Lessons · Electrical · measuring current, and the clamp meter
Amps are measured in the path, or around it
A meter in amps mode must be in series, so the whole current flows through it; a clamp meter reads the current from the magnetic field around one conductor without breaking the circuit.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
Nobody opens a live circuit to insert a meter on a 30 A dryer feed. The clamp is the tool. But clamp it around the cable instead of one wire and it reads zero, because the hot's field and the neutral's field cancel, and that zero has sent people looking for a fault that was not there.
How to think about it
For a clamp: AC amps, jaws fully closed around one conductor only, read. For an in-line ammeter: de-energize, lock out and test it dead, break the circuit, put the meter in the gap, restore, read, and never exceed the meter's fused limit. If you are not sure the current is under the meter's limit, use the clamp.
Worked example
Water heater: 3600 W at 240 V, I = 3600 / 240 = 15 AWhat the clamp should read on either hot.
Clamp around one hot: 15 AOne conductor, one field, one reading.
Clamp around both hots together: 0 AEqual currents in opposite directions cancel. Not a fault; a mistake in where the jaws went.
Wire passed through the jaws 5 times, reading 2.5 A: actual = 2.5 / 5 = 0.5 AWrapping a small current through the clamp several times multiplies the reading. Divide by the number of passes.
Your turn
A conductor passed through the clamp 4 times reads 2.0 A. Write the line for the actual current.
I = 2.0 / = 0.5 A
Solve one, graded on the server
The trap
Leaving the meter's lead in the amps jack and then measuring volts. In amps mode the meter is nearly a short circuit; across 120 V it blows the meter's fuse at best. Move the lead back to the volts jack the moment you are done.