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Old 02-13-2019, 02:28 PM
black rabbit is offline
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Replacing an old light switch with a motion sensor - wiring question


I'm replacing an older single pole (I think) light switch in my garage with a motion sensor. The house was built in the late 30's.

The cable coming out of the wall has three wires:

Red wire connects to terminal A on the top of the switch.

White wire connects to terminal B on the top of the switch.

Black wire connects to terminal C on the bottom of the switch.

With the breaker on, my cheapo voltage tester shows the following:

Black + White = Voltage

Black + Red = Voltage

White + Red = No Voltage

Any wire + Box = No Voltage

There is no ground wire connected to the box.

I am *reasonably* certain this is NOT intended to be a 3-way switch - I've never found an additional switch that controls the lights in question. Were ungrounded 3-way switches ever a thing?

My motion sensor requires that hot, load, and ground be connected.

Can I be reasonably certain that either Red or White are ground in this case? Any ideas on how I should figure it out, short (!) of calling an electrician?
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Old 02-13-2019, 05:14 PM
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It's always a bit hazardous to diagnose electrical issues through a forum, but I'll toss out a few questions.

First, are you measuring the voltage with the conductors disconnected from the switch, or when they are connected to the switch?

Second, if you have three terminals on the switch and none of them are obviously connected to each other, then have you removed the switch to check if it is actually a single-pole switch? (I'm assuming that your cheapo VOM has a continuity tester or you can use the resistance setting.) In other words, is one of the terminals a common that switches between the other two terminals?

Finally, do you see black, red, and white conductors in your light fixture? Or does the red disappear to somewhere?

It sure sounds like it was for a three-way installation. The voltage between black and white is right, the voltage between black and red sounds like the red is connected to the white...somewhere. No voltage between red and white fits with this. (No voltage measured to the box would be typical for that era.) Back in those days, an electrician would probably run the power to the light fixture, then bring down a black and red to the switch, using the switch to control the voltage back to the fixture on the red. However, a lot of electricians would jury-rig it using black or white instead of red...usually because they didn't have anything other than black or white readily available. Nowadays, electricians must bring the neutral (white) down to the switch, but that would have been uncommon back then.

I'd also suggest working on this by disconnecting the branch circuit at the light fixture so you can check continuity between your switch and the fixture. (Obviously, after turning off the branch circuit and capping those wires.)
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Old 02-13-2019, 05:22 PM
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Yes, ungrounded switches were a thing. The only way to tell for sure if this is a 3-way, is to disconnect the switch from all wires and use an ohmmeter, as mentioned above. Sounds to me like someone jury-rigged the wiring.
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Old 02-13-2019, 08:22 PM
black rabbit is offline
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Hm. The power going to the fixtures first makes sense. This is an attached basement garage. The cable on the switch box disappears behind the plaster in the ceiling. All of the other cables in that area are stapled to the wall and I can trace those circuits back to the panel.

The fixtures in question are actually closer to the panel than their switch is.

Looking even more closely the labels on the panel, it looks like that circuit serves several other completely unrelated rooms in the house.

My tester is just a $5 glow tester with probes.

I guess I'm going to call an electrician. Looking more closely, there's a bare conductor running alongside the outside of the cable that's attached to one of the screws on the box. I'm guessing that's supposed to be ground, and it's broken somewhere.
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Old 02-13-2019, 08:27 PM
Tim@T-Bonham.net is offline
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Quote:
Originally Posted by ZonexandScout View Post
Back in those days, an electrician would probably run the power to the light fixture, then bring down a black and red to the switch, using the switch to control the voltage back to the fixture on the red. However, a lot of electricians would jury-rig it using black or white instead of red...usually because they didn't have anything other than black or white readily available. Nowadays, electricians must bring the neutral (white) down to the switch, but that would have been uncommon back then.
And this would mean that there is no neutral available at the switch location. So that means modern switches like a motion detector, a timer, a lighted switch won't work in this switch box.

You'll need to run a 120V cable from the breaker box to the switchbox -- that can be difficult if the garage is a finished space.
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Old 02-13-2019, 10:24 PM
K2500 is offline
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If it's a three way it would make sense if you get voltage from black to red if red is headed through the light bulb.

How many light fixture in this garage?

A few things to check:

The color and number of conductors and cable in the fixture box controled by this switch and their suspected destination. If this three wire cable comes from the fixture box changing it up to have a neutral should be simple.

Other switch boxes in or near the garage.

Next to places or in boxes by other doorways into the same garage. Note that door ways could have been closed off and the three way switch that used to reside next to them bypassed and/or covered up.
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