Multiwire branch circuits

[cppoly]

For 3-phase 208/120V panels, if 3 consecutive circuits lets say #1, #3, &#5 (feeding receptacles in 1 circuit per office lets say) are in the same conduit and share the same neutral then these are considered a multiwire branch circuit and need a method for a simultaneous disconnect?

The NEC uses a multiwire branch circuit example that shows two hot conductors going to a receptacle and says a simultaneous disconnect is required to prevent someone working on a live circuit. This makes sense for a receptacle sharing two hot conductors since they are terminated very close to each other. But, does it make sense to do the same simultaneous disconnect for other multiwire branch circuits like the one I described?

 

[John120/240]

Yes it does make sense to disconnect all three hots in your situation. Say your replacing a

recptacle on circuit 3. Turn off #3. If you landed two neutrals on the recptacle, you can let the

smoke out of any thing connected downstream when you remove neutral from recptacle.

Hence the requirement for MWBC neutrals to be pigtailed.

 

[Strathead]

Letting the smoke out as John put it is the lesser of two evils in my opinion. Say you have two receptacles, both with loads (say a light) plugged in and turned on. Each is fed from a different circuit black and red, but the share the neutral. You are in the ceiling and want to install a new receptacle on one of the circuits, so you turn off just the black circuit. Then you disconnect the wire nut on the whites and they spring apart. If you are in sight, you notice that the other light went out and there was a slight spark. Or maybe you don't even get that clue. The white wire coming back from the red circuit receptacle is sitting there with 120 volts to ground, just waiting for a wet body to finish the connection.

 

[jumper]

The OP seems to be questioning 210.4(B), that he understands the reasoning behind 210.7(B), and did not seem to question 300.13(B).

 

[jaggedben]

The white wire coming back from the red circuit receptacle is sitting there with 120 volts to ground, just waiting for a wet body to finish the connection.

Yup. Seen it happen. Also once dealt with a situation where someone had connected a new light fixture to another circuit's neutral, thus in effect unintentionally creating an MWBC (albiet with two neutrals).

 

[charlie b]

Two significant changes on this subject were made in 2008:

• The requirement for simultaneous disconnection of two power sources serving more than one device on the same yoke was moved from 210.4(B) to 210.7(B), and
• 210.4(B) was revised to require simultaneous disconnection of all power sources for any MWBC.

These are two separate rules with two different hazards in mind. The first one is easier to visualize. Strathead described the second one quite clearly. You may need to sketch out the wiring, in order to visualize the hazard. In the sketch, draw the neutral wires from the source, and leading to each of the two outlets, being connected by a wire nut at the first outlet. This will help you see how, if you only turn off the "black wire circuit," then removing the neutral circuit's wire nut can expose you to a shock from the "red wire circuit."

 

[iwire]

All one needs to avoid letting the smoke out or a shock is experience and an amp clamp.

It is a poor code change and encourages more hot work, not less.

 

[cppoly]

So circuits #1, #3, and #5 should have a simultaneous disconnect, which leads me to my next question. How often do you see actually CB's connected this way from commercial panelboards? CB's that share the same neutral are done to cut costs for the contractor, but I've never seen handle ties installed in panelboards to accommodate the simultaneous disconnect rule.

 

[don_resqcapt19]

All one needs to avoid letting the smoke out or a shock is experience and an amp clamp.

In general I agree, but you could check the current on the grounded conductor and then open it and still get a shock or damage equipment if there is some automatically started device on the multiwire circuit. Also, if you are opening the connection on the grounded conductor, then I think that the OSHA rules would require that all ungrounded conductors associated with that grounded conductor be locked out.

It is a poor code change and encourages more hot work, not less.

It probably does, but then almost no hot work is ever permitted.

 

[don_resqcapt19]

So circuits #1, #3, and #5 should have a simultaneous disconnect, which leads me to my next question. How often do you see actually CB's connected this way from commercial panelboards? CB's that share the same neutral are done to cut costs for the contractor, but I've never seen handle ties installed in panelboards to accommodate the simultaneous disconnect rule.

That is one of the things that the local inspector always looks at, so around here you see the handle ties. He also looks at the required bundling of the multiwire circuit in the panel enclosure.

 

[iwire]

So circuits #1, #3, and #5 should have a simultaneous disconnect, which leads me to my next question. How often do you see actually CB's connected this way from commercial panelboards? CB's that share the same neutral are done to cut costs for the contractor, but I've never seen handle ties installed in panelboards to accommodate the simultaneous disconnect rule.

Lack of enforcement.

I just did a small office remodel with about 75 MWBC furniture feeds, even though they were existing the inspector insisted I install handle ties which we did as an extra to the contract.

 

[iwire]

It probably does, but then almost no hot work is ever permitted.

IMO, the rule was made to protect people from doing hot work that was likely not allowed to start with.

The fact is it puts us in a position to work hot when we would not have had to without the handle ties.

Here is an ugly but very common arraignment of a MWBC in my area, you could exchange the receptacles with lights, either way is common.

There is no danger in working on the one leg of this circuit after the split.

 

[roger]

So circuits #1, #3, and #5 should have a simultaneous disconnect, which leads me to my next question. How often do you see actually CB's connected this way from commercial panelboards?

Always in projects under 2008 and later.

CB's that share the same neutral are done to cut costs for the contractor,

They are actually more efficient in wasted power, voltage drop, and natuural resource conservative.

but I've never seen handle ties installed in panelboards to accommodate the simultaneous disconnect rule.

What code cycle are you under?

I agree with Bob that this rule creates more hot work on MWBC's and I will also say I use MWBC's where ever I can.

Roger

 

[Strathead]

IMO, the rule was made to protect people from doing hot work that was likely not allowed to start with.

The fact is it puts us in a position to work hot when we would not have had to without the handle ties.

Here is an ugly but very common arraignment of a MWBC in my area, you could exchange the receptacles with lights, either way is common.




There is no danger in working on the one leg of this circuit after the split.

This is a compelling argument, but it goes against everything our society has been propelling towards for several years. We must legislate against stupidity. Because there are idiots out there who don't know or forget that the neutral may carry current in a multiwire branch circuit, we must add one more layer to protect them. This is no different than putting a warning on a jar of peanut butter that it "May contain peanuts." or the McDonalds coffee "May be hot"

 

[roger]

There is no danger in working on the one leg of this circuit after the split.

This is where many don't get the concept.

Roger

 

[cppoly]

Always in projects under 2008 and later. They are actually more efficient in wasted power, voltage drop, and natuural resource conservative. What code cycle are you under?

Roger

I'm under the 2008 code. So a full 42-pole panel will all MWBC should ideally look like this:



But in reality, I just see this:

 

[charlie b]

There is no danger in working on the one leg of this circuit after the split.

This is where many don't get the concept.

You can safely work downstream of the split, by turning off only the one circuit you are touching. You cannot safely work within the junction box at which the split takes place, without turning off all three circuits. The difference between these two tasks is the concept that people need to get. I suspect that, like many other NEC revisions, this one was made because the decision makers did not choose to trust electricians to know the difference, and to treat them differently.

 

[iwire]

You can safely work downstream of the split, by turning off only the one circuit you are touching. You cannot safely work within the junction box at which the split takes place, without turning off all three circuits. The difference between these two tasks is the concept that people need to get. I suspect that, like many other NEC revisions, this one was made because the decision makers did not choose to trust electricians to know the difference, and to treat them differently.

I think we have here exactly the situation they have in the code making panels.

We have very knowledgeable, well meaning people that have little if, any hands on experience with a particular method deciding that they know better than the people that actually work with it based on 'what ifs'.

 

[Strathead]

I think we have here exactly the situation they have in the code making panels.

We have very knowledgeable, well meaning people that have little if, any hands on experience with a particular method deciding that they know better than the people that actually work with it based on 'what ifs'.

Really? I was with you up to this point. Who answered this question that way? I think people here are usually pretty good at answering questions with real world answers. I think, for example, of the answers to Horsegoer about his estimating ofr wall mount fixtures as a good example.

 

[jumper]

Really? I was with you up to this point. Who answered this question that way? I think people here are usually pretty good at answering questions with real world answers. I think, for example, of the answers to Horsegoer about his estimating ofr wall mount fixtures as a good example.

Reread Bob's/Iwire's answers, he was referring to NFPA CMP members.

Horsegoer's thread has nothing to do with this one.