Multi-wire Circuits

Mark Twenhafel

Hi --

I'm new to the trade, have begun reading the 2008 NEC (my boss bought it and made me pay for it), and I'll be having questions as I read it. I hope I can just post my questions here. Thanks.

First question:
210.4 is titled "Multiwire Branch Circuits". Does "multiwire" in this context refer to a multiple phase circuit? I.e., multiple hots with different phases and/or voltages? I'm assuming so based on 210.4 (B), which requires that "all ungrounded conductors" of the circuit be provided with a means to simultaneously disconnect all the ungrounded conductors of the circuit at the point where it originates.

Thanks. Some background, I'm doing simple residential, romex 200 to 600 amp service in 2000-10,000 sq ft new homes, stick frame construction.

Mark

Northernboy

A multiwire branch circuit is 2 hots and a neutral. Both hots are on the same phase. Single phase 240 is brought to the dwelling with a neutral tapped off in the middle of the 240 volt windings, making each hot 120 volts to the neutral and 240 between hots. Multiwire branch circuits are just used to "save" a neutral. You will normally run 1 12-3 w/g romex instead of 2 12-2 w/g. Both hots share the 1 neutral so the cost of wire is reduced.

jproffer

Not to be argumentative, but that's absolutely...180°...100%...wrong.

The hots MUST be on opposite phases and have means to be simultaneously disconnected. This way the nuetral only carries the difference between the loads. If both hots were on the same phase, it would carry the sum of the loads and (possibly...maybe even likely) be overloaded.

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Jim P.

GSE

Sounds like, Northernboy has a lot of jobs to go back to and make right.

thom

Actually, northernboy is correct, his description is accurate. Homes only have one phase (generally). What you guys are referring to is legs, not phases. The common would be for two hots on opposite legs (but the same phase).

However, a mulitwire branch circuit could consist of 2 or 3 hot wires which are on different phases with a single common. This would obviously be where 3 phase power is available, making it commercial or industrial.

Jprofer and gse, you guys might want to go back to school, and jprofer, don't be so adamant about things that you don't know.

jproffer

Touche'...but don't be so adamant about what I know or don't know.

A difference in terminology is a far cry from not knowing what I'm talking about.

Around here we call the "legs" phases...Phase A and Phase B. Yes it's understood that the "phase" is across the hots, and that residential power (around here...better qualify what I'm saying this time) is single phase.

Therefore...when he said "on the same phase" I (applying my own area's terminology) read that as wrong. Didn't mean to ruffle feathers.

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Jim P.

thom

well then, J, you didn't read his post. It specifically states that single phase is brought into the house.

Maybe you use sloppy inexact terminology, that doesn't make it correct. Just because you call two single phase legs phases doesn't make them phases.

jproffer

I do.

I'm sorry sir. From now on before I post anything, I'll run it by you for approval to make sure it's terminologically correct.

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Jim P.

GSE

Not to worry, the one that needs the schooling is thom.

http://www.inspect-ny.com/electric/multiwir.htm

There are plenty of guys out there that don't know what they are doing.

jbfan

Single phase 240volt is still out of phase on each leg!!!

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We are the people (our parents warned us about) (Jimmy Buffett)

Celtic

How about we skip the leg/phase angle and see what the NEC actually says:
No mention of phases, legs or arms

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thom

Decades ago in a class I took on electrical systems our instructor cited a single two phase system (it was a utility in a small town, I believe in NJ). He gave us a very brief description then told us to ignore it because we would never deal with a 2 phase system.

The problem with referring to legs as phases is the inexact terminology can in fact create a communications problem. What you intend to say may not be what I understand it to be.

Yes, I understand that two legs of a single phase system are out of phase, but that's using the word phase in two different contexts.

My point was, the original post had it correct. His explanation was correct and he correctly answered the question asked. When J jumped in, he added confusion thus making the correct not as clear as it was. This correct answer was, the point is to save a wire or save money. Of course sometimes the greater savings in in the reduced conduit size, but that's another argument.

mickeyco

Exactly how may degrees out of phase are the 2 hot legs?

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Some people climb mountains. I take out the trash. But we both do it for the same reason.

Andy in ATL

WTF !!

Did any one read that link???

Northernboy

This picture is for Jproffer and GSE

Attached Thumbnails

 

mahlere

What about in a 3PH situation? Still on the same phase?

Northernboy

I have never seen or heard of a 3 phase multiwire branch circuit. How exactly would you draw something like that out. Are you making it up as you go along?

mahlere

what are you talking about?

i have a commercial service, 3 phases (A, B, C) in a conduit I pull a hot from each of the phases and the neutral. Viola....a MWBC from a 3 PH service...

how long you been doing residential electrical work?

GSE

The real question should be, how long has he been pretending to be an electrician.

220/221

Hold on a minute.


So.....technically speaking, a single phase service has only ONE phase and a three phase service has 3 phases?

So you cant have a phase to phase short on single phase?


Aw crap! Way to confuse things for me.


I think I will still refer to the legs as phases for simplicity....unles I get into a technical issue.