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Old 10-05-2010, 06:43 AM   #1
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Can a Load Have Two Power Sources?


I have two fixtures that I would like to have the ability to turn on at any time. One is a motion detector light, whose bypass does not always work for me. It is now wired according to the instructions: power from switch goes into the motion detector, than on to the lamp. I was wondering if I could also install another wall switch that is wired directly to the lamp, bypassing the motion detector, so I could turn the darn thing on when I hear something outside, without doing the on/off/on/off/on "bypass" thing that is SUPPOSED to turn the light on. I realize that, at certain times, the lamp will have two possible sources of 115v power - but it seems to me that it would only draw the power it needs and be ok. The two sources of power would be on the same circuit.

The other fixture is a attic roof exhaust fan that has a thermostat. This has no option to turn on if the thermostat doesn't think that it is hot enough in the attic. Sometimes at night I would like it to run the fan for a while to draw in the cool outside air through the house while I open the windows and the pull-down stairs to the attic.

I don't know if I asked/explained this as clearly as possible, so feel free to ask for clarification.

Thanks in advance for your answers.


Last edited by Starlight; 10-05-2010 at 06:47 AM.
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Old 10-05-2010, 06:55 AM   #2
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Can a Load Have Two Power Sources?


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I have two fixtures that I would like to have the ability to turn on at any time. One is a motion detector light, whose bypass does not always work for me. It is now wired according to the instructions: power from switch goes into the motion detector, than on to the lamp. I was wondering if I could also install another wall switch that is wired directly to the lamp, bypassing the motion detector, so I could turn the darn thing on when I hear something outside, without doing the on/off/on/off/on "bypass" thing that is SUPPOSED to turn the light on. I realize that, at certain times, the lamp will have two possible sources of 115v power - but it seems to me that it would only draw the power it needs and be ok. The two sources of power would be on the same circuit.

The other fixture is a attic roof exhaust fan that has a thermostat. This has no option to turn on if the thermostat doesn't think that it is hot enough in the attic. Sometimes at night I would like it to run the fan for a while to draw in the cool outside air through the house while I open the windows and the pull-down stairs to the attic.

I don't know if I asked/explained this as clearly as possible, so feel free to ask for clarification.

Thanks in advance for your answers.
as for the light, most of the units i've installed have a switch sequence where as you switch it on-off-on and it manually overrides the sensor. check for it or replace rather then rewire it. with the fan you could just install a manual switch in parallel with the t-stat. no extra power source required in either case.

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Old 10-05-2010, 08:38 AM   #3
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Can a Load Have Two Power Sources?


As long as your proposed bypass switch is on the same supply circuit as the motion detector, your plan is fine. You are putting the the new switch in parallel with the motion detector. If either is on- the light will be on. Both must be off for the light to be off.
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Old 10-05-2010, 08:41 AM   #4
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Can a Load Have Two Power Sources?


The cleanest (least confusing) way to do it is to run a 2 conductor cable from the thermostat or sensor i.e. both ends as close as possible to the sensor (white wire on the line/feed side sensor terminal, black wire on the load side) then to your override switch.

That said, some folks want two motion lights to both come on when either one gets activated. This has been accomplished by running a single conductor (hot only, no neutral) from one light to the other light (same as running the single conductor from the load side of one sensor to the load side of the other sensor. Now both lights are fed by two sources namely the two sensors. In practice a two conductor cable is run with the white wire taped off and unused. Again, both lights must be fed from the same breaker and should be fed from the same cable going back to the breaker.

I think the correct method for the two motion lights is to disconnect one light socket from its sensor (not the location where the power comes in) and run both hot and neutral to the other light. Also run the hot jumper from the load side of the disconnected sensor to the load side of the still connected sensor.
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Last edited by AllanJ; 10-05-2010 at 08:59 AM.
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Old 10-05-2010, 10:04 AM   #5
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Can a Load Have Two Power Sources?


@TimPa Yeah, the light is supposed to override with on/off/on, but it doesn't always. The other problem, and the most annoying part actually, is that when you do this and get the light to come on, if something then triggers the motion detector it starts the timer and the light goes OFF in five minutes [my choices are one, three or five minutes]. The light is at the top of my driveway and it's frustrating when I'm shoveling snow.
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...with the fan you could just install a manual switch in parallel with the t-stat
Thanks, this got me thinking about how I can work this out. I could install a three-way switch on the line going from the existing switch into the fixture and run a three conductor [w ground] cable to the light. The power would connect to the common on the three-way. When that switch was up, the black wire would be powered and be wired to the light [past the motion sensor]. When the switch was down, the red wire would bring power to the motion sensor.

Thanks rjniles, I was hoping for that confirmation.

@ AllenJ: I think what you said is the cleanest way of doing what I need to do. Luckily I'm not trying to get two lights to work from either sensor, but that is an interesting solution.
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Old 10-05-2010, 10:26 AM   #6
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Can a Load Have Two Power Sources?


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Originally Posted by Starlight View Post
Thanks, this got me thinking about how I can work this out. I could install a three-way switch on the line going from the existing switch into the fixture and run a three conductor [w ground] cable to the light. The power would connect to the common on the three-way. When that switch was up, the black wire would be powered and be wired to the light [past the motion sensor]. When the switch was down, the red wire would bring power to the motion sensor.
That will certainly work but you will have no off position. You might be better of with a single pole, double throw, center off switch. Probably have to go to an electrical supply house rather than the typical big box. Or you could use a double combination switch. (2 switches on a single yoke).

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