I have a 12/2 with ground in the garage attic. I connected a GFCI outlet and a light switch on the LOAD side. The GFCI would not work. I looked at the LINE input to the GFCI, and an LED sensor showed current. However, when I checked with a multi-meter, it showed no voltage. Why does the LED show current and the multi-meter shows no voltage?
Listeners write in about ventilation and radon control and ask questions about tightening basement garages, ventilation solutions, and safer paint stripper.
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.
Start Free Trial NowGet instant access to the latest developments in green building, research, and reports from the field.
Start Free Trial Now
Replies
LED sensors can light up with a tiny amount of curent. I've gotten false positives just from having my forearm near a live piece or romex.
Check the neutral.
Neutral
Thanks! Good suggestion. I just checked voltage across 1) black and white; 2) black and ground; and 3) white and ground. There was zero voltage across 1) black and white; 2) black and ground. There was a 120 volt reading across white and ground. So, I am not sure exactly what is happening. There may be a junction box somewhere with white input connected to black, and black input connected to ground. This seems highly unlikely. The garage door opener is on the same branch circuit and it has no problems.
JC
Is there a switch somewhere that could operate that wire run in the attic? There's usually one switch in a house that no one knows what it does.............
There was...
I connected a switch across the white wire and the black wire; before connecting the switch, there was 120V across ground and white wire. There was no voltage across ground and black, or across white and black. I then found a loose wire in the attic and checked when the switch was ON. There was power on the loose wire. When the switch was OFF, there was no power on the loose wire.
Switch Loop
It sounds like you have a switch loop. The one code exception for the white wire always being the neutral is a swithch loop. The line connection goes to the fixture box (or switched outlet.) A single cable goes to the switch location. At the fixture box the line neutral (white) is connected to the fixture white (or identified) wire. The line black wire is connected to the switch loop white wire. The switch loop black wire is connected to the fixture black wire. At the switch box the white and black of the switch loop are connected by the switch. The switch then controlls the fixture. You can't interchange the white and black wires in the switch loop -- the fixture must be connected to both a black and white wire.
Neutral to ground voltage is not at all unusual. It simply means that some load is connected ahead of where you are testing. The neutral is connected to the hot across that load. Lack of voltage from the black to white wire can indicate that 1) The black wire is interrupted somewhere. 2) The white wire is interrupted somewhere (including being disconnected where you are testing). Lack of voltage from black to ground can indicate as 1 above or that the ground path is not contiuous. Ground wires should always be connected so that if a fixture is removed the ground path remains in tact. If you interrupt a circuit and proceed to test it the only information you can get is which end is the line side.