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Old 03-08-2010, 06:41 AM   #1
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Multiwire Branch Circuit


I was helping a friend modify the wiring in his basement yesterday. I saw that one of his hot feeds to the box was a 3-wire cable. I could not figure it out. Eventually I figured out that the black and the red wires in this 12/3 were connected to different breakers, and the white was going back to the panel. This is a MWBC, right? This house is only about 6 years old. I guess this installed did this to save money? This red wire passes through this switch box to feed his washing machine outlet. I guess this is to code, since it is on a separate breaker (though the white is shared with another circuit).

In any case, At some point, I do not remember when, this red wire gave me a tingling feeling. It wasn't a full 120v shock, just enough to let me know it was there. I tested it, and to ground it had something like 28 volts. What would lead to this wire having such small voltage?

Unrelated question...he has 12/2 wiring in his house. Some of his ground wires are a bit short and tough to wire together. Can I pigtail to 14 AWG copper for the grounds on a 20 Amp circuit, or do I have to use 12? This is just for the ground, all the current-carrying wires will be 12.

Thanks!
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Old 03-08-2010, 07:21 AM   #2
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You do need to use #12 wire on a 20a circuit

Yes MWBC - shared neutral...saves on installation costs for wire
The breakers should be side by side if installed correctly
They also should be tied together with handle bar



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Old 03-08-2010, 07:49 AM   #3
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I thought the ground wires can be smaller gauge than the blacks/whites? Is this only the case for larger wires (like for a subpanel)?

These breakers were next to each other, but each was a single breaker, the handles were not connected. Is this an issue?

How could I possibly have been getting 28v from one of the two hots?
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Old 03-08-2010, 08:17 AM   #4
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While handle ties are a good idea they were not required until The 2008 NEC code cycle.
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Old 03-08-2010, 08:19 AM   #5
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Can you add a handle tie later?

Wouldn't there have to be shorts on both circuits at the same time for the white wire to be overloaded?
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Old 03-08-2010, 08:30 AM   #6
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Yes, you can add a handle tie...I would in my own house
I actually only use 240v breakers to supply MWBC in my house
These supply (3) timers 12" away from the panel

28v could be phantom voltage...what kind of tester was used ?
Were both breakers off ?



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Old 03-08-2010, 08:34 AM   #7
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I used a fluke tester. There was definitely some voltage there, as I could feel it. I think one of the breakers was on when the 28v was present...which is odd...shouldn't it have been 120?

There is also a separate 2-wire cable in this switchbox from another separate circuit. This was off when the 28v was there.

Do they sell handle ties at big box stores? It's a SquareD box, not the homeline series.
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Old 03-08-2010, 08:40 AM   #8
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That's good to know that the handle ties are now required for 12/3s wired into two separate breakers. I had a couple of those in my house and when I was pulling out the existing wiring for replacement, I thought I had cut power to one of the circuits, but it gave me a nice pop when I started cutting into the other hot that I had not turned off.

That will teach me to always check both outlets on a plug circuit with my tester!!
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Old 03-08-2010, 08:48 AM   #9
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Do the breakers have to be removed to install the handle tie?
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Old 03-08-2010, 09:17 AM   #10
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Should not have to remove the breakers to install handle ties. 28V could be feedback through the neutral if the other circuit was still energized.
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Old 03-08-2010, 10:02 AM   #11
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What do you mean by feedback?
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Old 03-08-2010, 10:09 AM   #12
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The neutral could be loaded (backfed) through the load supplied by the still energized breaker.
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Old 03-08-2010, 10:37 AM   #13
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Quote:
Originally Posted by secutanudu View Post
I thought the ground wires can be smaller gauge than the blacks/whites? Is this only the case for larger wires (like for a subpanel)?
Table 250.122
Reducing the size of the equipment grounding conductor (EGC) starts at breaker 40 amp. 15 amp, 20 amp and 30 amp circuits must be the same size as the current carrying conductors.

Quote:
Originally Posted by brric View Post
While handle ties are a good idea they were not required until The 2008 NEC code cycle.
NEC 1996, NEC 2005 and NEC 2008
210.4 Multiwire Branch Circuits.
(B) Devices or Equipment. Where a multiwire branch circuit
supplies more than one device or equipment on the same
yoke, a means shall be provided to disconnect simultaneously
all ungrounded conductors supplying those devices or equipment
at the point where the branch circuit originates.

Disconnect Simultaneously = Handle Tie.

Handle ties on MWBC's are much more than just a good idea. It could save your life or someone else's. I would not like to be handling a receptacle that has been turned off at the breaker, only to find one side still hot.

I have always thought the handle tie was required as far back as I can think. But I have been wrong before. Do you have an article to back up your statement? Thanks in advance.
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Old 03-08-2010, 10:48 AM   #14
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Why would you wire a receptacle from two different circuits? If you plan to hook up two high-energy devices to the same duplex receptacle?

I am still having trouble undetstanding how 28v could backfeed through the live breaker...
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Old 03-08-2010, 11:06 AM   #15
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Quote:
Originally Posted by J. V. View Post
Table 250.122
Reducing the size of the equipment grounding conductor (EGC) starts at breaker 40 amp. 15 amp, 20 amp and 30 amp circuits must be the same size as the current carrying conductors.



NEC 1996, NEC 2005 and NEC 2008
210.4 Multiwire Branch Circuits.
(B) Devices or Equipment. Where a multiwire branch circuit
supplies more than one device or equipment on the same
yoke, a means shall be provided to disconnect simultaneously
all ungrounded conductors supplying those devices or equipment
at the point where the branch circuit originates.

Disconnect Simultaneously = Handle Tie.

Handle ties on MWBC's are much more than just a good idea. It could save your life or someone else's. I would not like to be handling a receptacle that has been turned off at the breaker, only to find one side still hot.

I have always thought the handle tie was required as far back as I can think. But I have been wrong before. Do you have an article to back up your statement? Thanks in advance.

That is only for devices on the same yoke, i.e. a duplex receptacle fed by a multiwire branch circuit with the tab removed.

2008 NEC 210.4(B) highlighted new provision

Last edited by brric; 03-08-2010 at 11:13 AM. Reason: additional
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