ECN Forum Forums General Topics of the Trade General Discussion Area multiwire circuit

The question may have originated with 225.7 which permits a similar principal for outdoor lighting equipment. However, as you describe it above, this is not a multiwire ckt. I, personally, would never do this. If for no other reason than it is not in accordance with conventional wiring practices. And that is a good enough reason for me.



[This message has been edited by Redsy (edited 01-14-2003).]

This Q has been run in both this BB and the MHE forum in the past.
The way it was originally asked by jdevlin,
it's perfectly legal. (without handle ties).
If it were in fact a multiwire circuit, with say 4 #14 hots (2 per Ø), and a #10 grounded conductor, it's perfectly legal. (the handle ties would only be required in a dwelling where the multiwire circuit supplies devices on one yoke, or one piece of equipment. Commercial, no handle ties if the loads are all line to grounded conductor.

The loss of the grounded conductor on a multiwire circuit, although it can lead to elevated voltages, will not put 240 volts to any single device unless the impedance on the opposing leg is 0 ohms.(dead line-noodle short). It is a series circuit once the grounded conductor connection is removed, with the loads on both legs in series.

This sketch might help explain what electure is saying.

The over-voltage resulting from an open neutral on a multi-wire circuit will depend on how well the load is balanced.

The sketch shows what I would consider an extreme case, and it definitely would cause damage.

In circuit A the voltages across the two loads are approximately equal because of the intact neutral conductor.(The line voltage drops are not calculated here).

With the neutral open (circuit B) the 10 ohm and 30 ohm loads are in series for a total of 40 ohms. There is a damaging overvoltage on the 30 ohm load, due to the unbalanced load. If the load were perfectly balanced, there would be no voltage difference.



Ed


[This message has been edited by electric-ed (edited 01-14-2003).]

I've always been taught that you can't use a common neutral for multiple circuits unless it is a multiwire circuit, however I can't find a code rule that says this. I have found rules that, in specific cases, permit a common neutral, but not rules to prohibit one. The rules that permit common neutrals are, 215.4(A)and 225.7(B).
Don

It seems to me the question is not about a multiwire circuit. Are you asking about running 2 #14 "hots" off one leg (1 single pole breaker) and only 1 #10 grounded conductor? If so, yes it would work although like you said, I cant think of any reason why to do it.

electure...ok maybe 240 volts was a little high.but i had a service call last year and if i remember right i had 190 to 200 volts on the receptacle.more than enough to burn out there appliances,etc.after checking the circuit i found and overheated neutral on the neutral bar.(loose connection.)like i said i don't like this practice.but that's just me.

You are correct it is not really a multi wire circuit. That was the best I could come with for a short topic. It is LIKE a multiwire with 2 breakers only both are on the same phase. This would mean that it is possible to have 30 amps on the neutral. Since #10 is rated 30 amps then would it be ok to use as a common neutral. No one could come with a code section that it would violate.
It was regarding a regular residential service. No 3 phase. Once again I would not attempt this is was just a discusion that came up.
I never thought this would cause so much discusion.

Thanks for all the comments.

Thank you, Ed.
I'm sure that made my raving and ranting much easier to understand!

I didn't mean to get carried away with the bolds, no offense meant to anyone.
I've seen the same "lost neutral" comment made 3 or 4 times in as many weeks on another electrician BB, and it's a shame...S

I believe that "multiwire circuits" are an exception to the rule of 1 hot = 1 neutral. I also am under the impression that the only place that different neutrals are allowed to come together is at the panels' neutral bus.
Even if I am wrong about the language of the code, I would still avoid this situation because if I turned off one of the feeds, and disconnected the neutral, I would have a "hot" neutral were any load on the other circuit be 'on.' That, in turn, would make it kind of hard to truly 'lock out' anyhting on the circuit being worked on.
So how do you do this legally? Install a sub-panel.
Also note that some panels -zinsco comes to mind- have "double" breakers draw from different legs of the panel.

John,
Belief #1 is correct. The impression, #2 is not.
Multiwire circuits are used extensively.
Scenario Subpanel. The subpanel in itself is a fed by a Multiwire circuit (the feeder).