12-3

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  #1  
Old 12-24-06, 08:29 AM
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12-3

Yesterday I posted a response regarding using 12-3 wire to run 2 circuits and then mentioned using 12-4. I was told never use 12-4 unless it is with 3 phase. I attempted to find out why this is so by starting another thread asking the question. The Moderator sternly rebuked me saying I already had my answer and why was I trying to find another answer and closed the thread. I sincerely would like to know the answer to the question.
To the Moderator, I was not trying to find another answer. I was asking why? I certainly would never run 12-4 after being told not to by a qualified person. I just wanted to know why this is not correct, and still do.
Here is the scenario: If you are wiring alternating circuits lets say in a kitchen using 12-3 there is only 1 neutral for 2 commons or hots and 1 ground. So, is it correct then that only 2 commons or hots are permitted for 1 neutral? Where does it state this in the NEC Code? Is there ever a situation in residential wiring where 12-4 is permissible?
I'm sorry if this an offensive response, I just want to know.
 
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Old 12-24-06, 08:49 AM
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Not offensive at all, the moderators do go a little frothy at the mouth sometimes, what you are refering to is a multi-waire circuit. When done correctly they are very useful.
 
  #3  
Old 12-24-06, 08:52 AM
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> So, is it correct then that only 2 commons or hots are permitted for 1
> neutral?

"Common" is another word for neutral.

In residential wiring, two hots may be used per neutral; this is called a multi-wire circuit. However there are a number of rules that govern multi-wire circuits, so they are not really a general purpose solution.

For example, the two hots of a MWC must be on different legs of the incoming service; and if each hot leg is terminated on the same yoke, a double pole breaker must be used. Moreover, the integrity of the neutral conductor cannot rely on devices, so wirenut splices and pigtails must be used.

Multi-wire circuits can make an otherwise straightforward wiring project of installing two individual circuits much more confusing. The advantages are that multi-wire circuits only use 2/3 of the wire of two normal circuits, and multi-wire circuits can decrease voltage drop which is an advantage on long distance circuits.

> Where does it state this in the NEC Code?

Multi-wire circuits are covered in many different sections.

> Is there ever a situation in residential wiring where 12-4 is permissible?

Yes; 12-4 cable can be used anywhere that could use (2) individual runs of 12-2. It is not intended for sharing neutrals; only for when you need two separate circuits going to the same place.
 
  #4  
Old 12-24-06, 08:59 AM
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I think the moderator answered your question, but here goes.

12/4 romex have blk, red, blu, and wht conductors. Only wht is allowed for a neutral. A 3 phase multiwire circuit would connect as follows: blk to phase one---red to phase two---blu to phase three---wht to neutral.

99.9% of homes do not have 3 phase services and can not have a 4 wire multiwire circuit. Homes have 240/120 single phase 3 wire services and hence can only have 3 wire multiwire circuits. Therfore, we use 3-wire romex for multiwire circuits in homes. Care must be taken to be sure that the"hot" blk and red conductors are on different legs at the panel connection. If they are not then the neutral will carry the total load of both circuits and not just the unbalanced portion.

It was mentioned that 12-2-2 romex has 2 wht and two colored conductors to give you 2 independent circuits(not mutiwire). Regular 12/4 romex has only one wht and three colors(blk,red,blu). With regular 12/4 we are allowed to use all 4 conductors as hots if we reidentify the white as a color such as blk and use it in switch legs. 12/4 would not normally find much use in home wiring. You are not allowed to ,say, reidentify the blu as white and try to use the cable as if it were 12-2-2.
 
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Old 12-24-06, 09:04 AM
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Did you mean two three wire circuits on 12-4? if so read on.

In a three wire circuit used to provide power to two 120v circuits the majority of the return current is handled by the hot legs so the neutral only caries the difference between the two 120v circuits.

Example: (Assuming 20 amp breaker/#12 wire) one circuit has a 15 amp load one has a 10 amp load the neutral would only carry 5 amps. This is well in it's range. In fact the most the neutral would ever would carry is 20a if on one circuit there was no load and a 20a load on the other. If you add a third hot the neutral would have to carry the full load of that circuit plus the other two. The neutral could therefore easily exceed it's maximum rating without tripping any breaker.
 
  #6  
Old 12-24-06, 09:12 AM
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No, I think you misunderstand me. Most homes have only 240/120 volt 3 wire services. The panels will only allow for a 3-wire multiwire circuit with the neutral carrying the unbalance.

However in industrial and commercial installations a three phase 208/120 volt 4 wire service or 480/277 volt 4 wire service is commonly found. In these panels we can install 4-wire multiwire circuits with the neutral carrying the unbalance of the 3 circuits.
 
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Old 12-24-06, 10:10 AM
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Axis,

A residence (unless extremely unusual) only has single phase service. The two legs of the 240 volts entering the house are 180 degrees out of phase.

When you run a multi wire circuit, that is when you use two hots, one from each leg of the service and a single neutral, the current on the neutral will be the difference between the current on each hot leg.

If you hooked up a third hot it wold be from the same leg of the service as one of the other hots wires. This would make the current on the neutral potentially double the allowed value, up to 30 amps or up to 40 amps. You could easily start a fire.

In an Industrial setting, where there truly is three phase service, things are different.
 
  #8  
Old 12-26-06, 08:38 AM
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Thank You

Thank you all for your responses to my question. I sincerely appreciate the info.
 
  #9  
Old 12-26-06, 09:35 AM
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The essential problem with using a 4-conductor cable with Black-Red-Blue- White #12 conductors for extending two 2-wire , 120 vollt Branch-Circuits-- Black & White, Red & "White" -- from a panel is that the Blue conductor cannot be "re-Identified" as a White conductor because of the guage-size- #12---- Art. 200.6.

An example where 12/4 cable with Black-Red-White-Blue conductors can be used in a residence is when wiring 2 or more fixtures when the routing of the cables is--- 2-wire "feed-in" cable to 3-Way Switch "#1"--- 3-wire cable, 3WS#1 to Fixture "#1", ---- 4-wire cable, F#1 to F#2--- 4-wire cable, F#2 to F#3--- 3-wire cable, F#3 to #3WS#2
 
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