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 Sunny B. 10-25-2011 06:41 PM

Just curious

Hey guys!!! Quick question, just curious. Is a split recept considered a MWBC?? I feel funny asking this but I don't know the answer so I have to ask.

 mpoulton 10-25-2011 06:56 PM

If the top and bottom are fed from different breakers and share a neutral, then yes. If it's just half-switched, then no.

 Code05 10-25-2011 09:30 PM

Sunny,

Always go to Article 100 to start.

Quote:
 Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.
As mpoulton stated, it depends on the circuit.

MWBC are used extensively in commercial. Every "full boat", 3 hots-ABC or B/RD/BL- and neutral is a MWBC. Two hots/ungrounded and a neutral are also a MWBC.

Classic other examples are dryers and ranges. Here is a good picture of MWBC.

http://i877.photobucket.com/albums/a...99596162_2.jpg

While not a branch circuit, every service is a multiwire circuit and will follow the same principles.

Something to note is the danger of a bad neutral in a multiwire circuit. Here is a good PPT tutorial on the subject.

http://www.code-elec.com/content/00/...st Neutral.ppt

 frenchelectrican 10-26-2011 02:53 AM

SunnyB.,

There is one major golden rules with MWBC it don't matter if single phase or triphase MWBC there is one item is never I will stress it never lift or disconnect the netural on MWBC until you have verify all the circuits on this MWBC is complety off otherwise you will know the result .,, Poof! magic smoke do show up.

So you can see why with MWBC you have to do the extra step when you do anything with it.

It don't matter which location you are in it wil be the same all way around. ( I live in France we do have MWBC there as well )

Merci,
Marc

 Sunny B. 10-26-2011 04:16 AM

Quote:
 Originally Posted by Code05 Sunny, Always go to Article 100 to start. As mpoulton stated, it depends on the circuit. MWBC are used extensively in commercial. Every "full boat", 3 hots-ABC or B/RD/BL- and neutral is a MWBC. Two hots/ungrounded and a neutral are also a MWBC. Classic other examples are dryers and ranges. Here is a good picture of MWBC. While not a branch circuit, every service is a multiwire circuit and will follow the same principles. Something to note is the danger of a bad neutral in a multiwire circuit. Here is a good PPT tutorial on the subject. http://www.code-elec.com/content/00/...st Neutral.ppt
Yes Derek. I am very familiar with a "full boat".

 Sunny B. 10-26-2011 04:18 AM

Thanks guys!! A split recep seemed to be a MWBC, I just never herd anyone refer to it as such. Makes since, and I will be sure to check article 100 before asking any questions in the future

 a7ecorsair 10-26-2011 07:04 AM

Quote:
 Originally Posted by Code05 (Post 756795) Classic other examples are dryers and ranges. Here is a good picture of MWBC.
Code 05
Are ranges and dryers considered MWBC?

 carmusic 10-26-2011 09:16 AM

Quote:
 Originally Posted by a7ecorsair (Post 756977) Code 05 Are ranges and dryers considered MWBC?
dont think so because they use 240V directly, neutral is often just there for electronics

 Billy_Bob 10-26-2011 11:36 AM

MWBC or Multi-Wire Branch Circuit is sort of a "magic trick" with wiring.

In a home this would be two separate 120 volt circuits. Normally you would need to run two hots and two neutrals for these two separate circuits...

However, if one circuit hot is on one phase of the breaker panel and the other circuit hot is on the other phase of the breaker panel, and they are both the same amperage, then a shared neutral can be used (the same size wire as one of the hots).

And this would be done to save on wiring costs.

The trick to this is you have alternating electricity or alternating current (AC). And the two black "different" wires coming into the house from the electric company (3rd is neutral). These wires, in a moment of time, are oppsite each other! So while one is "+", the other is "-".

They balance each other out.

Think of the neutral as a basket. While one wire is placing apples into the basket, the other wire is removing apples from the basket (opposite).

So if there are equal loads on both separate MWBC circuits (say both are using 6 amps), then there will be no (or little) electricity flowing on the neutral wire.

And the maximum load you would have on an MWBC neutral would be if no electricity was being used on one circuit and the maximum was used on the other circuit. And that would be just like a regular two wire circuit.

These are typically breakers next to each other in the panel like a 240 volt circuit. So each circuit is on a separate phase. Also so the two breakers can have a connected tie bar. Then BOTH circuits must be turned off - for safety. Old MWBC circuits may not have a tiebar! Best to turn off main power if not sure. (If one circuit is still on and you work on the neutral, you will get zapped by the other circuit using that same neutral!)

 a7ecorsair 10-26-2011 11:43 AM

Nice write-up but it didn't address my question...

 Billy_Bob 10-26-2011 11:54 AM

Quote:
 Originally Posted by a7ecorsair (Post 757142) Nice write-up but it didn't address my question...
I'll post that separately as this stuff can get to be confusing!

Anyway ranges and dryers are not considered MWBC's. When you say MWBC (in a home), I think two separate 120 volt circuits - and typically this is used for a garbage disposal/dishwasher outlet. Top outlet one circuit, bottom outlet the other circuit. And a 3 conductor with ground wire run - shared neutral. Red, White, Black, and ground.

I would consider a dryer or range to be a 240 volt circuit. And if one of these was using 120 volts for the controls, it would most likely be using just one hot leg.

 Billy_Bob 10-26-2011 12:07 PM

P.S. The difference is that a MWBC in a home is not using 240 volts whereas the 240 volt circuit is.

And I am not a circuit breaker guru, but it is my understanding that a 240 volt breaker is internally connected, so if one trips, the other trips as well????

Whereas for MWBC, it would be two separate 120 volt breakers which have a tie-bar added????

I tried searching for this info several years ago, but couldn't find anything. Anyone know? Does not matter? Does matter?

 a7ecorsair 10-26-2011 12:11 PM

 Billy_Bob 10-26-2011 12:19 PM

Quote:
 Originally Posted by a7ecorsair (Post 757171) Billy Bob, have you read through this thread? See post #3
Yes a dryer range is considered a MWBC.... But for forums such as these, when someone says MWBC, most people think of the shared neutral situation.

 Code05 10-26-2011 12:24 PM

Yes a dryer or a range is a MWBC.

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