Since we’re going to be measuring live voltage, observe the safety rules from part I of this series:
Use only one hand to hold the plastic handles of the meter leads, put your other hand in your back pocket so you don’t lean it on anything conductive
Be sure you don’t touch the metal tip portion of either meter lead
Don’t stand or kneel on wet ground while testing voltages. For most situations, dry sneakers will insulate you from the earth sufficiently, and if you’re doing this test on a dry stage then the wooden floor or carpet will protect you if something goes wrong.
But if you’re going to measure voltage at a waterlogged festival generator I suggest standing on a dry rubber shower mat or dry plywood so your feet are insulated from the ground. It’s cheap insurance.
Hot to Neutral
With nothing plugged in to the wall outlet, switch on the 20-amp circuit breaker at the power panel, set your meter to the 200 or 750 V AC setting and using one hand insert your meter leads into the left and right neutral and hot slots.
Remember not to rest your opposite hand on the metal box, as that can cause a shock through your heart if something goes wrong. That’s why electricians traditionally stick their unused hand in a back pocket.
It really doesn’t matter which side of the outlet gets the red or black meter lead since it’s alternating current (AC).
Measuring hot to neutral.
Since the neutral connection is at 0 volts and the hot connection should be around 120 volts, you should read somewhere between 115 and 125 volts on the meter display. If not, then something’s wrong with the power hookup.
If you measure 0 volts, then maybe you need to reset the circuit breaker, or if you have an outlet with a GFCI, remember to push the little reset button on the outlet itself. If it still doesn’t measure 110 to 125 volts, immediately contact the stage manager.
If you measure 220-250 volts, then that power outlet has been rigged inside the circuit breaker box to produce higher voltage. This is illegal and highly dangerous as you’ll surely blow up every piece of electrical gear you plug into the outlet. So, if you read 240 volts on the 120-volt outlet do not plug in your amp, and, again, immediately contact the stage manager.
Hot to Ground
If hot-to-neutral checks out around 120 volts (110 to 125 volts), then it’s time to test the ground, so plug one meter lead into the hot (shorter slot) and the other into the ground (U-shaped hole) connections.
Measuring hot to ground.
Since you’re reading from the ground connection, which should be 0 volts (less than 2 volts), and the hot connection, which should be around 120 volts (110 to 125 volts), your meter should show about 120 volts.
If you read 0 or something strange such as 60 volts, then the ground wire might be floating, which could cause a hot-chassis condition that will shock you when touching the strings of your guitar and microphone.