How to determine the hot wire in a wall light switch?

Old 05-11-13, 03:41 PM
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How to determine the hot wire in a wall light switch?

Hi. I have an in-wall light switch that is connected to three wires: two black wires and one ground wire. How do I determine which black wire is the hot wire?

I want to replace the existing switch with a timer switch. The timer switch has four wires: white (neutral), green (ground), red and black. The instructions say to connect the hot wire (one of the black wires connected to the existing switch) to the black wire on the new timer switch. Then connect the other wire (the other black wire that is connected to the existing switch) to the red wire.

What (inexpensive!) tool do I need to identify the hot wire? I found this item on Amazon: GB Electrical GET-213A Heavy-Duty 4-Way Voltage Tester. Is this what I need?

Old 05-11-13, 03:58 PM
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In a regular switch, there are only hots. Your neutrals (white) are probably tucked away in back of the box with a cap on them. A good analog multimeter is always a good tool to have around the house, and is inexpensive ($10+). You would uncap the neutrals, keep all other wires separated, then probe each of the two wires at the switch to neutral. One will be hot (line) the other will be load. Your white wire on the switch would go to your white bundle and capped. The ground to the grounding wire in the switch box, and the two blacks on the switch as described.

In some dimmers it doesn't matter which is line and which is load, but in some it does, so do as the instructions say.

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