210-4(b) states; "In dwelling units, a multiwire branch circuit supplying more than one device or equipment on the same yoke shall be provided with a means to disconnect simultaneously all ungrounded conductors at the panelboard where the branch circuit originated."
What does this mean? Does it mean that on a shared neutral circuit you need a double pole breaker or does the main suffice? Is a "yoke" here meaning a duplex receptacle?
I had a service call where the customer said they were tripping breakers on the upstairs circuits. I found a shared neutral circuit where both ungrounded conductors were under one 15 amp breaker. I seperated them by installing a double pole 15 amp breaker, one conductor under each leg. My reasoning was that if one leg tripped, both legs would trip making the neutral current 0. If you used seperate breakers, (on seperarte phases of course) and one tripped, you would still have neutral current. Also there is a chance of feedback through the other ungrounded condutor. Any comments?
Big A, The term "yoke" here means the strap that holds the outlets, switches, or what have you. A single gang box has one yoke, whatever is between the 6-32 screws. You can get a single recep, duplex, or even an old Despard type device with 3 on the yoke, but anything on that yoke that shares the neutral needs to be disconnected simultaneously.
If you had no multiwire circuits on a shared yoke here, there was no requirement for the double pole breaker. As long as the two circuits are on different phases, your neutral should be OK, as it was, of course, they only had one 15 amp circuit because both conductors were under the same overcurrent device.
What were they plugging in to their bedroom outlets that kept popping the breaker? Did you try any amp readings?
This could actually be one of the major cases FOR AFCI's, even though they are not required in VA. In July, VA will accept the 1999 NEC without the requirement for AFCI devices.
Re: On the same yoke#84556 04/10/0308:19 PM04/10/0308:19 PM
Big A: This code is designed to insure that if you run two phases of a multi wire circuit, (which is a neutral shared by two or more ungrounded conductors, on different phases) that both phases will be off at the same time.
Re: On the same yoke#84557 04/10/0308:30 PM04/10/0308:30 PM
Just want to add, that this is designed to protect persons from turning off a circuit and working on a device they think has no power. With out knowing before hand that there are two circuits on the device there is a serious shock potential.
[This message has been edited by russ m (edited 04-10-2003).]
Re: On the same yoke#84558 04/11/0306:19 AM04/11/0306:19 AM
Russ: That's exactly how I, and the local AHJ interpret it. I also know that others don't see it this way. If you are only allowed one neutral per screw for the reason that if you had two, one would still be carrying current when you tried to remove the other one, then I feel that the reverse ought to be true about the ungrounded coductors on a shared neutral circuit. Both "hots" should have to be disconnected at the same time.
George: The entire upstairs of this house was on this one 15 amp circuit. Space heaters, hair dryers, and an iron were all being used.
Re: On the same yoke#84559 04/11/0306:26 PM04/11/0306:26 PM