# Measuring AC Voltage Behind Light Switch

This is more of a theoretical question and not something I'm needing to do.

I understand the following, please correct me if I'm wrong;

1. Multimeter only measures the potential difference so
2. To measure the AC voltage passing through a light switch we need a neutral wire along with the live wire or
3. A ground can be used instead as neutral will be earthed somewhere and therefore be connected to ground anyway

So, from my understanding, are we basically treating ground as neutral then in this case?

Can a floating ground be used or must the ground be in the circuit of where the neutral is? I guess this is more a question of how the meter works.

What if no ground or neutral is present, am I out of luck?

TIA.

• If the switch is connected to a low resistance load (analog bulb) and with the switch off you connect the meter leads from a high impedance meter across the switch you will see near line voltage across the switch because you create a series circuit through the meter with most of the voltage drop seen across the meter, and almost none across the load. Commented Apr 16, 2021 at 17:33
• There is such a thing as a high impedance ground and isolated grounds but neither of these methods are floating they are both tied to earth and neither is a “normal” grounding method for residential. At least in the US. It is a requirement for the neutral and grounding system to be tied in the main panel. Power companies tie the center tap of 1 phase to ground to create our neutral. Commented Apr 16, 2021 at 18:56
• If you have a tungsten bulb I think you could just measure across the switch contacts; neutral will be connected to the wire going to the fixture through the bulb, which is itself just a wire, and hot will be connected to hot. With an LED bulb it may or may not work, depending on how the bulb is designed. Commented Apr 16, 2021 at 20:27

Yes, multimeters measure potential differences: phase to ground, neutral and also phase to phase.

Measuring at a switch, you'd need a hot and a neutral. You can measure to ground if a neutral is not in the box and while you'll get the correct voltage, you can't run a circuit off the two.

Grounds aren't normally floating but neutrals can be. If you have a hot, any ground will work it doesn't have to be from the same circuit.

If there is no neutral or ground at your switch or anywhere near it, you could take an amp reading and then get an accurate resistance reading of the circuit and work it from there: E=RI

If neither neutral nor ground is in the switch box, plug an extension cord into a convenient receptacle and measure between the hot in the switch box and the neutral in the receptacle end of the extension cord.