Multi-Wire Branch Circuits

[hbendillo]

This is for anyone who is familiar with the NEC requirements for multi-wire branch circuits. The code sections in question is NEC 210.4. The question of using multi-pole breakers or handle ties to disconnect the ungrounded conductors of a shared neutral circuit or using dedicated neutrals is what I would like to discuss. I want to know what engineers are doing in their construction documents. I have not used handle ties or multi-pole circuit breakers. I have chosen to use dedicated neutrals in multi-wire branch circuits. I just don't like the idea of turning off three circuits to work on one device or light fixture connected to the circuit. I don't like handle ties because they can be removed. I don't like multi-pole circuit breakers because they are more expensive.

If anyone is using handle ties or multi-pole circuit breakers, what has been the response of building owners and managers? Is disconnecting three circuits a problem for them? I am looking for solutions to balance cost and design integrity.

 

[jraef]

You do not have free reign on that decision in all cases. Here in California for example, if you have a duplex receptacle under the sink to power your garbage disposal and an instant hot water heater, each must have its own breaker. But because they are on the same receptacle, you must have handle ties on those breakers so that if one breaker trips, a service person is not being exposed to live power on that other circuit in a confined space. This is irregardless of the status of the neutral conductors. So most of the time that is done with a MWBC simply because the hardware will be the same either way but it saves on wiring.

"Will work for (the memory of) salami"

 

[jmbelectrical]

If you're providing dedicated neutrals, technically, I don't believe it's still classified as a multi-wire branch circuit. NEC Article 100 defines it as "A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system."

I generally avoid the use of multi-wire branch circuits. as they can pose hazards in some scenarios (See [URL unfurl="true"]http://ecmweb.com/content/multiwire-branch-circuits-can-be-dangerous[/url]) and can be problematic when serving highly nonlinear loads. When I absolutely must use them, I specify a multi-pole breaker, never handle ties, for the reasons you already mentioned.

If you wish to use multi-wire branch circuits while attaining some sort of a balance between cost and design integrity, why not go with multi-pole breakers and a shared neutral? In the case of a three-pole breaker, and assuming that your load is mostly linear, you'll only have three current-carrying conductors instead of six, so no ampacity derating is necessary. And, of course, there's material and labor savings from having two less conductors in the circuit. What's the motivation behind specifying dedicated neutrals? Concerns about harmonics?

 

[EEJaime]

hbendillo,

I stopped using multi-wire branch circuits a few years ago. The common trip requirement defeats the purpose of using multiple circuits in an area to assure there is some power available where a single circuit has a problem. I, like yourself use multiple individual circuits in the conduit homeruns. I think it provides far greater flexibility and clarity, especially when spaces are remodeled.

There is a cost premium, but the fact that the system is flexible and the elimination of 200% neutrals in certain environments. Along with the flexibility it provides, especially in light of California's new insane Energy code which includes all the controlled receptacles makes the expenditure worthwhile in my opinion. Many clients have agreed with me when it is explained to them.

Regards,
EEJaime