How to identify load or line wires in a light switch.


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Old 02-14-16, 08:30 PM
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Question How to identify load or line wires in a light switch.

Hi, I am installing four new GE smart light switches in my house but unlike regular light switches where it doesn't matter what terminal is used for the line or load, these switches have designated line and load terminals. So my question is how do I determine which black wire is line or load in each switch? I have a small multimeter that I use for my car usually for DC but it has AC on it also. Can you someone please help me with this?

Thanks!!
 
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Old 02-14-16, 08:33 PM
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Line black will show ~120v to ground or neutral.
 
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Old 02-14-16, 11:11 PM
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I've only ever probed DC wiring so I am correct in this process:

Turn my trusty little RadioShack multimeter to AC mode, with electric still on and the light switch off use either red or black probe pin to touch one of the two screw terminals on the current light switch and then touch the other probe to the metal mounting bracket area of the current switch as a ground. This should either read ~120v or zero. Then do the same thing but probe the other screw terminal on the current light switch.

The screw terminal that reads ~120v has the line connected to it and the screw terminal that reads zero has my load connected to it. Does this procedure sound correct? (especially the part of using any metal area as a ground)

Thanks!!
 
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Old 02-14-16, 11:59 PM
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If the box is grounded yes. If it isn't you will need to go to neutral. I would normally disconnect the wires first. If your multimeter is analog that will probably work but if it is digital you may get a false voltage reading of a few volts through the switch on the load side and ~120v on the line side.
 
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Old 02-15-16, 12:20 AM
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Thumbs up

Ok thank you for confirming Ray. It is digital (posted a picture of the one I have below) so if I see a few volts instead of zero I will not be surprised. My multi is so small I hope it can handle testing house AC electric without me getting fried! But this thing has given me many years of loyal service so I think I will be fine. Thanks again!

Radio Shack Multimeter 22-810


<img src="https://farm2.staticflickr.com/1702/25038817125_fe13ff3487_b.jpg" width="780" height="905"/>
Multimeter RadioShack Multimeter 22-810
by Rr Reo, on
 
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Old 02-15-16, 01:08 AM
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The meter is rated to 500v so you should be ok. Just make sure it's on voltage and not amps or ohms or the meter will retire early.
 
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Old 02-15-16, 01:32 AM
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Will do thanks. Actually I think the picture has the correct setting I should use 200v, coincidence lol.
 
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Old 02-15-16, 01:39 AM
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You are correct..... 200vac.
 
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Old 02-15-16, 04:35 PM
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You could use the 500 Volt setting. That is just the highest it will read.
 
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Old 02-15-16, 06:59 PM
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Well I installed today and used the multimeter with no issue and everything went well and works perfectly!!! Mission successfully accomplished!

What I found weird was that the line tested 10-12volts and the load tested 2-3volts. I had the setting on the meter as 200V in ACV mode exactly as pictured in the image I posted. Why did it cut off a zero? Usually it only does that if you pick the wrong range but 200v should have been perfect to test about 120v.
 
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Old 02-15-16, 08:08 PM
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Check the meter in a receptacle.
 
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Old 02-15-16, 09:04 PM
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I just did and it read 118v so I guess the meter is fine, not sure why it was showing odd numbers before.
 
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Old 02-15-16, 09:35 PM
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Did you only check to ground? If a reading is off you should always check to neutral. Just because something is metal doesn't mean it is grounded.
 
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Old 02-16-16, 12:34 AM
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That's probably it, I didn't check to neutral.
 
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Old 03-08-16, 07:33 PM
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With the switch turned off, you should see ~120V on the line wire when testing to ground and ~0V on the load wire when testing to ground. You may not have had a good contact point when testing, or (way worse) you've got an outlet or switch that is grounding out, reducing the potential between your line and ground.

As a side note, unless you are absolutely sure about the voltage, always start on the highest voltage setting on your multimeter. The lower number, in this case 200V, improves accuracy but if you test across a higher voltage, you will fry your multimeter.
 
 

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