120Vac Interface for Current Measurement

WARNING: The following project deals with mains voltage, which can cause serious injury or death. Should you choose to replicate any part of this project I am not responsible for any damage, injury or death that may result.[/alert-warning]

I wanted a way to easily and safely (or semi-safely) use my multimeter’s current measuring function (ammeter) on 120Vac devices.

To measure current with an ammeter you need to break the circuit and insert the ammeter so that the electricity flows through the ammeter on its way to the device being powered.


To do this easily and safely with devices powered via mains voltage (120Vac) I made this “interface.” One side of the circuit goes directly to the outlet, and the other is broken and goes to a terminal strip. An ammeter is connected to the terminal strip, completing the circuit and allowing the ammeter to measure the current being drawn from the device plugged into the outlet.

This was easy, took only about an hour to make, and all the parts can be picked up from any decent hardware store:

I started with a standard plastic surface mount switch/outlet box:


I used small angle brackets to attach the terminal strip to the box:

Drilled a hole for the wires to pass through to the terminal strip:

Wired up the outlet – this is wired the same way you’d normally wire an outlet except that one side (positive in this case) of the circuit is being broken by the terminal strip:


Measuring the current draw of a desk lamp (CLF bulb). Notice that I used longer screws on the terminal strip to allow alligator clips to be clipped on:

Now I can measure the current draw of a 120Vac device (or devices) easily and relatively safely using my multimeter. I say “relatively safely” because the terminals are exposed so caution should be taken when in use. Also, it should only be used for quick measurements, noting long term.

By measuring the current draw (amps) of a device you can easily figure out how many watts it’s pulling by multiplying the source voltage by the amps.

volts x amps = watts

My CFL bulb is drawing 0.12 amps, times that by 120 and we get 14.4 watts.

Adam ByersComment