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Subject - Sharing a Neutral
WK Jones I was looking to buy a lot of dimmers for my new house now under construction. I noticed a caution in the dimmer instructions: Sharing a neutral in the circuit with another phase may cause flickering. The Electrical Code is all about safety, and this sharing doesn't seem like a safety issue. However, is it a violation of the electrical code to do this or is it just poor electrical practice? I'm not a licensed electrician, but know most of the basics by doing basic wiring or helping out and watching others work. My guess is that this sharing probably could happen when the electrician ties all the neutrals together in a large gang switch outlet either accidentally or to save time and wire usage. An example might be when you have duplex outlets on a switch and overhead lights on a switch with both of their wiring ending in the same switch gang box, even though from different circuits. Instead of me guessing, my questions are: What are these dimmer instructions saying, what is the right way to wire to avoid this problem, is there a way to test for this potential error, and is the sharing of neutrals from different phases against the electrical code? The wiring in my new home is done and pulled to the panels, but won't be terminated at either end for a while--"trim out" is a ways away. It seems to me that if a circuit is a "circuit" that there should not be sharing even with neutrals on the same phase until they all meet in the panel.
Would really appreciate some advice. I am fussy about things like this; it is my new home.
Thank you.
Scott Vickrey Sharing a lights neutral with any high starting current load will cause the lights to flicker when the mentioned load is started.
You can see this in action by going to a almost any home's room, turn on the lights plug in a motor load like a vacuum cleaner and turn it on. See the lights blink. This may be slightly more noticeable with a dimmer switch installed. As for the sharing of the neutrals I'm not sure if you can do this in a residence. Someone else will have to answer that.
Dave Nix Scott has given you good advice about the flickering. There is no sure fire way to avoid it. Most of the time when the flickering occurs, we don't notice it because we are not paying attention and watching for it. Try what Scott suggested where you live now.

As far as the "sharing" of a "neutral" conductor, it is a common practice everywhere and nothing inherently wrong with it. It is not a code violation for the circuits you have mentioned.

Most experienced electricians will not share the neutral between lighting and receptacles. They will usually have two lighting circuits share the same neutral.

In my opinion, if there is another brand of dimmer that does not recommend the sharing, you may want to go with it!
WK Jones Thanks for your help. I assume when you say sharing of neutrals is an accepted practice, particularly in lighting circuits, you were assuming that all of the neutral sharing would be of the same phase leg from the panel, but not sharing a neutral with the other phase in a typical two phase residence.
iwire
quote:
Originally posted by WK Jones

I assume when you say sharing of neutrals is an accepted practice, particularly in lighting circuits, you were assuming that all of the neutral sharing would be of the same phase leg from the panel, but not sharing a neutral with the other phase in a typical two phase residence.


OK there are a couple of ways to share a neutral.

One way is in what the NEC calls a Multiwire Branch Circuit

Article 100
quote:
Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.


In a Multiwire branch circuit the ungrounded conductors (the hots) must be from separate phases.

In a single phase system that would mean A, B, N.

In a 3 Phase system that would mean A, B, C, N.

Hooked up like this the neutral will only carry the imbalance of the hots.

If Phase A & B both have 10 amps of load there is no imbalance and the neutral carries 0 amps.

If Phase A has 5 amps of load and phase B has 15 amps of load the imbalance is 10 amps and that is what the neutral will carry.

That is the accepted and code compliant way to share a neutral.

Now the other way to share a neutral

A much less common and IMO not code compliant way to share a neutral would be to have the hots on the same phase.

Done this way the current on the neutral adds up, if you had 2 20 amp circuits each with 20 amps of load on them the shared neutral would carry 40 amps and unless sized for this much load will be overloaded.

Also this second way IMO is a violation of 240.8 Fuses or Circuit Breakers in Parallel.
David Hyatt I wire is correct about sharing the neutrals. It is common to run a three wire romex two circuits and one neutral. In a new house this is probably not as common. Wiring a new house not much need in doing this. Your dimmers should be fine. I have never read the instructions on a dimmer, but the only way I would see this being a problem as mentioned above is if there is a heavy load on the lighting circuit. The lights will flicker weather or not there is a dimmer or not. It will just be more noticeable with a dimmer. I suggest putting that out of your mind its nothing to worry about. You building a new house is stressful enough to worry about this problem. Good luck.