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While working to switch a single switch out for a double (putting a fan and a light on separate switches), I came across an interesting condition -- to make this switch cold, I have to turn off four separate breakers. I'm at a loss to understand why anyone would do this, if there are any risks to keeping it as-is, and if there's anything to be aware of when splitting this into two switches. If anyone could shine some light on this, I'd be very appreciative!

I've drawn out the conditions as best as I can tell, but I haven't tested each individual wire on the patio switch.

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    How are you determining when the switch is "cold"? Did you find connected devices no longer work (if so, what kind of devices?), voltage measurement, non-contact sender, etc.? Your method may affect the conclusion, and you might need multiple methods to narrow down the situation. – blarg Mar 29 at 1:59
  • @blarg I was using a multimeter with a 'live' setting, and touching it to the contacts on the switch. – Itinerati Mar 29 at 10:19
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    You may be detecting phantom voltage. I have seen 2 circuits crossed in the field but never 4 the chances of this are very remote. Read up on phantom voltage and you might find why you detect voltage. For them to be connected all 4 would have to be odd or even breakers, if some are odd and some are even it will be phantom voltage. – Ed Beal Mar 29 at 15:40
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    I'm not positive what the "live" setting would be. Does it measure a voltage (~120 V), or continuity (often read with a green LED or a tone)? – blarg Mar 29 at 15:44
  • @blarg I'm not entirely certain...I was using the 'live' setting of this multimeter: amazon.com/…. I'm starting to think Ed might be right, though -- only one of the breakers also trips the 'NCV' setting, and with that it adds an orange light to the 'live' setting, in addition to the green lights that it was doing. – Itinerati Mar 29 at 23:55
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I will copy this as an answer as I believe it is most probable.

You may be detecting phantom voltage.

I have seen 2 circuits crossed in the field but never 4.

The chances of this are very remote. Read up on phantom voltage and you might find why you detect voltage. For them to be connected all 4 would have to be odd or even breakers.

if some are odd and some are even it will be phantom voltage because if connected odd and even would be a direct short. There is a possibility of a back feed through a 240v device but this would be as rare as all 4 actually being connected in the field. So the most probable answer is phantom voltage.

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