I have a low voltage reading at a wall box where I plan on installing a thermostat. Essentially, the reading at this box is about 60-65 volts instead of the 110-120 volts expected.

The circuit is a 20 amp afci protected circuit which goes to a j-box and then serves 3 branches - one to a fan/light, one to the outlets and one to this j-box.

The first 2 branches read 120 volts, but the 3rd is only about 62 volts.

Why?

 

When measured with a digital voltmeter, voltage readings such as 62 are almost always "phantom voltage". The reality is that one of the two wires you're measuring voltage between is an open circuit. Something is either wrong with the circuit, or wrong with your testing procedure.

 

What type of thermostat is is that you need to install? Is it a line voltage stat for electric heat? If so, this needs to be a dedicated circuit, not tapped of a circuit feeding other devices.

 

Another point to ponder, if the thermostat is a standard low voltage one, you can't install it in a box which contains line voltage.
The phantom voltage you are getting is probably involved with a three way switch application. Unless something isn't working properly, don't get too excited over it. As mentioned earlier, if it uses line voltage, it has to be dedicated.

 

I don't recall the brand off hand, but it is a line voltage thermostat. The draw will only be 850 watts, so I'm not sure why it couldn't be on a circuit handling other devices. I do have a Lutron Fan/Light switch on another branch of this circuit that I suppose could be causing the phantom voltage (particularly if it's not wired correctly).

The readings are without the thermostat installed (just checking line voltage) so I suspect the Fan/Light Switch is wired wrong but then I wouldn't expect to get proper voltages at the outlets on the 3rd branch. Hmmmm........

Must be something between the j-box in the attic and the thermostat box on the wall. I don't see any other way for 2 branches to have 110 while the 3rd measures just 62.

By "Open Circuit" do you mean a loose connection or broken wire?

 

First, you are using the term "branch" very loosely. Tell us about what color wires and how many. What used to be in this box ? a switch?
What did it switch?

 

This is an entirely new circuit.

I ran 12/2 wg (Black/White/Bare Copper)from the panel (20 amp AFCI breaker) to a j-box in an attic space. From that point there are 3 sets of 12/2 wg (Black/White/Bare Copper) [all like color wires connected with wire nuts] going out to:

1. a series of duplex outlets wired in parallel.
2. a switch box that has a Fan/Light combination switch. A 12/3 (Black/Red/White/Bare Copper)wg goesfrom this box to the Fan/Light.
3. a switch box that will have the thermostat. A 12/2 wg (Black/White/Bare Copper) goes from this box to 2 outlets which radiant heating panels will be plugged in. The incoming Black and White will be connected to the corresponding "Line" connections of the Thermostat and the outgoing to the "Load" connections.

I call each of the above a "branch". If this is incorrect terminology I apologize.

I don't know how I can be any more clear about this.

 

You have a J-B with one 2-wire "Feed-in" cable, and three 2-wire "Feed-out" cables, and there is not 120 volts across the Black/White wires at the termination-point of the 2-wire "Feed-out" cable to the thermostat O-B.

I suggest a "continuity" test. Dis-connect the Black & White wires of thermostat "Feed-out" cable at the J-B, and connect the Black & White wires together. Check for Zero Ohms across the Black & White wires at the thermostat O-B with your multi-tester.

 

Thanks.
That's really what I needed to know.