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Old 04-21-2009, 09:17 PM   #1
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Motion switch wired to two circuits


I am replacing an old microwave motion switch which controls my two porch lights. It was wired from a three position (on, off, auto) wall switch. Much to my surprise, the wall switch and motion switch are powered by separate circuits. When the switch is in the on mode, power goes through the wall switch to the lights and they are on. When in the auto mode, power comes from a separate circuit, through the motion switch, then through the wall switch to the lights and they come on when motion is detected. It may have been wired this way to allow the on/off/auto feature before the newer infrared motion switches provided this internally. My quandry is that the lights only have one neutral, which is through the wall switch. So, when in auto mode, the hot is coming from one circuit and the neutral from another. Is this kosher? If not, I'll have to run a new neutral wire from the wall switch to the motion switch.

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Old 04-22-2009, 01:11 PM   #2
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Motion switch wired to two circuits


It is far from kosher.

A neutral may return only the current supplied by the accompanying hot wire. Neutrals from different branch circuits may not be tied together up in the house at outlet boxes, etc.

I suggest you disconnect the hot wire that does not go with the neutral that is being used, tape its end, and figure things out from there.

Power feed and neutral to manual switch (existing)
Switched feed and neutral from manual switch to light (existing, for always on)
Need two hot conductors and a neutral between manual switch and motion switch. You need to send unswitched power out to motion switch and get switched power back to manual switch's box to tie to switch's third position for auto setting. The neutral is needed, too, to operate the motion switch mechanism.

Two wires are needed between motion switch box and manual swtich box if the motion swith has built in on-off-auto.

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Last edited by AllanJ; 04-22-2009 at 01:29 PM.
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Old 04-22-2009, 07:31 PM   #3
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Motion switch wired to two circuits


Thanks AllanJ. The new motion switch does have a built in on-off-auto. However, wouldn't I still need three wires between the wall switch and motion switch, a hot wire to the motion switch, a return switched hot, and a neutral to operate the motion switch? Alternatively, could I use the neutral from the other circuit to operate the motion switch? I know this violates the rule mentioned in your first paragraph but I doubt is uses much power. The neutrals from each circuit are not tied together.
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