Troubleshooting an Electrical Circuit - Is This an Expected Voltage Drop?

Question

I am troubleshooting an electrical problem and am having trouble understanding the readings I am seeing on my digital multimeter. I am a DIYer with just the basics of electrical engineering knowledge, but would appreciate comments on what I am seeing and the physics behind it. Would this be expected based on Ohm's law?

Below is a schematic of my circuit which supports three electric baseboard radiators controlled by a single wall mounted thermostat. The heat had stopped working and when I removed the thermostat I found 2 tiny baby mice in the electrical box! After removing the mice I wanted to test to see if there might be any anomalies indicating chewed wire damage inside the wall and got the voltage readings shown on the diagram.

For those outside of the US and Canada, note that we get 220V by going across the two 110V wires from the line source, whereas our 110V is done by going from one of the two 110V sources to the neutral. So I understand the 120V reading on the first measurement.

I am not clear however as to why I got the other two readings. Note that all of these readings were taken with the thermostat switch open.

My question: Are the 20V and 70V readings I am seeing to be expected voltage drops due to the resistance of the radiators? Or could it be an indication of some line damage hidden in the walls?

Answer 1

Ohms law disagrees. Since the thermostat is open, current is zero, so voltage drop due to Ohms law (V= I * R) is (V= 0 * R) is (0) that's (zero.)

So, the "70V" reading should be 240 (you're doing the common but wrong for 5 decades or more bit of calling 120/240 110/220) and the "20V" reading should be 120V. The "120V" reading is as it should be.

Both "70" and "20" are probably "phantom" voltages (high impedeance meter showing voltage capacitively or inductively coupled onto a wire) and probably indicate a disconnected wire from the other side of the 240V supply to all the radiators (since any one radiator being connected should cause the correct voltage to appear at these points, if the diagram is accurate.)

One thing to look out for is that many North American resistive heat thermostats are actually wired to open both sides of the line, which would be one way for the other side of the line to be disconnected from the radiators. That's not what you've drawn, but if you oversimplified the drawing, it might apply.

Answer 2

I found 2 tiny baby mice in the electrical box!

Because you're running the box or panel with missing knockouts, a code violation. KO covers are 30 cents.

On your 70V measurement, since the switch is open, and your voltmeter is taking micro-amps, the resistors/radiators are functionally deleted and you can now see that your voltmeter probes are now directly across the "220V" source.

Thomas Edison started with system voltage of 100V and did voltage bumps to 105V and 110V to increase system capacity while not instantly blowing people's light bulbs. It had reached 110V when Edison lost the War of the Currents, so Tesla/Westinghouse adopted 110/220V split-phase as a national standard and filled every advertising channel with the call to hook up to 110V. Sometime around WWII they started another series of bumps and arrived at 120V around Vietnam, but in slang speech, 110V and 220V stuck like Kleenex in a Xerox.

That's how you can have a "220V" source but 120V measured value to ground.

If you are in a 120/240V split-phase country, or a 120/208V or 127/220V location, you should be seeing 110-127V between either phase and ground. Apply the above logic that the resistors functionally disappear when not under load, and you can see your phase-ground voltage should be 120V-ish, not 20V.

I suspect a lost phase. Either in the circuit or at the electric service.