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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

migrated from electronics.stackexchange.com Oct 30 '17 at 18:40

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

  • This will probably be migrated to DIY. You should state what country's electrical standards are likely to apply because this is an international site. Add your location to your user profile too. – Transistor Oct 30 '17 at 18:39
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    Also note that if "apartment" means a unit that you rent (not own), then doing anything with the electric is probably a big no-no. The apartment maintenance person might be able to change a fixture for you, but could refuse to make any changes. – JPhi1618 Oct 30 '17 at 20:07
  • If the fixture worked why would you think it was wired wrong? See Dave's answer below. It is very common and most electricians and manufactures use red as the switched leg. – Ed Beal Oct 30 '17 at 22:20
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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).

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    One example of this being used would be in a ceiling fan where the fan motor is wired to black (always on) and is controlled by the pull chain only, and having the light kit wired to the red which is controlled by the switch. – JPhi1618 Oct 30 '17 at 20:32
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Neutral must be white or gray, them's the rules in North America.

Equipment safety ground must be bare wire, green, or green/yellow worldwide.

Hots can be any other color: black brown red orange yellow pink blue purple.

However, you have two types of hots often involved in a lamp circuit: always-hot (you probably don't want that for a lamp), and switched-hot (which is hot with switch "on").

When working in multconductor cable, you are stuck with the standard colors that come in cable: bare, white, black and sometimes red. And so a "conventional practice" is to use black for always-hot and red for switched-hot, because it's most often used in places where that red wire will be in the cable.

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