DIY Home Improvement, Remodeling & Repair Forum > 2 circuits on a 3 wire

04-15-2009, 08:48 AM
DaveyDIY
Extreme DIY Homeowner

Join Date: Apr 2009
Posts: 82

Quote:
 Originally Posted by kok328 Ok, I'm lost now (not really) but, you see my point.DaveDIY says:"12-3 = 2 hots & a neutral & a ground If fed out of phase the load blances against each other on the neutral IE 15a on one, 10a on the other - neutral carries 5a If on the same phase the loads ADD for the neutral IE 15a + 15A = 30a on the neutral - TOO MUCH!!"TripleD says:"However at no point along those circuits can the neutrals touch. I and others like me volunteer ourselves to assist people in a safe way. I am simply trying to do that for you. Are your neutrals touching?" Also, I'm curious, DaveDIY, why would you want to nusiance trip a non-offending circuit w/a two pole breaker? Why would you run a 240V circuit when the destination is not 240V?"Under NEC 2008 a multi-wire circuit is required to have a double breaker so that both hots shut off. This is the preferred method as it automatically puts the 2 hots on opposite sides - "out of phase"" I'm not totally sure what you mean by a "multi-wire circuit" but, I don't believe what vdotmatrix is doing, constitutes a multi-wire circuit or were you referring to my input on sharing a neutral. This still does not constitute a "multi-wire circuit". Again, I'm curious and would like to be enlightened because if what your saying is true, I've got a ton of wiring problems in at least 4 out of 5 plants I work in.
It s a new change in NEC 2008
When you are running a multiwire circuit you are sharing the neutral between the 2 circuits. You do not have to go back & fix pre-existing wiring. In 2005 only the connected bar was required between the breakers.
But 2 problems were identified:
#1 People were not installing the bar, or it was removed
As a result the breakers couild end up on the same phase & the power doubles on the shared neutral from the 2 circuits
#2 One breaker could be shut off, the other left live
Since this is a shared neutral power could flow on the circvuits that is shut off via the shared neutral

A 240v breaker is now required to prevent this
You are not supplying 240v, you are supplying 120v to 2 different circuits & using a shared neutral

As long as you have 2 circuits & seperate neutrals you are fine
This is only for Multi-wire circuits that share a neutral
Neutrals from 2 different circuits in the same box should not touch
All of the grounds are supposed to be tied together in the box

Hope that clears it up?

I have 3 setups using multiwire circuits
Each one is fed w/a 20a 240v breaker, power goes to a 40a dual 120v timer
After the timer there is a seperate run of 12-2 (GFCI protected) to outlets in my front garden & around the house
My multiwire circuit is only 12" long as the timers are connected to my panel
I knew of the upcoming change for 2008 so I used the 240v breakers
It makes sense, if the breaker is shut off then I know all power in the timer is off

The code is always changing - updated every 3 years
The wording can be very hard to understand
I've been in discussions with electricians, Inspectors & others on certain aspects of the code. In some cases each one has a different view of what the code actually is saying

By code you can run a 20a 120v circuit - UF wire - 12" deep without conduit. So long as it is GFCI protected befor eit goes in the trench. I wanted to run 2 circuits in the trench (6" wide), both 20a GFCI protected. Everyone I ran it by said that was perfectly acceptable
My Inspector did not fully agree, he said I needed 12" between the circuits in the trench. So I ended up running conduit

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04-15-2009, 08:58 AM
speedy petey
Lic.Electrical Contractor

Join Date: Mar 2006
Posts: 410
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Quote:
 Originally Posted by kok328 I'm not totally sure what you mean by a "multi-wire circuit" but, I don't believe what vdotmatrix is doing, constitutes a multi-wire circuit or were you referring to my input on sharing a neutral. This still does not constitute a "multi-wire circuit".
A "multi-wire branch circuit" is the correct term for a shared neutral circuit.
This is two (or three in the case of a 3-phase MWBC) hots sharing one neutral where the line conductors are placed on different "legs" of the panel as Dave described.
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04-15-2009, 09:44 AM
kok328
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Join Date: Nov 2007
Location: Grand Blanc, MI
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Thanks Dave, now I understand.
So I guess the gist of it is, do not create a multi-wire circuit where you can not tolerate one circuit tripping the other.

Thanks Again.

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