see a cable with two hot wires and just one neutral. For a conduit with more than two hot wires you would need to do some circuit testing to find out what neutral goes with what hot wire.

The double breaker unit holding the two hot wires that share a neutral must be positioned in the panel so you have 240 volts across the hot terminals of the two breakers.
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Old 09-20-2011, 05:07 PM   #13
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Quote:
Originally Posted by AllanJ
Usually you find a shared neutral just by looking. You see a cable with two hot wires and just one neutral. For a conduit with more than two hot wires you would need to do some circuit testing to find out what neutral goes with what hot wire.

The double breaker unit holding the two hot wires that share a neutral must be positioned in the panel so you have 240 volts across the hot terminals of the two breakers.
We usually tape the circuits with the neutral that it's sharing. I only could imagine to have to test to find everything. I bet it's a nightmare
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Old 09-20-2011, 05:23 PM   #14
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We usually tape the circuits with the neutral that it's sharing. I only could imagine to have to test to find everything. I bet it's a nightmare
That only became code in the 2008.
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Old 09-20-2011, 06:00 PM   #15
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Quote:
Originally Posted by a7ecorsair View Post
Reviewing your original post:

Did these two whips originate at the same junction box?
The one 30 amp breaker supported both the oven and cooktop?
No. There were two whips and each had a double-pole 30A breaker (4 positions in the panel).

Quote:
Originally Posted by a7ecorsair View Post
Four separate circuits would mean you have four new 20 amp breakers in the panel. What happened to the 30 amp breaker that was in the panel to support the old appliances?
The 20A breakers will be replacing them.


Last edited by BobSmitt; 09-20-2011 at 06:08 PM. Reason: Corrected a misspelling
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