atcomsystems.ca/forum Forums General General Telephone & Systems Information A question for any prior sparkies out there

Well, I've seen more than a few threads of telecom man vs. sparkie man, so I thought I'd throw a question out to reverse the situation a bit.

After 8 years, I'm finally finishing my basement. I've run out of 12/2, but have plenty of 12/3.

I need to run 2 more circuits from the panel.

One of the circuits will only have 1 receptacle on it (for a mini fridge).

The other circuit will have 4 outlets on it.

What I'm wondering is - although code (NEC 201.4) apparently permits it - can I run 12/3 from the panel and use the neutral as common for both circuits (multiwire branch circuit)?

Code says yes, as long as I provide a device (yoke) to trip both breakers if either of them trips. As far as I can tell, this requirement is to protect people working on the circuit later, thinking it's deenergized when it's not.

But I don't understand why something like this would be legal. It seems to me the neutral will be carrying the load of both circuits, and since the neutral doesn't have overcurrent protection, would be subject to overheating. On the other hand, a circuit is a circuit so I suppose the breaker would trip if either circuit were overloaded, regardless of the fact that they share a neutral.

So, I'm torn between having 175' of unused 12/3; probably never to be used again, or going up and buying another 75' of 12/2.

So the question. If it were your house, would you:

1) Go buy some more 12/2.
2) Use a multiwire branch circuit.
3) Use the 12/3 to feed the 2 circuits, but leave the 3rd (red) conductor unused (i.e. make the 2 runs with seperate 12/3 runs).

Any thought/opinions appreciated!

- Matt

It's OK to use the 12/3 and as you stated in the code, they must be controlled by a double breaker so BOTH circuits are shut off together.

Feed it to the first outlet (fridge) using the black or red as the hot, then carry on to feed the other circuit using the other colour as the feed. The unused conductor must be capped at both ends. I always tape the caps as well to make sure the caps stay on.

Thanks Dave!

This will save me time as well as material. This particular run back to the panel is a bit complex, with a couple places where it's gonna be a bit of a pain to fish the wire.

Thanks again -
Matt

But I don't understand why something like this would be legal. It seems to me the neutral will be carrying the load of both circuits...

What is important for you to understand is that the two circuits- the red and black MUST be on different phases in the panel. When that is done and if you have equal loads on both circuits the neutral current will be zero. If the loads are unequal the neutral current will be the difference between the two, so it can never be more than full load on one leg and zero on the other. That's why it's legal.

If you put both legs on the same phase then you will have the condition you describe. You can easily overload the neutral- that is illegal and a fire hazard because with two 20A breakers you could have 40A on the neutral.

As to a two pole breaker, that should put both legs on different phases but dual mini breakers will not. So watch what you are doing. The way to check is to take your meter and make sure you have 220V between your red and black.

You do not have to use a two pole breaker instead of a couple of singles on a multi-wire branch circuit unless you are on the 2008 NEC but it isn't a bad idea. If you split wire receptacles and put both phases on the same yoke then you always have to use a 2 pole common trip breaker.

-Hal

If you look at your panel you will see that you (probably) have two hot legs, usually a black and a red coming in, but only one neutral (white) shared by both, As Hal said, as long as you don't put the two wires on the same phase you won't overload the neutral.

On the last big electrical job I worked on the neutral was overbuilt going into the panel. So a panel that had 2/0 Hot legs might have had a 4/0 neutral. They also spec'd separate neutrals for critical circuits.

Sam

They do that for loads that produce high harmonics such as from switching power supplies in computers and UPS's. The harmonics don't have the simple phase relationship that a clean load does so the harmonic currents can add on the neutral and over load it. That's why you have to oversize the feeder and branch circuit neutrals when you have predominant loads like that.

-Hal

Thanks Hal, Sam!

Fortunately there is one unused dual 20A breaker in the panel. I'll meter it to make sure I'm on different phases. I'm fairly certain it's not a mini breaker, but best to make sure.

Thanks again for the explanation and tips!

- Matt

Hal -

It was a job for the State Government in the old Post Office building next to Ground Zero. Lots of cubicles, lots and lots of personal computers shown on the plans. Harmonics was indeed the reason.


Sam

One last follow up: 226v read across the 2 breaker poles, so looks like I'm good to go. Before this post, there was probably a good chance of me putting both circuits on the same phase (seeing as I didn't really understand how it worked)... so - Hal - thanks for keeping me from burning down my house!!!!!

Next weekend: Sheetrock. Oh boy.

Just watch out for that Chinese stuff!

-Hal