# Common voltage?

Joined Jul 19, 2007
3
Okay, this is probably really simple but I always wanted to know.
What is the definition of common voltage or the common wire?
How does it vary between DC and AC? I know what it is when
I'm measuring wiring but I would like a real explanation and maybe
some history (common to all components in a circuit?)

Thanks.

#### recca02

Joined Apr 2, 2007
1,212
frm what i know,
it is a common point for termination of loops,
often it is also a reference point for voltages at other points.

#### Dave

Joined Nov 17, 2003
6,969
Typically it means that a point, this could be a lead or connection point, that is "common" to, or in this sense "shared-by", two other elements within a circuit.

Dave

Joined Jul 19, 2007
3
Okay, sorry for the delay here in following up.

So, you can have common going to ground or common voltage (live).
For instance, in a motor each of the coils is connected to a common
point so is the voltage common to each coil as they go to ground,
and if reversed (as some motors do?) the voltage being applied to
each coil and going to a common ground. Or am I confused between
a DC motor (reversible) and a three phase motor?

Or am I off base here? I know it's a term used so much, but for me
it's like the term 'irony' in that everyone knows what it is but only a
few can explain it.

Dave and recca02, your explanations are clear but I am just looking for examples of
application in electrical systems so that I have it right in my head.

#### techroomt

Joined May 19, 2004
198
i like to use the analogy of a (negative ground) car voltage system. all of the circuits are built such that a load will be supplied voltage with respect to a common point, which here happens to be connected ultimately to the car frame and cathode of the battery. so this point can be called common. however, that point, if measured to earth ground is open, or floating. but yet it provides the return path point for all of the load loops. many dc circuits have a common point that is not tied to earth ground.

however few, if any, ac circuits that i know of utilize a floating neutral, as neutral is tied to the earth ground at the main entrance panel.

motor reversal schemes vary. a 3 phase ac motor can be reversed by interchanging any of the 2 phase leads. not all dc motors can be reversed.