Multi Wire circuits

[dsteinhorn]

I am wiring a shop with a new subpanel from which I am running several 20 amp outlets on two new circuits to machines that will not be operated at the same time, since I am the only one working in the shop.

Can someone explain how it works electrically and phase-wise to use one hot wire from the A and B sides of the panel for each of the two new circuits but only one white/common wire for both of the circuits? Is it because the A and B sides are 180-degrees out of phase, so their return current will travel in separate phases through the single neutral wire? In the past, I have always used a dedicated white/common wire for each circuit, i.e. four wires plus one ground for two circuits, which is easy to do with romex, but I am pulling cable in conduit for this job. Given conduit fill guides, I was considering two multi-wire circuits to reduce the number of current carrying conductors and save a little money on copper.

Thanks for your clarification of this issue.

 

[PJB12]

You are exactly correct, Dseinhorn!
When Phase A is moving "forward" and pushing electrons down the grounded circuit conductor (neutral), Phase B is moving in "reverse", moving electrons away from the neutral conductor. The neutral carries only the unbalance.

With Single Phase:
If you have a load of 10 amps on Phase A and a load of 10 amps on Phase B, the load on the neutral will be zero.
If you have a load of 10 amps on Phase A and a load of 12 amps on Phase B, the load on the neutral will be 2 amps.
The 2 amps is the "unbalance".

That is why we can wire, for example, a 400 amp service with a neutral sized for 400 amps, not 800 amps. (Ignoring upsizing due to harmonics.)

With Three Phase:
You have Phase A, Phase B and Phase C. They are 120 degrees apart.
The formula for the unbalance is In(A) = A2 + B2 + C2 – A*B – A*C – B*C
In(A) = Neutral current in amps.

This answer is very simplified. Power factor is ignored. (P = √3 Vl × Il × cos φ where cos φ = power factor is a start), instantaneous current, lead and lag, plus more are also ignored.

Code Reference:
When you install the circuit with the shared neutral, NFPA-70 requires a handle tie be placed across the two circuit breakers so that when someone turns one off to work, both sides are dead. If not, one could remove a device from Phase A and be energized by the neutral because something on Phase B is in use. It also applies to split duplex receptacle outlets.
(You don't have to use a two-pole circuit breaker, Just the tie is sufficient.)

Happy Wiring!
Paul

 

[dsteinhorn]

Paul,

Fantastic simplified answer but I appreciate the formal formula explanation.

From a practical point of view in a home wired system, is there any reason not to run a dedicated neutral for each circuit, if I have room in the conduit for it? That could avoid confusion in the future and make the subpanel cleaner without the handle tie, which might look like a 220 v double breaker.

Thanks for giving a comprehensive response.

 

[PJB12]

From a practical point of view in a home wired system, is there any reason not to run a dedicated neutral for each circuit, if I have room in the conduit for it?

No, not at all!

My house is loaded with EMT and IMC. (I don't play well with NM cable)
For most receptacle outlet circuits I put individual neutrals. But this was when #12 thhn & thwn were about 1-1/2 cents a foot. Today, I'd be more inclined to share. (I have a few shared on purpose.)

At work, I share neutrals as a rule. On three phase circuits, this saves the fill of 2 conductors, which may prevent using larger pipe and boxes. The downside is that later a semi-trained maintenance person might move a breaker to add something and have it end up on the same phase as the other half of the circuit. I've had service calls for burned neutrals often.


A good habit is to wrap one layer of electrical tape to bind the line side conductor, the neutral and (if possible) the ground wire inside the load center, any junction boxes along the way and in the device box. This will make "who's who?" easier later, especially should the load center gutters become crowded.

Ideally, instead of the tape, a label with identifying words or code may come in handy.

Three Hints:
Watch your conduit fill. From Post #1, you are being wise and already watching.
But should someone else view this thread; See NFPA-70 Table 1 Chapter 9. (It's probably on line somewhere. There might even be an online calculator for conduit fill.)

If the conductors pass through a junction box and are not spliced in the box, put a loop around the box perimeter instead of going straight through. That loop might come in handy one day when you want to add a circuit.

Monitor your box fill calculations. NPFA-70 Article 314.16 et. al.

Paul

 

[dsteinhorn]

I like the loop idea for the conductors traveling through a junction box. I will use electric tape of different colors to designate the various circuits, since I am not using red/blue/black separate hot wires to designate them.

Thanks again. I sent your previous reply to my MIT educated astrophysics son with the caveat for him to always kill the power with the disconnect to the main panel, if he ever works in there on his own. In the end, electricity, plumbing, and cardiovascular physiology are all similar but each one has its own nuances and risks that one needs to keep in mind. CV physiology is much easier.

 

[PJB12]

That's a good warning to your son about turning off the main. If the main is in the panel where he is working, it must be remembered that the wires coming out of the main circuit breaker are still live. (Assuming he has the ubiquitous back-fed breaker instead of bus lugs)

Your color tape plan is good. Craft store tape works well and can be 1/4" wide if desired. Electrical color tape often peels itself after time- especially blue and purple.

If it is easier for you, there are numbered wire tags available. You can then make a chart of which number is which circuit. I use the ones on the roll shown below. I hang it on the tool belt (yeah, I'm old school) and on a load center or panelboard screw for handy access.

 

[dsteinhorn]

That's a good warning to your son about turning off the main. If the main is in the panel where he is working, it must be remembered that the wires coming out of the main circuit breaker are still live. (Assuming he has the ubiquitous back-fed breaker instead of bus lugs)

Your color tape plan is good. Craft store tape works well and can be 1/4" wide if desired. Electrical color tape often peels itself after time- especially blue and purple.

If it is easier for you, there are numbered wire tags available. You can then make a chart of which number is which circuit. I use the ones on the roll shown below. I hang it on the tool belt (yeah, I'm old school) and on a load center or panelboard screw for handy access.

The new unanticipated issue is solar power coming into the bus from panels which needs to be disconnected or the bus will still be hot even if the main disconnect is cut. I learned that when we put solar in a few years ago.