Switch loop uses neutral wire

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  #1  
Old 03-08-06, 06:36 PM
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Switch loop uses neutral wire

Hi,

I just replaced a light fixture and discovered the previous homeowner has it switched by a loop of the neutral wire instead of the hot. While this is certainly not to code, is there really anything to worry about? It is otherwise wired ok, properly grounded, etc. I'm tempted to leave it alone since it's working fine and not giving trouble, but could be swayed if there is really a safety issue.
Thanks.
 
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Old 03-08-06, 06:52 PM
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How do you conclude the neutral is switched? There are many schemes to switching. Perhaps more detail will help you and us.
 
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Old 03-08-06, 07:39 PM
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Switching the neutral is not the most serious of hazards. But it is a hazard nevertheless, and there's no reason to tolerate it since it is so trivial to correct. The same applies to polarity reversals. Both of these conditions increase the risk of electrical shock.

Many people equate "white wire" with "neutral". Unless you clearly understand the difference, don't change anything.
 
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Old 03-09-06, 07:30 AM
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I have a degree in electrical engineering. Given that background, my question may seem trivial, but I have a deep respect for the NEC and the experts here who know it well. The code covers many practical issues and hazards that I would not think of.

Anyway, the fixture has 2 2-wire Romex cables in it's box. The white wires and grounds are connected, while the fixture itself is connected to the 2 black wires. Obviously, one of the blacks has to be connected to neutral in order to energize the device.

In looking at the switch, there is one Romex cable in the box with white and black wired to the switch. With the switch open, voltage measurements are 120VAC between white and black and between black and ground (black would be getting voltage via bulb filaments), and zero volts from white to gnd as expected. With the switch closed, all voltages in the switch box go to zero of course and the light goes on.

So, this is clearly a switch loop where the previous do-it-yourselfer wrongly wired a switched neutral. Now, I decided to leave it alone, as I can't be certain what else is going on inside the wall, and it has been that way for years. I am curious, though, what the actual hazard is, minor though it may be, other than perhaps confusion.

Thanks for the responses. Great forum. Keep up the good work!
 
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Old 03-09-06, 08:04 AM
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This problem should be trivial enough to correct. It's possible that someone made an assumption that the black wire was ground, basing this on DC applications. This mistake happens a lot, with people even carrying over the terms positive and negative and applying them to the cables.

The necessary changes are all at the light. First make sure that the incoming power black wire is definitely the hot and that the white is definitely the neutral. Then rewire correctly.

The danger here is small. If care is taken that power is always turned off at the breaker before working on a fixture and that the wiring is understood before proceeding, then there probably won’t be any mishaps.

However, if someone turns off the switch but not the breaker and assumes that the power is turned off to the light, then they might get zapped sticking their fingers in to the box. They might assume that the switch leg is the hot and therefore think they can touch the other wire safely.

The bigger issue is that this will confuse someone later on, perhaps even you. The next person who owns the house may want to change the fixture, perhaps installing a ceiling fan and light. If attempt to follow directions and wire according to the instructions, they might end up feeding the neutral into the hot wires and the hot into the neutral. They could really mess things up with the wrong connections.
 
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Old 03-09-06, 08:40 AM
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Originally Posted by kenbert
In looking at the switch, there is one Romex cable in the box with white and black wired to the switch. With the switch open, voltage measurements are 120VAC between white and black and between black and ground (black would be getting voltage via bulb filaments), and zero volts from white to gnd as expected. With the switch closed, all voltages in the switch box go to zero of course and the light goes on.
The way you describe it, it could also be that the switch loop is wired using the wrong color as the feed. If the black were fed, as opposed to the white, you would get the same scenario.

Do the same measurements without the lamps in place. If you get any voltage at all the neutral is not switched. If you do not get any voltage then the neutral is switched.
 
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Old 03-09-06, 11:30 AM
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I believe that it is pretty conclusive that the neutral is switched, since the voltage on the switch terminals goes to ground potential when the switch is closed. If the switch were in the hot leg, then both switch terminals would become hot when the switch was closed.

Depending upon the way that the lamp is wired, there is an additional significant danger: the outer screw shell of the bulb holder may be energized, and would be energized even if the switch is off. This would present a shock risk every time the bulb was changed.

-Jon
 
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Old 03-09-06, 12:24 PM
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Actually winnie you're right. I was fixated on the first part and lost sight of the last sentence.
 
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Old 03-09-06, 04:00 PM
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The fix is simple. Split the two wires and the black feed from the fixture. Connect the black feed to the switch white wire. Connect the white feed wire to the fixture white wire.
 
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