MWBC Question Again

[goldstar]

I'm trying to understand the difference in the CMP's thinking between 210.4(B) and 210.7 (2014 NEC). Under 210.4 if I run a MWBC to a 2-gang JB and install two separate receptacles, this section infers that I have to use a 2-pole CB to disconnect all un-grounded conductors. Yet under 210.7, if I run two separate 120V branch circuits to a 2-gang JB and install two separate receptacles I am not required to use 2-pole CB's unless I snap off the tabs on the receptacles and install opposing circuits on the same yoke.

If the thinking was to disconnect all un-grounded conductors for safety reasons in the case of 210.4(B) why wouldn't the same logic be applied to 210.7?

 

[infinity]

The common disconnecting means for two circuits on one device has been around for decades, the MWBC disconnecting requirement is newer. The prior has to do with working on a single device and having both circuits de-energized. The latter has to do with ensuring the neutral is not energized if only one circuit is shut off.

 

[goldstar]

The common disconnecting means for two circuits on one device has been around for decades, the MWBC disconnecting requirement is newer. The prior has to do with working on a single device and having both circuits de-energized. The latter has to do with ensuring the neutral is not energized if only one circuit is shut off.

So, if I run two separate 120V circuits into a 2-gang JB and land the circuit wires on two separate receptacles, can I then use s/p breakers ? Or, am I required to disconnect all power to the JB ?

 

[jumper]

So, if I run two separate 120V circuits into a 2-gang JB and land the circuit wires on two separate receptacles, can I then use s/p breakers ? Or, am I required to disconnect all power to the JB ?

Two individual SP breakers without handle tie is fine.

 

[infinity]

So, if I run two separate 120V circuits into a 2-gang JB and land the circuit wires on two separate receptacles, can I then use s/p breakers ? Or, am I required to disconnect all power to the JB ?

As Jumper stated two single poles is fine since you do not have a MWBC. There is no requirement to disconnect all of the power to a box.

 

[Dennis Alwon]

You run into this alot with switch boxes where 3 switches may be on one circuit and then you may have a 3 way fed from another circuit

 

[goldstar]

As Jumper stated two single poles is fine since you do not have a MWBC. There is no requirement to disconnect all of the power to a box.

I understand that and I can buy into the logic behind the Code section. However, from the standpoint of safety, I don't see the logic if you have 2 separate circuits when you have separate neutrals. If you run an MWBC into a 2-gang box and are installing 2 duplex receptacles, you're splicing the neutrals under a wire nut and terminating the tails onto each receptacle. What's the difference, from the standpoint of safety ?

 

[jap]

I understand that and I can buy into the logic behind the Code section. However, from the standpoint of safety, I don't see the logic if you have 2 separate circuits when you have separate neutrals. If you run an MWBC into a 2-gang box and are installing 2 duplex receptacles, you're splicing the neutrals under a wire nut and terminating the tails onto each receptacle. What's the difference, from the standpoint of safety ?

If wired correctly perhaps.

But,

If someone were to jumper the neutral from the 1st receptacle to the 2nd where the removal of the 1st receptacle would disconnect the neutral to the 2nd receptacle, if something was plugged into the 2nd receptacle, you stand the chance of throwing 240 volts across the 120v load plugged into the 2nd receptacle in the MWBC scenario.

That would not happen if the 2 receptacles were fed from 2 separate circuits with 2 separate neutrals.

Jap>

 

[GoldDigger]

The common trip or handle tie requirement when sharing a neutral is not just about securing all power to an outlet (not just receptacle) box, but also for working with the neutral at any intermediate boxes along the way. If you turn off just one breaker to de-energize the specific circuit you are working with, you could be exposed to full line voltage on the "neutral" when you open it at an intermediate junction box.
If the circuit is an uninterrupted home run, then the primary concern is at the outlet box. But there could still be a risk of injury when working with the neutral at the panel where it originates.

PS: If you turn off one side of an MWBC, there is no risk of putting double voltage on a connected load even if you interrupt the common neutral!

 

[infinity]

I understand that and I can buy into the logic behind the Code section. However, from the standpoint of safety, I don't see the logic if you have 2 separate circuits when you have separate neutrals. If you run an MWBC into a 2-gang box and are installing 2 duplex receptacles, you're splicing the neutrals under a wire nut and terminating the tails onto each receptacle. What's the difference, from the standpoint of safety ?

If the two circuits are from a MWBC then de-energizing one circuit will not remove the load from the neutral for the other circuit and this can create a shock hazard if the neutral is opened. If there are two separate neutrals then when you de-energized one circuit you can safely open the neutral from that circuit.

 

[goldstar]

Judging by some of the replies posted here it's looking more and more like the CMP decided to make 210.4(B) a "one size fits all" code section and I'm also guessing it was done for safety reasons. However, in my scenario there's one MWBC running to one 2-gang JB where the neutral will be spliced to 2 pig tails, each of which get landed on 2 separate duplex receptacles. From the standpoint of safety I can't understand the requirement to use a 2-pole breaker to disconnect ALL un-grounded conductors yet allowing s/p breakers to shut off each circuit separately if I ran an extra neutral wire. There are no JB's in between - just 1 conduit run to one JB which is less than 50' away and within sight of the breaker panel (if that means anything). The only difference would be is if you are working live in that JB. Then there's 240V present so a total disconnect would make sense. But, then you would have to ask "Why are you working it live?"

BTW, I would also like to point out that we can't use the word IF in a scenario. That small word can make any scenario work in your favor. For example : IF a bank employee left the front door open and IF that same bank employee left the bank vault door open and IF I were so inclined I could walk into that bank and take all the $$ I could handle (IF I were so inclined)

 

[jap]

However, in my scenario there's one MWBC running to one 2-gang JB where the neutral will be spliced to 2 pig tails, each of which get landed on 2 separate duplex receptacles. From the standpoint of safety I can't understand the requirement to use a 2-pole breaker to disconnect ALL un-grounded conductors yet allowing s/p breakers to shut off each circuit separately if I ran an extra neutral wire.

Because with a MWBC and no handle ties shutting off only 1 of the breakers does not shut off all of the power associated with that circuit.
That circuit consists of 2 ungrounded conductors and 1 grounded conductor.

It boils down to Off should mean Off when shutting off circuits, and, that includes the neutral associated with those circuits.

In a MWBC without handle ties in your case, if someone shut off one of the circuits and something was plugged into the other, there would be 120 volts on the return neutral in the panel.

Therefore it did not disconnect all of the power associated with that circuit like it would if seperate neutrals were pulled for each circuit.

JAP>

 

[jap]

And the word if does matter.

That's the only way we ward off 90% off construction accidents to begin with.

If the current gets too high the breaker trips
If you step on the brake the car better stop
If you mom yells at you to do something you'd better get to it.
If the heat melts the glass at the sprinkler head the water better come out.
If I flush the toilet the water better go down the drain.

and so on....


JAP>

 

[goldstar]

Because with a MWBC and no handle ties shutting off only 1 of the breakers does not shut off all of the power associated with that circuit.
That circuit consists of 2 ungrounded conductors and 1 grounded conductor.

It boils down to Off should mean Off when shutting off circuits, and, that includes the neutral associated with those circuits.

In a MWBC without handle ties in your case, if someone shut off one of the circuits and something was plugged into the other, there would be 120 volts on the return neutral in the panel.

Therefore it did not disconnect all of the power associated with that circuit like it would if seperate neutrals were pulled for each circuit.

JAP>

Sorry, I guess I'm just not thinking out of the box here. I understand your explaination completely if it were installed say for a kitchen with multiple outlets. Let me try one more time. You have a 10' length of conduit with only one 2-gang JB at the end of it and the other end connected at the breaker panel. You can see the breaker panel - you can walk over 10' and shut a breaker off if you wish. What difference would it make at the 2-gang JB if I had a common neutral or if I ran 2 neutrals ? In the case of a MWBC I'm required to use a 2-pole breaker and disconnect power to ALL un-grounded conductors but in the case of 2 separate circuits (of opposing phases) if I install separate neutrals I can use s/p breakers. I just don't see the logic in the Code requirement in my specific instance.

 

[goldstar]

And the word if does matter.

That's the only way we ward off 90% off construction accidents to begin with.

JAP>

If the current gets too high the breaker trips
It's not IF the current gets too high - Its WHEN the current gets too high
However, you can ask WHAT IF the current gets too high and the breaker doesn't trip ........?

If you step on the brake the car better stop
It's not IF you step on the brake - It's WHEN you step on the brake
Again, you can ask WHAT IF I step on the brake and the car doesn't stop.....?

If you mom yells at you to do something you'd better get to it.
WHEN

If the heat melts the glass at the sprinkler head the water better come out.
WHEN

If I flush the toilet the water better go down the drain.
WHEN the toilet is flushed the water WILL go down

I know we're playing with semantics here but I think there are many parts of the NEC that create "what if" scenarios, the possibilities of which could be million to one shots.

 

[infinity]

Sorry, I guess I'm just not thinking out of the box here. I understand your explaination completely if it were installed say for a kitchen with multiple outlets. Let me try one more time. You have a 10' length of conduit with only one 2-gang JB at the end of it and the other end connected at the breaker panel. You can see the breaker panel - you can walk over 10' and shut a breaker off if you wish. What difference would it make at the 2-gang JB if I had a common neutral or if I ran 2 neutrals ? In the case of a MWBC I'm required to use a 2-pole breaker and disconnect power to ALL un-grounded conductors but in the case of 2 separate circuits (of opposing phases) if I install separate neutrals I can use s/p breakers. I just don't see the logic in the Code requirement in my specific instance.

If you have a two wire circuit (hot/neutral) and you open the neutral you have a shock potential from the neutral. The part you're missing is that for a MWBC you are sharing the neutral, if you shut off one of the two circuits and open the neutral you can get shocked because the other circuit on that shared neutral is still energized (the same as the 2-wire circuit in the first sentence). If you have a two wire circuit (hot/neutral) and you shut off the hot the neutral has no shock hazard.

 

[goldstar]

If you have a two wire circuit (hot/neutral) and you open the neutral you have a shock potential from the neutral. The part you're missing is that for a MWBC you are sharing the neutral, if you shut off one of the two circuits and open the neutral you can get shocked because the other circuit on that shared neutral is still energized (the same as the 2-wire circuit in the first sentence). If you have a two wire circuit (hot/neutral) and you shut off the hot the neutral has no shock hazard.

I understand your explanation completely. I'm not asking the CMP to re-write this Code section. Just looking for some clarification.

If you have a two wire circuit (hot/neutral) and you open the neutral you have a shock potential from the neutral.

Please explain how the neutral can become open and where and to whom the shock hazard might be if I have a 10' length of conduit with a receptacle installed at the load end ?

The part you're missing is that for a MWBC you are sharing the neutral, if you shut off one of the two circuits and open the neutral you can get shocked because the other circuit on that shared neutral is still energized (the same as the 2-wire circuit in the first sentence).

Again, please explain how the neutral becomes open. It is wire nutted together with two pig tails to each of 2 receptacles. Where is the shock hazard and to whom ?

If you have a two wire circuit (hot/neutral) and you shut off the hot the neutral has no shock hazard.

Why should this be any different than the precautions taken with a MWBC if you have 2 circuits with separate neutrals in one 2-gang JB ?

Now, if you tell me that this Code section is designed to prevent any stupid HO's or otherwise unqualified personnel from getting shocked - then I'll buy into that explanation and reasoning completely.

 

[jaggedben]

I understand your explanation completely. I'm not asking the CMP to re-write this Code section. Just looking for some clarification.
Please explain how the neutral can become open and where and to whom the shock hazard might be if I have a 10' length of conduit with a receptacle installed at the load end ?

Again, please explain how the neutral becomes open. It is wire nutted together with two pig tails to each of 2 receptacles. Where is the shock hazard and to whom ?
Why should this be any different than the precautions taken with a MWBC if you have 2 circuits with separate neutrals in one 2-gang JB ?

Now, if you tell me that this Code section is designed to prevent any stupid HO's or otherwise unqualified personnel from getting shocked - then I'll buy into that explanation and reasoning completely.

The MWBC handle-tie rule exists because if the neutral is opened at some point upstream of a load that is on a leg that is not turned off, it can shock you or arc flash. This may not be likely if you are working at a receptacle on the tail end of conduit run, but that is not the only place you're ever going to be working on an MWBC. You need to understand the danger that the code rule is trying to address, not keep asking how it relates to an example that may not apply.

There are two different code rules for two different dangers that are not the same thing.

 

[jap]

WHEN the toilet is flushed the water WILL go down

Now I know for sure your living in a fantasy world if you believe there's no question that will happen every time.

Jap>

 

[LarryFine]

Please explain how the neutral can become open and where and to whom the shock hazard might be if I have a 10' length of conduit with a receptacle installed at the load end ?

Again, please explain how the neutral becomes open. It is wire nutted together with two pig tails to each of 2 receptacles. Where is the shock hazard and to whom ?

Why should this be any different than the precautions taken with a MWBC if you have 2 circuits with separate neutrals in one 2-gang JB ?

The code has no way of knowing whether any given installation consists of a conduit run supplying one 2-gang box or a multi-outlet run with many boxes and/or outlets. The concern with a shared neutral is that someone could believe they've de-energized a complete circuit when flipping one breaker.

Suppose you turn off a breaker with the intent of relocating that circuit's neutral to another neutral bus terminal or even another neutral bus. If that neutral is shared by another hot wire with a load, it becomes energized the moment it's loosened from the bus. The same thing could happen in any box.

It's an intentional or even accidental opening of a shared neutral that is of concern, which, again, could happen if someone mistakenly thinks they've de-energized a complete circuit by opening a single breaker. A handle tie is for manual operation, while a common-trip mechanism is for automatic operation.

Yes, an intact neutral theoretically poses no threat, but a neutral pathway could be interrupted without realizing another circuit is still dependent on it. The same concern does not exist when each hot wire has its own corresponding neutral because opening one breaker does indeed de-energize an entire circuit.