# 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
1,214
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
6,970
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.