MWBC

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SceneryDriver

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Electrical and Automation Designer
I was informed this afternoon that MWBC circuits are apparently no longer permitted in residential installs, as of the '11 code cycle. I know they're fraught with issue with all the new arc fault regs, but I didn't think it meant that there was an outright prohibition. I haven't gone digging through my code book yet, but that doesn't sound right.

I'm personally not a fan of them, as a loose neutral can place wildly unexpected voltages across things that are expecting a nice steady 120V. That said, I didn't think they're illegal now. Thoughts?



SceneryDriver
 
They are not illegal in resi installs. Recent code changes expanding the requirements for AFCI and GFCI have made using them more difficult.

I personally think they are the most elegant of all circuits. I never knew a day of wiring without them. I have them all over my house and never had a lick of trouble. Consider how many millions of MWBC are in service today and the amount of material and energy saved by using them. There is nothing not to love. The hysteria that surrounds them is sad.
 
What has changed is the requirement that the source of such an MWBC be either a single breaker with common trip on all poles OR single pole breakers with a handle tie so that manually opening one breaker opens the rest.

The rationale is that when you go to work on one leg of an MWBC you do not want to be surprised by current (and voltage when open) on a common neutral.

Because this requires shutting off all legs of the MWBC for relamping or other operations instead of just one and because it means that you must momentarily shut down the other legs to reset a tripped breaker on one leg, MWBCs are not as attractive as they were earlier.
 
GoldDigger said:
What has changed is the requirement that the source of such an MWBC be either a single breaker with common trip on all poles OR single pole breakers with a handle tie so that manually opening one breaker opens the rest.
...
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I don't recall seeing that as a requirement anywhere. Please provide verification.
 
As others stated with the advent of the afci's it is not advisable to use a multiwire branch circuit. Originally some manufacturers did not make a dp afci but now they are available. The problem with using dp afci's is that if one circuit trips then you lose power to the other circuit also which IMO, is a bad idea.

You can, however, run a multiwire branch circuit and feed circuits that are required to be afci but as stated above handle ties or use of dp breakers would be needed.

210.4(B) Disconnecting Means. Each multiwire branch circuit
shall be provided with a means that will simultaneously
disconnect all ungrounded conductors at the point where
the branch circuit originates.
Informational Note: See 240.15(B) for information on the
use of single-pole circuit breakers as the disconnecting means.
Click to expand...


240.15(B) Circuit Breaker as Overcurrent Device. Circuit
breakers shall open all ungrounded conductors of the circuit
both manually and automatically unless otherwise permitted
in 240.15(B)(1), (B)(2), (B)(3), and (B)(4).
(1) Multiwire Branch Circuits. Individual single-pole circuit
breakers, with identified handle ties, shall be permitted
as the protection for each ungrounded conductor of multiwire
branch circuits that serve only single-phase line-toneutral
loads.
(2) Grounded Single-Phase Alternating-Current Circuits.
In grounded systems, individual single-pole circuit
breakers rated 120/240 volts ac, with identified handle ties,
shall be permitted as the protection for each ungrounded
conductor for line-to-line connected loads for single-phase
circuits.
Click to expand...
 
Personally the only problem i've encountered with MWBC's are the grouping of them , which any decent spark will do. I was glad to see the recent 210 grouping inclusion ....~RJ~
 
SceneryDriver said:
I was informed this afternoon that MWBC circuits are apparently no longer permitted in residential installs, as of the '11 code cycle. I know they're fraught with issue with all the new arc fault regs, but I didn't think it meant that there was an outright prohibition. I haven't gone digging through my code book yet, but that doesn't sound right.

I'm personally not a fan of them, as a loose neutral can place wildly unexpected voltages across things that are expecting a nice steady 120V. That said, I didn't think they're illegal now. Thoughts?



SceneryDriver
Click to expand...

99+% of residential services in the US are MWBC. As others have written, not illegal to use inside a house either. Ovens and dryers have required MWBC for 20 years. MWBC arent very common in resi here for some reason, even in days past. I suspect that is because it's easier to keep track of one circuit than two... or NM was cheap enough to run 2 runs where one would do...or more $$$ in install. In higher end homes with a ton of switched receps and 3 ways, having another hot in there could get confusing...and blow box fill.

In commercial/industrial, you'd be crazy to pull a neutral with each phase conductor, yet it's mostly commercial/industrial job specs that are banning MWBC from what Ive read on this forum over the years. go figure.
 
infinity said:
Are you saying that a mult pole CB wouldn't be manufactured with a common trip?
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Independent trip are not all that uncommon. It will be marked “Independent Trip” or “No Common Trip.”

A lot of independent trip breakers look like two or three single pole units riveted together... because that's what the manufacturer did, slapped on a handle tie, and put on the appropriate label.
 
JFletcher said:
99+% of residential services in the US are MWBC. As others have written, not illegal to use inside a house either. Ovens and dryers have required MWBC for 20 years. MWBC arent very common in resi here for some reason, even in days past. I suspect that is because it's easier to keep track of one circuit than two... or NM was cheap enough to run 2 runs where one would do...or more $$$ in install. In higher end homes with a ton of switched receps and 3 ways, having another hot in there could get confusing...and blow box fill.

In commercial/industrial, you'd be crazy to pull a neutral with each phase conductor, yet it's mostly commercial/industrial job specs that are banning MWBC from what Ive read on this forum over the years. go figure.
Click to expand...
MWBC is multi wire branch circuit. A service is usually multiwire but is not a branch circuit.

The reason the commercial/industrial specs are moving away from MWBC,s is once they required handle ties or common trip, designers didn't want a trip on one leg taking out the othet leg(s) of the circuit. Dwellings were effected some by handle tie rules, but the bigger factor was AFCI requirements and either unavailability of multi pole AFCI's or higher price of them.
 
Smart $ said:
I don't recall seeing that as a requirement anywhere. Please provide verification.
Click to expand...

I think that this is a matter of terminology. I was not thinking of a dual or quad breaker assembly with handle ties as a "single breaker". If you know of multipole breakers with a single handle (not just a permanent handle tie) that are not common trip I stand corrected.
 
GoldDigger said:
... If you know of multipole breakers with a single handle (not just a permanent handle tie) that are not common trip I stand corrected.
Click to expand...
I do not know of any offhand, and I'm not going looking, but a single handle (not just a permanent handle tie) on any multipole breaker is a very, very good common trip type indication. :D
 
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