Was having a discussion with a buddy about wiring using a shared neutral. I don’t like to do it for a couple of reasons. First – if someone switches the phases on the red or black wire the neutral can be undersized for the load. He was not really concerned about this but I always worry about someone in the future not knowing what was done with this wiring scheme and moving breakers around.
The second reason has something to do with the neutral being lost or damaged and the two hot legs now acting a 240volt source. I wasn’t sure how to explain this and was wondering if this is really a concern. I can’t remember where I read this and couldn’t find anything in a search.
Any ideas on using a shared neutral. I know it saves a little wire but really not that much.
Thanks.
Replies
For Romex,it seems the price for 14-3 is almost twice as much as 12-2. I don't know why since the copper goes from 3 wires to 4 wires. So I don't think there's much savings there.
But in commercial work, where we often use three phase, it becomes a matter of three circuits [or hots] sharing 1 neutral versus using three white wires. Thus a savings of two wires. And it also helps with wire derating and conduit fill.
For the lost neutral scenario, it depends on the resistances of each component of each operating device on each f the branch circuits involved.
~Peter
One argument in favor of the shared neutral is that, for the shared length of the circuit, worst-case voltage drop is half that for individual circuits. This is because the neutral carries no current (at full load) and therefore produces no voltage drop.
It can be done but you have to make sure that the hot leads are attached to opposite hot bus bars. That way if both circuits are used at the same time, the return load on the neutral is zero, If used separately the return on the neutral would be 110v. If you put both hot leads on the same hot bus and used the two circuits at the same time your return on the neutral would be 220v. Usually the spaces for the circuit breakers are stagered so that opposites run directly underneath each other on the panel. Side to side placement will give you the same hot bus. Be carefull.
As for the 240 volts ... What's the voltage between a hot wire and the neutral? 120. The voltage between two hot wires? 240.
Now, assume the neutral is lost. Also, assume one hot wire powers a high resistance appliance, such as a space heater. Assume the other hot wire powers a low resistance item, like a plasma TV. If both are turned "on,' you have a series resistance circuit.
Look at it this way. The normal electrical path is hot >>> load >>> neutral.
Instead, you would have hot >>>> load >>>> load >>>> hot.
Ohm's law will show a larger voltage drop across the TV set. That is, the low resistance load will be exposed to 180, 200, even 220 volts, while the heater is exposed to the remaining low voltage. The cheap heater survives, and the expensive TV dies.
That's a high price for a loose wire nut.
I have two reasons for avoiding 'shared neutrals' in homes.
The first is that Romex, by it's construction, makes it easiest to have a separate neutral, and ground, for each circuit. All are wrapped up in that nice jacket. The cable leaves the panel and goes to the load, without any extra junction boxes.
The second is that a home is far more likely to have the electrical worked on by less qualified persons. I want to reduce their chances to screw things up.