hot common????

Thread Starter


Joined Aug 26, 2006
my boss and i have been having a disagreement, about the common wire in a three wire split phase 240 a.c. circuit. the argument is , first , that the neutral wire and common wire is one in the same.. second, that this wire is only used for return voltage and is never a live wire... he says that is he taks a voltage reading from the wires connecting to the capacitor wich are ( fan , commmon , compressor) he says if he takes a voltage reading from one of the legs and the common wire he gets 240 volts, therfore he believes the common is a hot wire..
can you please explain this to me .. and also explain that the common and neutral wire is the same thing... thanks.......:eek:


Joined Apr 20, 2004

AC common and neutral are equivalent terms. It is not live. It is for the return of current in the circuit.

A voltmeter measures a voltage difference between two points. If you see 240 volts between any phase and neutral, then it follows that the neutral is at ground potential. If it were live, there would be a lesser voltage differnce between it and the voltage leg being metered. If you have an earth ground close by, place the meter leads on it and neutral, and see what voltage shows up.

Joe S.

Joined Aug 28, 2006
please see my comments to this link:
It has some application here.
If the neutral is carrying current then you need a safety ground, example a split phase welder might have a 120v wire feeder attachment, then it requires a safety ground, whereas a compressor may not have an active neutral and likely would not, but if that neutral is carrying load then a seperate safety ground is required. same deal with 3 phase, it's no longer permitted to run 3 wire 3 phase here anymore. Bottom line error in the favor of safety, money is replaceable, lives are not.