MWBC Question

[dicklaxt]

210.4 covers it and its pretty clear what has to be done.My question is,what condition exists that would determine when a MWBC (2 pole breaker) would be used and for what reason? Here's another thought if a MWBC were in use does it have to have two associated neutrals?

dick

 

[petersonra]

as far as I know it is only about eliminating one or more wires. you don't have to count the single N as a CCC either, which might matter now and then.

 

[480sparky]

Not sure what you mean by your first question, other than it's done because 1. It's legal and 2. it's cheaper in some cases.

As for 'two associated neutrals', if there's two hots and two neutrals, it's not an MWBC. It's just two circuits.

 

[jap]

210.4 covers it and its pretty clear what has to be done.My question is,what condition exists that would determine when a MWBC (2 pole breaker) would be used and for what reason? Here's another thought if a MWBC were in use does it have to have two associated neutrals?

dick

As far as I know you dont ever have to use a MWBC if you dont want to.
The purpose of disconnecting both circuit with a handle tie or a 2p breaker is because 2 circuits share a common neutral.If you have 2 circuits with 2 associated neutrals then I dont believe you have a MWBC but 2 seperate circuits with thier own neutral.

This is just me talking off the cuff.

 

[Smart $]

as far as I know it is only about eliminating one or more wires. you don't have to count the single N as a CCC either, which might matter now and then.

But you do have to count it if it's a 3-wire MWBC on a 3? 4W wye system.

 

[Smart $]

...My question is,what condition exists that would determine when a MWBC (2 pole breaker) would be used and for what reason?

Less wire, less I?R losses, and subsequently less cost.

 

[dicklaxt]

But you do have to count it if it's a 3-wire MWBC on a 3? 4W wye system.

That just opens more doors,then in the lighting mode that would be a common neutral MWBC with a 3 pole breaker now we are saving the cost of 2 neutrals but adding in the cost of a 3 pole breaker,,,,,,,,where the economics fall,I don't know which is cheaper?

dick

 

[dicklaxt]

Less wire, less I?R losses, and subsequently less cost.

I'm having a hard time thinking this money saving approach would be something the code writers would consider,got to be another reason for it's use.

dick

 

[jumper]

That just opens more doors,then in the lighting mode that would be a common neutral MWBC with a 3 pole breaker now we are saving the cost of 2 neutrals but adding in the cost of a 3 pole breaker,,,,,,,,where the economics fall,I don't know which is cheaper?

dick

Dick, MWBCs do not have to have common trip breakers, 2P or 3P, a listed handle on SP breakers for the circuit is legal.

 

[jumper]

I'm having a hard time thinking this money saving approach would be something the code writers would consider,got to be another reason for it's use.

dick

Are you asking why the section was revised requiring simultaneous disconnect of the the MWBC?

 

[dicklaxt]

Dick, MWBCs do not have to have common trip breakers, 2P or 3P, a listed handle on SP breakers for the circuit is legal.

True enough but do they make a handle for 3 single pole breakers used for a 3 phase 4 wire distribution with loads connected at line to neutral?

hmmmmm

dick

 

[jumper]

True enough but do they make a handle for 3 single pole breakers used for a 3 phase 4 wire distribution with loads connected at line to neutral?

hmmmmm

dick

They make them for certain panels, but not all, especially older older ones-you gotta use a 3P breaker.

 

[Smart $]

I'm having a hard time thinking this money saving approach would be something the code writers would consider,got to be another reason for it's use.

dick

I think your looking at it backwards. MWBC are an inherent wiring method for any 1? 3W or 3? 4W wye system. All Code writers can do is implement safety restrictions on usage.

 

[dicklaxt]

Aha thats where the kicker lies I don't reason it that way as 210.4(c) says it shall only supply line to neutral loads,,, so a dryer range etc couldn't be a MWBC so thats back to the question when is it used? I'm not trying to be contrary or the Devils Advocate...I'm not finding an answer.

dick

 

[Dennis Alwon]

IMO, the basic reason to use a mwbc is the use of one neutral instead of 3 neutrals and the lower count fill in the conduit. Multiply that by hundreds of circuits in builds and the cost savings is very high.

Three pole handle ties are available for all manufacturers products produced today- afaik

 

[kwired]

Aha thats where the kicker lies I don't reason it that way as 210.4(c) says it shall only supply line to neutral loads,,, so a dryer range etc couldn't be a MWBC so thats back to the question when is it used? I'm not trying to be contrary or the Devils Advocate...I'm not finding an answer.

dick

Read the exceptions to 210.4(C).

The second exception basically allows any combination of line to line or line to neutral load to be supplied if there is simultaneous operation of all poles.

 

[jap]

I'm having a hard time thinking this money saving approach would be something the code writers would consider,got to be another reason for it's use.

dick

They're not telling you to use it, they know its allowed, and a common practice to use,so they are addressing the safety issue of it.
The use of the Handle Ties or a common trip breaker requirement on a MWBC is to disconnect all of the circuits sharing the neutral so that if you open the neutral you dont catch the load.

 

[dicklaxt]

Read the exceptions to 210.4(C).

The second exception basically allows any combination of line to line or line to neutral load to be supplied if there is simultaneous operation of all poles.

Okay thats the qualifying answer,,,,,,,,now we can erase this post ,I'm satisfied.

I still think the authors must have been looking for something new to dwell on that day of origin.:thumbsup:

dick

 

[Smart $]

Aha thats where the kicker lies I don't reason it that way as 210.4(c) says it shall only supply line to neutral loads,,, so a dryer range etc couldn't be a MWBC so thats back to the question when is it used? I'm not trying to be contrary or the Devils Advocate...I'm not finding an answer.

dick

Actually a dryer or range circuit can be...

(C) Line-to-Neutral Loads. Multiwire branch circuitsshall supply only line-to-neutral loads.


Exception No. 1: A multiwire branch circuit that supplies
only one utilization equipment.

Exception No. 2: Where all ungrounded conductors of the
multiwire branch circuit are opened simultaneously by the
branch-circuit overcurrent device.

...but I don't see any advantage to calling a dryer or range circuit a MWBC.

In my experience, mwbc wiring was taken advantage of as much as possible. Generally, there had to be a compelling reason (whether real or theoretical :huh to not run mwbcs. I haven't been involved in that part of the industry since before the simultaneous disconnect rule was implemented, so I don't know what the norm is now.

 

[dicklaxt]

Me either and when I was looking it was on the other side of the fence..It's fun to learn tho.

dick