Weird exterior light switch wiring (72 volts???)

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Old 09-23-18, 05:44 PM
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Weird exterior light switch wiring (72 volts???)

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This is a drawing from the electrical box in which there is 2 switches. It's a house from 1961 with ungrounded wiring in this part of the house.

I was about to install a programable switch for my exterior lights consisting of 2 lights in the soffits and 1 by the address. The lights were wired through 2 switches, basically one just enable the other one but the 3 lights just worked when the right switch was and then the left one could work. When I opened the box I notice wires coming in from 3 sides as the schematic in the drawing. 72 volts was coming from the bottom right wires and there was a jumper between each switches. I just removed the 72 volts wires and wired everything to just 1 switch with the 120 volts wires. Everything works fine but I thought that was quite weird.

Any thoughts on what might be going on?
 
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Old 09-23-18, 05:55 PM
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You measure 75 volts to what..... hot ?
If that is tied into the neutrals like you show then there should be no voltage on it.
The problem is there is no ground there to measure to.
 
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Old 09-24-18, 01:06 PM
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I took the wiring appart and the measurement are made between hot and neutral.
 
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Old 09-24-18, 04:55 PM
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Sorry.... I didn't catch it at first.

Look at your diagram. The hot comes in to the left switch. Then you have it coming out switched to feed the right switch and then to the light. That is incorrect. The hot wire needs to feed directly to both switches and then the lights connect to the second screw on each switch. The 72v you were measuring was with the lights in series.


This is how it should have been wired......
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Old 09-25-18, 02:48 PM
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Maybe I'm slow, but I still don't get where the 72V is coming from, and even what's the point of 2 switches when 1 would do the job.
 
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Old 09-25-18, 03:05 PM
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It's because you're putting 120 volts across TWO light bulbs.
If they're both 100 watt, then it's ~ 60v each.
if it's 100 watt and 75 watt then you'll get slightly different voltages.
 
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Old 09-25-18, 05:31 PM
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Nothing was going through the lights, the 72V was with the lights off and disconnected of the switches.
 
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Old 09-25-18, 05:35 PM
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Then the 72v was a ghost (induced) voltage picked by a sensitive digital meter
due to a dead wire being in close proximity to a hot wire.
 
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Old 09-25-18, 06:09 PM
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I had never heard of that before. After reading a bit about ghost volages, is there a way to verify if this is my issue without a multimeter that has a low impedance option?
 
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Old 09-25-18, 06:18 PM
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We often recommend a cheap, $7-$15, analog instead of a digital for AC work. Its high impedance will bleed off ghost voltage.
 
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