ECN Forum Forums Code Related Discussion NEC & other Code issues Multiwire circuit neutrals

I never understood the logic of trying to take this potentially dangerous shortcut. Code or no code, the neutral needs to handle the maximum potential unbalanced current. Period. In the case of two 20A circuits, that's 40A, in which case the neutral needs to be #8 and that's not what you get with 12/4.

Think about it... it's your license. And if something goes wrong, guess who the lawyers point the finger at? And they're not gonna want to hear you've done it that way for years and there never was a problem. It's the potenital they'll nail you on. And if you say there's no possibility of that happening, you'll then be labeled as incompetent.

What is so difficult about running dedicated neutrals, or a correctly sized neutral in the case of conduit? I understand copper is getting expensive, but since when is expense an excuse for shortcuts?

Run a dedicated neutral and this way you'll have nothing to worry about.

Joe

"I never understood the logic of trying to take this potentially dangerous shortcut. Code or no code, the neutral needs to handle the maximum potential unbalanced current. Period. In the case of two 20A circuits, that's 40A, in which case the neutral needs to be #8 and that's not what you get with 12/4."

If your (2)20A circuits are on opposite phases you would never have more than 20A on the neutral. The neutral will only see the difference between the (2) loads, not the combined total. Circuit A= 15A + circuit B=10A the neutral would carry 5A. Nothing dangerous about it.

It depends on the job.

The types of jobs I work at generally have homeruns in the hundreds of feet and many times 100s of circuits.

Using correctly wired multiwire branch circuits does the following for me.

1)Use less matrial, copper, cables and EMT.

2)Reduce the number of current carrying conductors in a raceway.

3)Significantly reduce voltage drop.

4)Reduce the number of terminations.

There is also plenty of office furniture (office cubes) that come pre-wired with multiwire branch circuits. The only way to code compliantly supply an existing multiwire branch circuit is with a multiwire branch circuit.

It is 'my license' and if I can not safely wire a multiwire branch circuit I have no business having a license.

Bob

For a visual explanation of a single phase Multi-Wire Branch Circuit see the follwing graphics.

A Neutral carrying the current of an imbalanced Multi-Wire Branch Circuit.



A true Neutral



Additive current from one leg



Roger

Of course there's no problem if they're on opposite phases. Question is, will it remain intact?

For example, NYC code requires #12 to be used on 15A lighting circuits. Does that make sense, when #14 is fine? The only reason I can see would be the possibility of Harry Homeowner or Harry Handyman (who it should be noted can't do any electrical work in NYC) by the substituting a 20A breaker into the lighting circuit, or a wiring change where part of the lighting circuit gets on a 20A feed.

I don't know if this is the actual reason, just my educated guess. It just doesn't make sense, if #14 can carry 15A, why is #12 mandated? (And it's not like #10 is mandated on 20A, 20A #12 is okay in NYC -- length of run aside.)

Same principle can be applied to multi-wire with shared neutral. Panel is tight on breakers, and the home owner decides to finish the basement, either himself or with a GC who does "everything". Hmmm, let me put some tandem breakers in and move this breaker (the one for the multi-wire circuit) over here (onto the other phase). Now if that multi-wire circuit is overloaded, it's possible the fire department will be stopping by.

Also, with a shared neutral, you now have 240V in a box somewhere. So let's say someone piggybacks a new 120V circuit on the two hot legs, then plugs something in this new circuit. I don't think those books at the orange box mention anything about multi-wire circuits.

I guess I should've clarified the screw-up factor. I know the folks here know all about multi-wire requirements, and would do it correctly. But how often does your original nice, neat work stay intact over the life of the installation? Something always seems to get hacked up.

Now I'm not one of those folks that believes we need to protect folks from themselves, but not sharing neutrals sure can eliminate a lot of potential future problems.

In view of the foregoing, I still say ideally it's best to run a dedicated neutral.

Just my 2 cents worth...

Joe

I like using multi-wire branch circuits because it can save time and money during the installation. The fella's on this board sold me on the idea a long time ago. Once you understand about the unbalanced load on a nuetral on a 3-wire circuit -- including proper installation of the OCPD's -- IMO you are good to go.

In my view it is entirely depends on the job a hand.

I am willing to bet you would look at this differently if your job was pipe and wire with multiple homeruns that are a couple hundred feet long.

All these big box stores springing up do not have panels in every aisle.

You are adding significant costs to a job like this using dedicated neutrals.

JMO, Bob

Although I have no intentions of doing this, when this subject came up before I posted this, taking Bob's Handbook chart a level further.





Can anyone come up with a Code reference to "shoot it down"?

Okay, Bob you got me, LOL! It would be ridiculous to use dedicated neutrals in pipe; one properly sized neutral would be the way to go. What a PITA it would be trying to track down which neutral goes to where.

Electure, that second illustration from Bob's Handbook with the incremental increase in neutral sizing works for me. The wire is properly sized for any potential unbalanced current throughout -- though you might have trouble selling that to a "difficult"
inspector, who might insist on #4 straight through.

Of course, there's still the GFCI and finicky computer equipment issues with shared neutrals -- which is one of the reasons I prefer dedicated neutrals -- but that's another issue, and I don't want to go back and beat that dead horse.

I gotta say, you guys ARE good!

Joe

Scott, no code referance to shoot it down, but the T&M factor could very well work against you.....