ECN Forum Forums Code Related Discussion NEC & other Code issues Multiwire Branch Circuit

George,

You said that the building disconnect was a DPST switch, so let me assume for the moment that it's a snap switch (as allowed for a residential detached building disconnect). Then:

1) Wouldn't that be considered two circuits on a common yoke, leading to the requirement for a handle tie?

2) How do you handle the requirement to be able to disconnect the system neutral from the building wiring neutral? A wire nut in the switch box?

3) Since there's more than one branch circuit, the building requires its own grounding electrode, right?

(I think my own question (3) tells me that my first assumption is probably wrong, and that the DPST is actually something like a molded-case disconnect with ground and neutral bars. If so, ignore all this...)


[This message has been edited by John Crighton (edited 06-06-2005).]

Read the exception to that one. In this case a multiwire branch circuit is considered a single branch circuit.

John, I didn't say what you think I said.

As far as your 1st question, a multiwire branch circuit is one circuit, see 210.4(A) and notice the wording shall be permitted to be considered as multiple circuits.

Roger

[This message has been edited by Roger (edited 06-06-2005).]

Roger, I meant to address that to "George" -- Sorry for the confusion. I've fixed it.

I don't have my NEC at hand, but I'll have a look at 210.4. Thanks.

I'm still wondering about the breaker tie...


[This message has been edited by John Crighton (edited 06-06-2005).]

Electricmanscott,

Thanks for pointing out the multiwire circuit being a single circuit. I missed that one. I guess that's why you have achieved that superhero status!

Harold

Take a look at 225.33 Number of disconnects.

225.36 Exception from SUSE rating.

225.38 Snap switches as disconnects.

225.30 Multiwire branch circuit is one circuit.

For safety, I agree with John Crighton on the need for a handle tie with the multiwire branch circuit feeding the double pole switch.

But, the book says...

"more than one device or equipment on the same yoke" 210.4(B)

A double pole single throw switch is only one device, right?

So, no handle tie required?

shortcircuit

shortcircuit, thanks for making "the coin drop." I completely missed that a DPST switch is only one device.

That's sort of a hole in the code, isn't it? It's common for a two-pole switch to control a line-to-line load, in which case the handle tie is required anyway. In this rare case where it controls two line-to-neutral loads on a multiwire branch circuit, the lack of a handle tie would seem to be just as hazardous as if the switch were two devices.

It seems I learn something here every day. Thanks, all.

One more thing -- 210.19(A), FPN 4 recommends a 3% max voltage drop on branch circuits, and the city of Los Angeles makes it a requirement. Assuming a 16 amp max load on a 20 amp circuit, that's only 58 feet of an awg-12 pair. It's easy to exceed that when running to a detached garage, as I recently discovered.

Ok, two more things -- Can anybody explain the reason for 225.38(C)? In this case, with a snap-switch disconnect, is this met by just splicing the neutrals with a wirenut in the switch box?


[This message has been edited by John Crighton (edited 06-07-2005).]

I think you satisfy the code by simply using 2 single pole switches for the <less than six> disconnects.

BTW Florida is putting the FPN voltage drop in the energy code. I am not sure how you are going to enforce that with anything but fixed in place equipment.
I suppose you could use the article 220 load calcs and try to deduce the current in every leg of every circuit but, like flying the F117, you couldn't do it without a computer.

Not if you are doing 30 a day.

[This message has been edited by gfretwell (edited 06-07-2005).]