Multiwire branch circuit question

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lesaew

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So I have a question about multiwire branch circuits. What scenarios would one choose to run two ungrounded conductors sharing a grounded conductor on a two pole breaker. Rather than just using 2 breakers and have independent grounded conductors?

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Anybody wanting to use a very efficient smart wiring method. It saves not only on the initial installation but overall performance.

Roger
 
There is no advantage to doing this, except maybe saving the cost of a 2nd neutral conductor, or not exceeding conduit fill. I hate the thought of one circuit tripping another when there is no problem on the 2nd one. I recently did a job in some science labs, splitting mwbc's where multi tripping had been a problem.

Also consider that losing a neutral can put 208 or 240 on both circuits.

Not worth doing if you don't have to.
 
roger said:
Anybody wanting to use a very efficient smart wiring method. It saves not only on the initial installation but overall performance.

Roger
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:thumbsup:


lesaew said:
So I have a question about multiwire branch circuits. What scenarios would one choose to run two ungrounded conductors sharing a grounded conductor on a two pole breaker. Rather than just using 2 breakers and have independent grounded conductors?

Sent from my SAMSUNG-SM-N920A using Tapatalk
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Every time I want to save on pipe fill and materials, reduce the number of current carrying conductors and derating, reduce voltage drop, and basically every time I get the chance.

I try to use handle ties instead of two or three pole breakers.
 
Pulling 12 or 14/3 is many times easier than pulling 2 12 or 14/2. Material cost is lower, lower/lighter conduit fill, lesser derating. Sure if you lose the neutral you can have fried gear, but keep in mind nearly all residential services use 2 hot and one common grounded conductors.

Pulling a x/3 conductor when you only need x/2 also a cheap way to have a spare circuit w/o pulling an entirely new run, which may not be economical or possible w/o putting lots of holes in finished drywall.
 
480sparky said:
Oh, do tell us how this works! I'd love to get 480 volts out of a 120/240 system just by leaving out the neutral.
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He's referring to losing a neutral on a resi system giving a theoretical 240V on one 120V load, or 208V on a 3ph system, or a 1ph pulled from a 3ph. Both circuits is either circuit, not both at the same time.
 
jmellc said:
I hate the thought of one circuit tripping another when there is no problem on the 2nd one.
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I hate that too. I hate the code change that brought it about. I hate 210.4(B) so much it is one of the only code references I know by heart. Over a century of safe wiring practice hobbled up by a useless change.

I recently did a job in some science labs, splitting mwbc's where multi tripping had been a problem.
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That sounds like a load problem not a circuit problem.
 
Has anyone gone back to using 14-3/12-3 home runs in resi? 2 pole afci's used to be not available/hard to get/expensive, but now (at least for homeline which I am most familiar with) 2 poles are the same cost as 2 singles.
 
electrofelon said:
Has anyone gone back to using 14-3/12-3 home runs in resi? 2 pole afci's used to be not available/hard to get/expensive, but now (at least for homeline which I am most familiar with) 2 poles are the same cost as 2 singles.
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No but we seldom use MWBC in residential. 1 12/3, 2 12/2s and a device blows box fill on even a 22in3box. Spare circuits that are pulled to a crawlspace or attic get 12/3 tho.
 
JFletcher said:
He's referring to losing a neutral on a resi system giving a theoretical 240V on one 120V load, or 208V on a 3ph system, or a 1ph pulled from a 3ph. Both circuits is either circuit, not both at the same time.
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So when the clerk at the store says, "Would you like either paper or plastic?" you expect to get your groceries to be sacked in both paper PLUS plastic?
 
jmellc said:
There is no advantage to doing this, except maybe saving the cost of a 2nd neutral conductor, or not exceeding conduit fill. I hate the thought of one circuit tripping another when there is no problem on the 2nd one. I recently did a job in some science labs, splitting mwbc's where multi tripping had been a problem.

Also consider that losing a neutral can put 208 or 240 on both circuits.

Not worth doing if you don't have to.
Click to expand...
There is no reason to use a common trip device. A handle tie meets the code rule, and most of the time will not cause the other breaker of the pair to open if one is clears a fault.
 
don_resqcapt19 said:
There is no reason to use a common trip device. A handle tie meets the code rule, and most of the time will not cause the other breaker of the pair to open if one is clears a fault.
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Given the choice, I would never install a common-trip multipole breaker for a MWBC [except for 210.4(C) Exceptions].
 
don_resqcapt19 said:
There is no reason to use a common trip device. A handle tie meets the code rule, and most of the time will not cause the other breaker of the pair to open if one is clears a fault.
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A man once said Tis better to keep quiet and be presumed stupid that to open your mouth and prove yourself stupid. But here I go, So. it this statement saying that "handle ties" won't trip both breakers and a "common trip" will. I've never paid this much attention or given it consideration in my layouts.
 
don_resqcapt19 said:
There is no reason to use a common trip device. A handle tie meets the code rule, and most of the time will not cause the other breaker of the pair to open if one is clears a fault.
Click to expand...

tadavidson said:
A man once said Tis better to keep quiet and be presumed stupid that to open your mouth and prove yourself stupid. But here I go, So. it this statement saying that "handle ties" won't trip both breakers and a "common trip" will. I've never paid this much attention or given it consideration in my layouts.
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My experience is mostly with QO and Homeline. Those lines the handle tie will not trip other poles if only one trips. You will end up turning off the other pole(s) to get the tripped one to the full off position when resetting it though.
 
kwired said:
My experience is mostly with QO and Homeline. Those lines the handle tie will not trip other poles if only one trips. You will end up turning off the other pole(s) to get the tripped one to the full off position when resetting it though.
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Never paid it attention, Yes, one IS tripped and one IS off. THanks for that. 30 years and still learning...or am I remembering?
 
Another answer to OP's question is the neutral conductors of MWBC's may not count as a current carrying conductor when it comes to ampacity adjustments in a raceway.

I used to run a lot of 15 or 20 amp 120/240 three wire circuits in a 3/4 inch raceway - 8 hots 4 neutrals - only 8 CCC's still allowed for using 12 or 14 AWG conductors. Run 8 hots and 8 neutrals - you have to derate those conductors to 50% and will need to increase conductor sizes, and will be increasing to 1 inch racweway as well.
 
In addition to the obvious reduction in number of CCCs counted for derating, there is a quite significant reduction in voltage drop on a balanced circuit.
With two circuits and two neutrals, the VD on. 120V circuit is twice the drop in the hot conductor.
With an MWBC the voltage drop on each circuit is just that of its own hot wire.
An earlier post stated that the MWBC is more efficient, and this difference in voltage drop is where that comes from.
Cost is also lower for less copper.

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