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Discussion Starter · #1 ·
Im working at a school and we are changing out all the lights in the school. We are using the old whips to hook up the new ones. While installing we have just been turning off the switch so it will not be hot, but the neutral has a load on it. To try and get the load off I went around and switched off all the lights in the other rooms around the one im working in. I couldnt get rid of the load so I just went ahead and hooked it up hot but while I was working on it and wiring the neutral the lights in the room im in would flicker. I was wondering is the neutral being back fed? Is this what happens when you put more than 3 circuits on one neutral?
 

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Exactly what is your trade? Ever hear of a multi wire branch circuit? You shouldn't be working hot to the switch.
 

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Discussion Starter · #3 ·
Exactly what is your trade? Ever hear of a multi wire branch circuit? You shouldn't be working hot to the switch.
Electricians helper.

I dont think so.

Im not working hot to the switch I turned all the switches off and hooked the light to the switch leg which is off. The neutral was hot and was causing the lights to go on and off.
 

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Electricians helper.

I dont think so.

Im not working hot to the switch I turned all the switches off and hooked the light to the switch leg which is off. The neutral was hot and was causing the lights to go on and off.

Unless the breaker is off, your hot to the switch. A MWBC is more than one usually two ungrounded conductors sharing the same neutral. If your up on a ladder/scaffold and someone walks in the room, the first thing is to hit the light switch, which could be bad if the wire is in your hand at the time. To be safe at your level of the trade you need to kill all the breakers that feed these lights.
 

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I don't understand... the switch is turned off, so the lights are off, but the neutral is hot causing the lights to come on. ;)
 

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I don't understand... the switch is turned off, so the lights are off, but the neutral is hot causing the lights to come on. ;)
I took that as the neutral was feeding through the fixture he was working on and the connection was loose to the next fixture.
 

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Might be a switchloop with the wrong (black) wire switched. I don't know if it has anything to do with being backfed (meaning the switch is supplied from connections at the fixture,) but it isn't unusual to encounter situations where the feed is at the switch and two wires go to the fixture, one through the switch. It could be the black wire which means turning the switch off breaks the path, but not the feed. You aren't supposed to switch the neutral.

I have encountered many instances where a switchloop's white wire is connected to the hot wire at the switchbox, but not taped to show it is a hot wire. I always tape the white wire when I make up a switchloop.

Assume nothing! Fact is, I just got zapped the other day by a hot white wire, part of the switchloop to the fixture. My fault. I put my "barker" on it and assumed it was barking at some low, stray voltage. It wasn't.
 

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Discussion Starter · #10 ·
Unless the breaker is off, your hot to the switch. A MWBC is more than one usually two ungrounded conductors sharing the same neutral. If your up on a ladder/scaffold and someone walks in the room, the first thing is to hit the light switch, which could be bad if the wire is in your hand at the time. To be safe at your level of the trade you need to kill all the breakers that feed these lights.
Were not able to turn all the breakers off all the time because they are still having classes in the school. What is the difference in a MWBC and 3 hot conductors sharing the same neutral?
 

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Might be a switchloop with the wrong (black) wire switched. I don't know if it has anything to do with being backfed (meaning the switch is supplied from connections at the fixture,) but it isn't unusual to encounter situations where the feed is at the switch and two wires go to the fixture, one through the switch. It could be the black wire which means turning the switch off breaks the path, but not the feed. You aren't supposed to switch the neutral.

I have encountered many instances where a switchloop's white wire is connected to the hot wire at the switchbox, but not taped to show it is a hot wire. I always tape the white wire when I make up a switchloop.

Assume nothing! Fact is, I just got zapped the other day by a hot white wire, part of the switchloop to the fixture. My fault. I put my "barker" on it and assumed it was barking at some low, stray voltage. It wasn't.

The volume of the ticker I use gets louder as the voltage increases, lower than 90 and no sound.
 

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Discussion Starter · #13 ·
Might be a switchloop with the wrong (black) wire switched. I don't know if it has anything to do with being backfed (meaning the switch is supplied from connections at the fixture,) but it isn't unusual to encounter situations where the feed is at the switch and two wires go to the fixture, one through the switch. It could be the black wire which means turning the switch off breaks the path, but not the feed. You aren't supposed to switch the neutral.

I have encountered many instances where a switchloop's white wire is connected to the hot wire at the switchbox, but not taped to show it is a hot wire. I always tape the white wire when I make up a switchloop.

Assume nothing! Fact is, I just got zapped the other day by a hot white wire, part of the switchloop to the fixture. My fault. I put my "barker" on it and assumed it was barking at some low, stray voltage. It wasn't.
I dont think its a loop. There are two black wires going down to the switch I figured one was the hot and one was the switch leg.
 

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It should be obvious if the feed is to the fixture or to the switch, but if the lights are on a MWBC and if the switch is on to one side, the neutral disconnected from the panel will have line voltage on it. Common hazard with MWBCs. The good part is that if you complete the circuit, you'll be in series with whatever load is still on, so you won't be dropping all the voltage.

Two black wires to the switch? Do they both connect to the switch? If they do, it is probably a backfed switchloop.
 

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I doubt there's any white wire being used as unidentified hots if it's 277v in a school. My money's on it being piped, not NM'd or MC'd.
 

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I agree, but I didn't see that it was a 277 volt circuit. Doesn't matter, I bet you're right about it being piped.
 

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Were not able to turn all the breakers off all the time because they are still having classes in the school. What is the difference in a MWBC and 3 hot conductors sharing the same neutral?
Tim--You and I are at about the same level, so don't take my word as necessarily correct. If I'm wrong, I hope the guys here will correct me.

The difference between a MWBC and 3 hot conductors sharing the same neutral:

A MWBC is frequently encountered in NM (Romex) wiring, which is not allowed in most commercial occupancies. It is just 12/3 w/Ground, or 14/3 w/Ground. 3-conductor nonmetallic cable (NM, or Rome) has two hots, one neutral and one ground. (The proper names are: "ungrounded conductor" for hot; "grounded conductor" for neutral; and "grounding conductor" for the ground--the bare or paper-only covered wire, which is also called the "equipment grounding conductor" or "EGC".)

A MWBC is properly connected at the breaker box so each of the hots is on a different leg of the service. In typical 240/120 Volt, 1 Phase service, the power company brings in two hots and one neutral. The two hots come from one phase of the three-phase transmission and distribution power. These two hots are at opposite polarity, which means this: the AC goes from +120 volts, through zero, down to -120 volts, back to zero and back up to +120 volts. But timing is important. One of the hots is always at -120 when the other one is at +120, and vise-versa. They are synchronized and opposite. Sine waves that are 180 degrees out of phase. Sparkies often will say these two hots are "phases", but don't confuse this with actual 3-phase service. In proper 3-phase service, there are three hot conductors, each one is a sine wave, and they are each 120 degrees out-of-phase with each other. To distinguish the two legs of 240/120 single-phase from proper three-phase, I call the two legs "polarities". Anyway, to get 240 volts at a receptacle, you connect the two hots, because it is 240 volts across them. To get 120 volts, you connect one hot and one neutral.

So, to finish up what a MWBC is, when pulling homeruns in a dwelling using NM (Romex), sometimes you will pull a 12/3 or 14/3, because that gives you two hots sharing a single neutral. Each hot gets breakered separately at the panel. The neutral goes to the neutral bar and the ground to the ground bar. Now, here's the kicker. If you turn off the breaker to one of the hots, the other hot is still completing its circuit on the shared neutral. So, just because you have de-energized at the breaker, the shared neutral can still bite you, because if the other hot of the MWBC is still energized, this second hot will be energizing the shared neutral. The 2008 NEC addresses this by requiring that the two legs of a MWBC be breakered with a tied, or double-pole, breaker--one of those double breakers where both handles are connected together so they are always both on or both off.

So, that is one way a neutral can still be hot. By the way, since you were de-energizing at the switch (i.e., turning the switch off to de-energize at the fixture), the circuit feeding that fixture would still be hot up to the switch--which is what Random was talking about.

Now, the difference between a MWBC and three hot conductors sharing a ground is the difference between what I wrote above and what is written below:

Three conductors sharing a ground sounds awfully like it could be three-phase, four-conductor service. There are different ways to connect this. One way is when three-phase service is provided and the phases are wye-connected with a neutral being brought from the center of the wye. Other ways were more common in older days, and you say this is an old building with old wiring, so this building may have a different type of connection. But, in any case, three hots and one neutral sounds like the three hot conductors of 3-phase service with a neutral being gotten somehow. (Corner-grounded delta was more common in the old days, and there is still a lot of it out there.)

If you have three-phase, four-conductor service, it's easy for the neutral to carry current to bite you. In a perfect world, all the loads will be evenly balanced across the three phases--but, in the real world, the neutral will carry the difference in loads across phases. So, if phase A has 12 amps, phase B has 8 amps and phase C has 2 amps, there will always be current in the neutral, since the three phases are not balanced in load. The exact numerical number is some kind of vector sum that I don't know how to do--but it will be much more than 10 milliamps, which is enough to hurt, or 50-100 milliamps, which is enough to kill.

Anyway, hope that helps. You can always Google anything I didn't explain well enough. And, I hope the journeymen and masters here will correct any errors I made.

Be safe out there, Tim.

Peace out,
Mark T.
 

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Discussion Starter · #19 ·
It should be obvious if the feed is to the fixture or to the switch, but if the lights are on a MWBC and if the switch is on to one side, the neutral disconnected from the panel will have line voltage on it. Common hazard with MWBCs. The good part is that if you complete the circuit, you'll be in series with whatever load is still on, so you won't be dropping all the voltage.

Two black wires to the switch? Do they both connect to the switch? If they do, it is probably a backfed switchloop.

There are two black wires going down to the switch in a pipe. One is the hot and one is the switch leg. When I turn off the light switch one of the blacks goes dead this one is the switch leg, this is the one im hooking to the whip that is going down to the light.
 

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Were not able to turn all the breakers off all the time because they are still having classes in the school. What is the difference in a MWBC and 3 hot conductors sharing the same neutral?
There isnt a difference. That is a multi wire branch circuit. Its a common practice in lighting as your doing. So what your saying is that you cant turn off the breakers for the light circuit your working on?
 
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