I have a fixture to change in my apartment. There is a black, a white and a red wire in the junction box.
The old fixture was connected to the white and red wires only. This led me to believe that something is not normal, because i expected it to be connected to the white and black wires at least and i though the red wire was the ground and could have been neglected by the person who installed the old fixture.
Anyways i tested with a voltmeter in order to find out witch wire is the hot one.
I get a reading on the voltmeter when i test the white and red wires together and when i test the white and black wires but not when i test the black and red wires. This tells me that the white wire is the hot one, when it should be the black wire.
But it gets really weird when i turn off the light switch, and this is where i really don't understand what's going on. If the switch is off, i get a reading on the voltmeter when i test the black wire with the white only. I expected to see no reading at all when the switch is off. Additionally, i expect the hot wire to show a reading on the voltmeter when tested with the neutral wire and with the ground. In this case, only the black and white wires combination shows a reading and neither does when tested with the red wire. This is really inexplicable to me because either the black or the white wire has to be hot.
To recap, here are the results of my tests in a more simple form :
The switch is on:
- Red + white : 120V
- black + white : 120V
- red + black : 0V
The switch is off :
- black + white : 120V
- red + black : 0V
- red + white : 0V
I'm a complete noob when it comes to electricity, but this doesn't look normal to me according to what i have read online. A simple solution would be to connect the new fixture the same way as the old one, on the red and white wires while ignoring the ground, but i don't feel safe doing this.
Clearly some of my assumptions must be wrong or there is a problem with the junction box… What can i do about this? Is there a safety hazard?
Thanks for your help
Best Answer
The wiring sounds perfectly normal to me, for US standards:
white wire = neutral
black wire = hot (unswitched)
red wire = hot (switched)
When the switch is on, the black and red wires are effectively connected together at the switch, so the voltage between them is zero. If you look inside the box where the switch is, you'll see that it is connected between the black and red wires.
When the switch is off, the red wire is not connected to anything at all, so it shows zero volts to any other wire (because of the finite impedance of the voltmeter).