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I want to replace a light switch in my home with a dimmer.

Each switch has one wire going into the wall and the other pigtailed with the corresponding wire from the other switch.

I am testing with the lights off.

I want to figure out which is the hot and which is the load wires before removing the old switch.

I have never used a multi-meter with AC power and the one I have is not very fancy. When I test the 2 terminals on the current switch, I get around 105 V (I guess there is something wrong with how I am making contact). When I reverse the leads, I still get the same value, not the negative.

I wanted to make sure the box is properly grounded. I thought that the single wire going into the wall was the hot; but there is no voltage difference between that wire and the box. There is a difference of 120 V between the pigtailed wires and the box. Again there is no difference when reversing the leads.

Are these pig-tailed wires hot and the single wires load?

The wiring diagram from the dimmer's instructions:

wiring diagram

The wall box has 2 light switches in it:

enter image description here

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    I let an electrician answer but the black wires tied together should be the line (power supply) and the single black out of the switch should be the load to the fixture. – Alaska Man Feb 26 at 0:44
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First off, good for you. You want to figure out what's hot and what's the load wires before disconnecting everything. With the power off, pull the switches out of the box and pull out the bare copper ground just a bit. If there's a group of white wires, neutrals, you can remove the wire nut but don't untwist the wires. Make sure none of the groups of wires are touching each other. Now you can turn the power back on. Take the common (usually black) probe from your multi meter and touch the group of white wires and take the + probe, usually red, and touch it to the black wires one at a time where they are screwed into the switches. With the switches off, when you get 120 Volts +-, those are your hot or feed wires and those will be pigtailed to both switches if one breaker controls both switches. The other wires connected to the switches are the loads.

Once you determine the hot wires by doing the above, take your black probe and put it on the bare copper ground wire and the red probe on a black hot wire. You should get 120 Volts+-. Now you know you've got a ground and if it's connected to the metal box, the box is grounded too. Make sure your meter is set to "AC Voltage" and the proper range. Since it's AC and not DC, you'll get the same readings when you reverse the probes.

Turn the power back off and connect your dimmers according to the diagram, recap the white wires, and connect your switch ground wire to the ground from the dimmers and push the switches back into the box and screw them in. Turn the power back on and test everything.

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The answer is in your question:

Each switch has one wire going into the wall and the other pigtailed with the corresponding wire from the other switch.

A simple switch has hot (aka line) and switched hot (aka load). It is common, but not required, that multiple switches in a box share the incoming hot wire by pigtailing together. So that is your hot wire, which fits with it showing 120V to ground. That leaves the "wire going into the wall" to be load.

The one catch with a dimmer (or smart switch or timer or motion detector, etc.) is that you may require neutral. In a setup like yours, you should find a set of white wires wire nutted together - those are the neutrals. If your switch needs neutral, you add another wire to that group of wires, possibly needing a larger wire nut.

AC is alternating. It flips back and forth between positive and negative 60 times per second (50 in most places outside US/Canada). Your meter handles that automatically (provided it is set on AC V), so you actually can't tell the difference, with a typical meter, between "hot to neutral" and "hot to ground" - they will both show nominally 120V.

If you are still reading 105V, I suggest checking a few standard receptacles. If you get 105V consistently then your voltage is just low - i.e., a brownout (perhaps not technically a brownout, depends on permitted tolerance in the system). However, if you find some receptacles read 105V and some read 135V then that is a sign of a much bigger problem.

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