Just to start from the beginning, I was installing a new LED light fixture, in a dining room. Stupid mistake, as I didn't turn off the breaker, (thinking-switch off-no current to light).....I was wrong (not the first time). I connected the Black hot to the light, then grabbed the white wire on the light and pulled it up to meet the white wire in the box and this is when it let me know this wasn't real smart. It gave me a shock, so confused as I was, I put my meter on it. Is this because I was in between two neutral wires? Switch was off, although it's a Lutron smart dimmer, not just a standard switch. I then went to the light switches, that had recently been updated (same type) and when I check the neutral to ground they show some voltage when separated, but when I put them all back under a wire nut and check, the voltage goes to zero. I didn't think there would be any voltage, but it varies from 5 to 30 on the neutrals, WHEN they are separated, but goes to zero when they are all connected back and you check them to ground. Shows zero at the panel (neutral to ground) and 120 volts hot to ground at panel, as well.
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Not sure but it might act like a lost neutral. Usually when a house has a suspected lost neutral, the main recommendation is to get the power company out there yesterday because of the danger.– crip659Feb 5 at 21:10
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I was thinking that too, but the white wire from the light goes into the wall box and is tied to the other white neutral wires.– BradFeb 5 at 21:26
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Now you know, some white are not neutral, they are part of switch loop (switched hot) –– RuskesFeb 5 at 21:41
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I think you are mistaking, the white at the light goes to the switch, then check with volt meter from neutral to the ground and it should be zero– RuskesFeb 5 at 21:43
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1I always do the safety test. Briefly contact the neutral to the ground and nothing will happen, or you have a spark and just saved your life.– RuskesFeb 5 at 22:07
2 Answers
A disconnected neutral which is connected to a device which is connected to a hot has full voltage on it.
The device will drop that voltage when the neutral is connected, but when the neutral is disconnected, no current flows, so the voltage on the floating neutral is whatever the line voltage is, as none is dropped across the device, because no current is flowing across the device.
Turn off the circuit, next time. Be happy you got a next time. Those are in limited supply when you are careless around electricity.
"Smart switches" are always "on" to the extent that they need to power themselves to have "smarts" even if the position of the switch is "off." Use the circuit breaker.
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1Some smart switches do have a mechanical switch which fully disconnects them... but you should still never work on a hot circuit.– keshlamFeb 5 at 22:19
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Many white wires in switch boxes are actually hot
The "lamp(s) and switch(es)" group is a little network if you will. Supply power (always-hot and neutral) is brought to only one place in that network.
When power is brought to the lamp, neutral stops at the lamp. The switch only needs "always-hot" and "switched-hot", and that arrangement is called a "switch loop".
- Switch loops installed per NEC 2011 (the year), use 3-wire cable+ground. Black and red are the hots, white is the neutral. This neutral wire is for smart switches.
- Old switch loops use 2-wire+ground cable. White is supposed to be "Always-Hot" per Code, and be marked with black tape to indicate it is not neutral. Black is "switched-hot". This is often ignored.
So if you assumed white is always neutral, that may well be what gotcha.
Sophisticated "switches" don't fully turn off.
Switches don't normally need power. The power to throw them comes from you. But smart switches do need power - that's why NEC 2011 requires a neutral. So do motion sensors, lighted switches, dimmers etc. etc.
Most of these "power themselves" by leaking power through the light bulb (since most of them cannot count on neutral being in the box on pre-2011 installations). As such, "off" isn't really "off".
Wires going through the box? Interrupt at your peril.
Very often, when power comes into the lamp, it also continues onward to other lamps or outlets. In that case you will see 2 black wires joining a wire to the switch, and you will see 2 white neutrals joining a neutral wire to the light.
If you separate those 2 hots or neutrals, you put yourself in the middle of power going to other lights and outlets.
Now normally, you would have no reason to mess with the black "hot" bundle, since "always-hot" does not go to the lamp. However, the lamp does need neutral. So you typically have a wire-nut joining 3 wires - the two supply and onward neutrals, and a wire heading to the lamp.
And I bet you separated that wire nut, separating the supply and onward neutrals, so you could add the white wire from the lamp. When you separated them, you got "in the loop" of power going to other downline outlets.
That's why you turn off the breaker.
And given all the weirdnesses and troubles I see in pre-existing wiring, honestly, I'd recommend shutting off the main breaker. You never know when someone will have stolen a neutral, installed a MWBC without handle-tied breakers, or a variety of other defects which can energize neutrals on circuits that are definitely off.
Yet still, presume every wire is hot, and that the wiring in your house is part of a greater conspiracy to try to kill you. Don't handle wires more than needed.
