2 switches, one light

Originally Posted By: apolillo
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Fellow Inspectors,


Nine year old townhouse. Both front bedrooms had 3 wall switches by the door. No ceiling fan, just one light fixture in the ceiling and one outlet switched. The first and second switch both turned on the ceiling light. I did not turn them both on at the same time.


My question is what could have happened if I had? I wrote it up for evaluation / repair by a Lic. sparky.


Anatol


Originally Posted By: phinsperger
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I see these often where it was originally wired for a ceil fan but where one was never installed.


It is likely that both circuits are from the same service leg and splits at the switch box giving them a potential between each other of 0V and a potential to the neutral of 110V. If so, nothing is going to happen if you turn on both switches other than the light coming on.

Although unlikely, it is possible that each switch is served from a diferent service leg with a potential between them of 220V. If so turning on both switches would cause a short and hopefully an overload protection device (breaker) would trip.

If the switch coverplate was removed you should be able to tell by the way it wired if the wires are from a single feed and split in the box.

Either way, as you recommended, it should still be corrected.


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Paul Hinsperger
Hinsperger Inspection Services
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Originally Posted By: Jay Moge
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if served by 2 legs, both switches on would give a 220v, but wouldn’t it blow the nearest device that acts as a fuse. ie: the bulb? a 60 watt bulb with 220v will burn at 120 watts (give or take) and the element couldn’t handle all that juice and POP. giving an open cuircet and eliminate it’s own problem. a bulb, afterall, is nothing more than a fuse with a tungston element encased in inert gas tight bulb. icon_cool.gif


Originally Posted By: pdickerson
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Not to be too picky, but if you double the voltage potential, you double the current (V=IR, assumes the element resistant is constant, which is isn’t, but close enough). Because Power = Volts x amps, and both volts and amps are doubled, the power increases by a factor of 4 when the voltage is doubled. Its a squared function. If you have a bulb that is rated at 60W/120 volts, it will consume 240W at 240 volts…for a very short period of time.


Originally Posted By: Greg Fretwell
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If we are going to be picky, one side of the light will always be connected to the grounded conductor. If you really had 2 phases there it would be a bolted line to line fault on the black wire.