Voltage low at Thermostat box

Reply

  #1  
Old 12-26-05, 11:34 AM
Member
Thread Starter
Join Date: Jan 2005
Location: United States
Posts: 455
Question Voltage low at Thermostat box

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?
 
Sponsored Links
  #2  
Old 12-26-05, 02:15 PM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
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.
 
  #3  
Old 12-26-05, 02:40 PM
Member
Join Date: Nov 2004
Location: CA
Posts: 2,041
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.
 
  #4  
Old 12-26-05, 03:11 PM
chandler's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,968
voltage low at thermostat box

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.
 
  #5  
Old 12-27-05, 02:32 PM
Member
Thread Starter
Join Date: Jan 2005
Location: United States
Posts: 455
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?
 
  #6  
Old 12-27-05, 05:27 PM
Member
Join Date: Nov 2004
Location: CA
Posts: 2,041
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?
 
  #7  
Old 12-28-05, 09:22 AM
Member
Thread Starter
Join Date: Jan 2005
Location: United States
Posts: 455
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.
 
  #8  
Old 12-28-05, 11:43 AM
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
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.
 
  #9  
Old 12-28-05, 02:05 PM
Member
Thread Starter
Join Date: Jan 2005
Location: United States
Posts: 455
Thanks.
That's really what I needed to know.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'