# How to measure voltage AC using multi-meter

In the process of looking at why my doorbell is not working, I used a multi-meter to check Voltage AC. (I am very basic at electricals though).

How would you check the voltage AC on a wire? Where does the black and red leads have to go. I am told that the black lead needs to go to your earth cable. Is this so? If you don't have access to earth, what do you do? Can black go to a large metal object?

Thank you.

• one lead per wire works just fine, you can probe the diff between them all if you have more than a pair Sep 18, 2018 at 21:11

First, make sure you multimeter is set to measuring AC voltage. If your meter has multiple AC voltage range (e.g. 1000, 100V, 10V), use the largest one.

Make sure the black lead goes into the "COM" jack and the red lead into the the jack with a letter "V" (or something like "VΩ", but not "A" or "mA").

When measuring, remember voltage is always measured between the black wire and the red wire. So in your earlier question, you could measure something like "the voltage between (1) and (2)" by touching (1) with red and touching (2) with black. It does not make sense to say "the voltage of (1)" without mentioning a reference.

Of course you can measure the voltage between a point in a circuit and the ground, by sticking the black wire into the ground wire ("earth cable"). But if the circuit itself is not grounded, or the "earth cable" is not actually connected to the ground, that voltage doesn't mean anything.

You can't measure voltage on a wire.

Current travels in loops. Measuring voltage on one wire is like measuring a battery by touching only one end. You know why that can't work, right? Same applies to all power everywhere.

Now some people (electronics geeks where Vss=GND; mains electrical guys who are accustomed to a master EGC/GES) tend to think of all voltage as relative to some absolute point because they've spent too much time in systems that were designed that way and they've fallen into bad habits. You have to measure 2 points.

And your meter has to be correctly set AC or DC for what you are sensing. Because both are present at once (usually, one is 0 volts) and you only care about the one that matters.