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#76171 - 12/22/00 06:26 PM MULTI WIRE CIRCUITS
doc Offline
Member

Registered: 11/25/00
Posts: 119
Loc: Texas
I do not like this type of circuit but am in a tough spot and have no choice but to do one so is this correct: 2 hot wires 1 grounded conductor 1 ground and the breakers go on different phasses side by side and tied together.The company I work for has ask me to put one in , and after explaining to them why to me these are dangerous circuits they have agreed that they would rather not have them in the plant so it will get removed in the near future but for now.I know I didn't need to give all this other gibberish
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#76172 - 12/23/00 04:39 AM Re: MULTI WIRE CIRCUITS
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
Doc,

If I understand it correctly this is a standard multiwire circuit and very common. (2 120v circuits) If the two "Hots" are sharing the same grounded (neutral) they must be on different phases to prevent overload. You didn't mention if this is in 1 cable or in a raceway, but having a common tie-handle (2 pole breaker) is considered safer by many and is often required.

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#76173 - 12/23/00 06:54 AM Re: MULTI WIRE CIRCUITS
sparky66wv Offline
Member

Registered: 11/17/00
Posts: 2339
Loc: West Virginia
If you think about it, all 120/240V systems are "multiwire"... If you loose the grounded conductor at the meterbase, disconnect, transformer, etc. a 240V series circuit will do interesting things to VCRs, TVs, Refrigerators, and just about anything plugged in at the time. Just had a slightly corroded "neutral" destroy a customer's VCR and quite a few bulbs the other day... a little sandpaper and some antioxidant and now I'm a hero...
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#76174 - 12/23/00 07:02 AM Re: MULTI WIRE CIRCUITS
gto6t7 Offline
Member

Registered: 12/16/00
Posts: 30
Loc: Machias, ME
Several years ago, while in a code upgrade class, the question came up regarding multiwire branch circuits based on Article 210-4. It became evident what the code requirement was that when the two phases supplied two or more devices on a single yoke and only in a dwelling did it need to have the breakers connected. An example is a duplex receptacle that is split so that the top is fed by one phase and the bottom is fed by another, must be fed by breakers that are interconnected but only in a dwelling. If you had a 2 gang box with two receptacles and one duplex is fed by one phase and the other duplex from another phase then the breakers are not required to be connected. Your situation appears to be outside of the requirement as it is not a dwelling. The untrained, unsuspecting and unauthorized may have a problem with a multiwire branch circuit due to the hadzard of lifting a neutral they think is from a circuit that is dead. The code is a minimum and it's not a bad idea to have a common trip breaker for a multiwire branch circuit.

Dave

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#76175 - 12/23/00 02:25 PM Re: MULTI WIRE CIRCUITS
doc Offline
Member

Registered: 11/25/00
Posts: 119
Loc: Texas
That is the very reason I don't like them is because of some unsupecting person taking it loose,sparky you are correct on multi at the pole and meter base and panel but there you know that the neutral is for 2 phasses while out in the houise or a plant a lot of people do not realize what they are or how to deal with them
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#76176 - 12/23/00 08:58 PM Re: MULTI WIRE CIRCUITS
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
Doc,

For these reasons it would be safer to have a 2 pole breaker controlling the multiwire circuit. When 1 circuit is turned off the other one will be also. So there will be no problem when disconnecting the neutral.

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#76177 - 12/24/00 02:14 AM Re: MULTI WIRE CIRCUITS
dandy467d Offline
Junior Member

Registered: 12/24/00
Posts: 8
Loc: auburn, ny usa
In many applications 3 conductors share one neutral, would you use a three pole breaker?
Safety considerations are great, but common sense has to enter the picture at sometime. We recently worked in a nursing home, 3 circuits shared 1 nuetral- circuit 1 fire alarm panel- circuit 2 receps near alarm panel- circuit 3 receps in patient room. With a 3 pole breaker a overload on circuit 2 would turn of circuit 3 and possibly shut off important medical equipment. Not the way i would circuit this area, but we need to be realistic- cost saving is very important in todays world. I geuss the point off all this is, take a minute and find out what everything is before you discconnect anything.

[This message has been edited by dandy467d (edited 12-24-2000).]

[This message has been edited by dandy467d (edited 12-24-2000).]

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#76178 - 12/24/00 06:48 AM Re: MULTI WIRE CIRCUITS
gto6t7 Offline
Member

Registered: 12/16/00
Posts: 30
Loc: Machias, ME
Dandy the application you used must follow the strict requirements in article 517, "health care facilities", specifically 517-30(c)(1) which would indicate the circuit you called #3 not be for the receptacles in the patients room designated as a life safet branch circuit or part of the emergency system. Logic is a big part of deciding what will be feed by a multiwire circuit. I'm glad my last post left me some wiggle room to see that it is not always good to have a common trip breaker but I feel it is always essential to have a qualified person (as discussed in another thread) work on the installation and maintenance of multiwire circuits and in fact all electrical systems.


Dave T.




[This message has been edited by gto6t7 (edited 12-24-2000).]

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#76179 - 12/24/00 06:53 AM Re: MULTI WIRE CIRCUITS
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
Dandy,

Hello!

I was just trying to point out that from a safety standpoint of "less than qualified" persons working on the Electrical system the common tie handle is the safest. I areas where you would not want crital circuits being turned off or tripping off with others I believe single pole breakers are best. These areas should also have qualified personnel installing and servicing the system so it is not a problem. The only area I have seen this required is in residential work in some jurisdictions.

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#76180 - 12/24/00 07:13 PM Re: MULTI WIRE CIRCUITS
doc Offline
Member

Registered: 11/25/00
Posts: 119
Loc: Texas
this has opened up a bigger disscussion than I thought it would , so let's go here for a second if you put 2 breakers on same phase and then run you 2 hots and one neutral is this a code violation or is it just dangerous or is it ok to do this and would you have to tie breakers together or not
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