How to test for power before you touch a wire

The four steps

1. Turn off the breaker. If the panel directory is missing or wrong, plug a lamp or radio into an outlet on the circuit and flip breakers until it goes off. For a light, turn the light on first.
2. Prove the tester works. Hold your non-contact tester to an outlet you know is live, or use its self-test. A tester with a dead battery says "dead" about everything.
3. Test every wire in the box. Take off the cover plate, unscrew the device and pull it forward without touching the screws. Hold the tester tip against each wire and each screw, whites included.
4. Test again with two leads. A non-contact tester can miss. A two-lead tester or multimeter set to AC volts gives a number: black to white, black to bare, white to bare should all read zero.

What a live box reads

Between	Live circuit	Dead circuit
Black and white	About 120 V	0 V
Black and bare ground	About 120 V	0 V
White and bare ground	Near 0 V	0 V
Black and red (240 V circuits)	About 240 V	0 V

Traps that catch people

• Two circuits in one box. Kitchens, hallways and double switch boxes often hold wires from two breakers. Test after the first breaker is off; if anything is still live, find the second breaker.
• A shared neutral. Some homes run two circuits on one white wire. That white can carry current from the other circuit even with your breaker off. Do not open a splice of whites until both breakers are off.
• A switch that is off is not a dead box. With the switch off, power still reaches the switch box (and, with power at the light, the light box).
• Testing the wire nut, not the wire. The tester needs to be close to the copper. Test each wire on its own.