Wiring question, why would a switch have no power when ON and power when OFF?

(Ron) #1

Why would a switch read 0 volts when the light is turned on and 120 volts when the light is turned off ?

I am considering installing a zwave dimmer on a ceiling light in my extra bedroom. First I opened the box and tested the wiring. But my results have me stumped and thinking the switch will not work.

The case is when the switch is turned On, there is no power going through the switch. Reading with a volt meter. Seen here

When I turn the light Off the switch does have power flowing. Seen here.


This is confusing to me and has me wondering how this light is wired.

NOTE: I found a similar situation with a three way switch, when the switch is on, zero power any wire on the two switches, when the switch is off there is power flowing to one switch or the other. I chalked that one up to three way wiring I didn’t understand but this one is a single switch light.

(Paul Haskins) #2

You’re measuring it wrong - that red and black is a loop. Measure the black to neutral /and / or to ground.

(Edward Pope) #3

When measuring the switch on and off you are measuring the potential difference. The switch is more or less a short when you throw it on, So it should read closed, and no voltage difference across the switch. With it open you should read the total difference between the two points (120V)

(Ron) #4

Thanks guys, I knew i could get help here. I understand. Any good tips on sites or places where I can go to learn hot to test using a multimeter better. I never used to have one but my house is wired weird and I totally need to learn more.

I used the meter successfully to figure out with the power off where the wires in another box were going using the Ohm meter but I didn’t know how to use the Voltage/Amp correctly. I need to find a site/book/resource to learn how to use this thing :slightly_smiling:


If there’s a Home Depot near you, many offer a class on how to replace a light switch which is a really good way to learn about switch wiring and how to use the tools. They won’t likely cover networked switches, but you’ll still learn the basics which will then make all the networked resources easier to understand. :sunglasses:

(Ron) #6

Sounds good. I have been switching wires in previous homes for years with no issues but my current home is wired in very “creative” ways. With more standard wiring I have never needed a multi tool but with this house I need it every time I do anything !


You probably already found this, but here’s the Home Depot link for their free classes. You have to pick a local store to see which classes are offered there when.


(Ron) #8

Yes found it. They had perfect ceiling light and switches class but it is same time as my dogs vet appt. She gets first priority

(Michael) #9

Lowes offers classes too but honestly YouTube has a ton of videos as well. From what you describe it sounds like you are comfortable with single pole wiring but need to learn about 3 way wiring as your pictures are of a three way switch where you have 2 switches for the same fixture. There are many resources on YouTube and online for 3 and 4 way wiring.

(Ray) #10

It’s actually quite simple. What you are measuring is a floating voltage. Let say you have line hot going into a light bulb and the switch is connecting to the neutral side of the light bulb.
When the switch is in the off position. What you are measuring is the line voltage going across the bulb to neutral.
When you turn the switch on. You no longer see the voltage because you just completed the neutral circuit for the bulb.

(Ron) #11

Yea I tried YouTube, I am not really looking for how to wire a three way, I know how and I know there are a few different ways (power to switch, power to light etc. What I am trying to learn is how do I identify what I have in my box when I open it. I found for example my GE dimmer and add on switch do not work in my Gym room because these switches only work with standard power->switch1->switch2->light configurations where there is only a traveler between switch1 and switch2. My light is a different configuration but I don’t know how to identify which with the multi meter.

But more specifically I need to learn how to use the meter in standard applications. It works find sticking it in an outlet to check it is working. But as explained to me above I was using it wrong to test my switch. Because I was placing black and red on opposite terminals of my switch I was reading power when off and no power when on which apparently was false due to my error using the multimeter. I should have held red to the line-in hot wire and the black to either neutral or ground. Something I have learned in this thread :slightly_smiling:

So long story short (I know too late for that) I want to learn all the various setting of my Multi Meter and how to use them. The manual that came with it explains NOTHING and yet it has all kinds of fancy features. For example why in the heck does it measure temperature ?

Thanks, that has been explained to me and I get it now. Makes complete sense but maybe not intuitive :slightly_smiling: at least wasn’t for me. This is the kind of thing I want to learn more of, using the multi meter and what the results mean.