Voltage readings in 14-3 Romex connected to combo switch

[Stephen Tashi]

TL;DR

Should the voltage between the red and white wires in 14-3 Romex depend on the positions of both switches in a combo 2-pole switch?

In a house (USA) there is "combination switch" ( e.g. https://www.acehardware.com/departm...utlets-and-plugs/switches/3235199?store=16359 ) containing 2 two-pole switches. [Edit: correction: 2 single pole switches]. The electrical diagram for the circuit ought to be the following:

Should the voltage between R and W depend on whether switch S2 is open?

The input to the switch is the black wire from a 14-2 cable. The white wire of the 14-2 is connected to one end of the white wire of a 14-3 Romex cable. One end of the red wire of the 14-3 cable is connected to switch S1. One end of the black wire of the 14-3 cable is connected to switch S2. ( The 14-2 is an older type of cable that does not contain a ground wire.)

The other ends of the 14-3 cable are not yet connected to anything. When I measure the AC voltage from R to W with S2 open, I get about 120V or 0 V depending the position of S1. However, with S2 closed, I get about 60 V with S1 open. Does this have to do with induced voltages?

The 14-3 cable is about 20 ft long. I assembled a bench top version of the circuit using about 1 ft of 14-3 cable. With switch S1 open, I get about 3 V beween R and W with switch S2 open and about 6 V between R and W with switch S2 closed.

 

[DaveE]

Summary:: Should the voltage between the red and white wires in 14-3 Romex depend on the positions of both switches in a combo 2-pole switch?

Does this have to do with induced voltages?

Yes. Not induced, that implies magnetic field coupling due to current flow, which you don't have. It's really capacitive coupling inside the cable, most likely.

Voltage readings to a "floating" circuit node (i.e. disconnected from everything) are ambiguous. If you connected this configuration to whatever loads you intend, then you would get reliable (eg. 0) readings.

 

[Tom.G]

Summary:: Should the voltage between the red and white wires in 14-3 Romex depend on the positions of both switches in a combo 2-pole switch?

However, with S2 closed, I get about 60 V with S1 open. Does this have to do with induced voltages?

With the 'other' end of the 14-3 not connected, there is not any current flow to create magnetic coupling. You are seeing capacitive coupling between the wires in the 14-3 cable.

You are probably using a digital voltmeter for the measurements. These have a very high input impedance so it doesn't take much stray capacitance for a significant voltage to show up.

Due to their much lower impedance, an older analog, moving coil, meter would read little or no voltage under the above conditions.

Cheers,
Tom

 

[wirenut]

If you use a meter that loads the circuit (ie a "wiggy") you will see very different readings.