multiwire circuits

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stew

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I ran in to a new construction service for a 4000 square foot house today which in the fierst place seemed like it should have ebeen a 320 anyhow. In the c/h panel (br breakers) the left hand side was breakered all the way down with doubles. They had put a single cicuit on the topmost double and then used multiwire circuits all the way down to the last double where the bottomost was also a single circuit. They did the same thing on the other side with the 15 a circuits ie: first one a single and then down the line with multiwires and the bottomost a single a gain. (fewer 15 circuits tho) 3 afci 15 and 1 2pole 30 and 1 2pole 40. ) I was there to install a 5 ton ac unit which had a min cir amps of 35 so used a 40 mbreaker and # 8 wire. Had to move one afci to accomplish this and now the panel of course is totally maxed out. Poor desigign not usuing a bigger panel . The question is however as the way this is wited it does not allow for handle ties on the multiwires as they are of course on alternate breakers. Is this allowed? I thought all multiwires in a residential had to be handle tied no?

[ June 14, 2005, 11:14 PM: Message edited by: stew ]
 
Re: multiwire circuits

210.4(B) Dwelling Units. 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. :D
 
Re: multiwire circuits

Originally posted by stew:
I was there to install a 5 ton ac unit which had a min cir amps of 35 so used a 40 mbreaker and # 8 wire.
As Charlie already answered your question :) I would like to comment on the 40 amp breaker.

From my experience a 40 amp breaker seems a little small for a unit with an MCA of 35.

Was the maximum overcurrent device listed as a 40?

I would expect a maximum overcurrent device of 45 or 50 amps on a unit with a MCA of 35 amps.

If someone short cycles the unit the breaker may trip before the self reseting overloads in the unit open.
 
Re: multiwire circuits

I would add one thing. The code refers to standard size breaker. So if the name plate calls for a 45 you must use a 45 not 50. More & more manufacturers are starting to list 25,35, 45 as max breaker. I don't remember this from the past but I don't think the inspectors checked as close either. And on a heat pump you must get the AC people to mark the over current protection for the unit's heat strips. This would be great expect you don't see them all the time. I think that should be part of the AC rough and not try to get the electrician to make sure it's done. Just make sure you comply with what ever the manufacturer wants. That makes it legal. If it doesn't work out right that's someone else's problem.
 
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