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drewdin 01-17-2013 07:44 AM

Basic 3-wire switch wiring diagram
I am planning on turning a regular switch to a three wire, two switch scenario. Can anyone provide me with a basic wiring sketch? I am going to do a google search but i trust you guys more, thanks!

nealtw 01-17-2013 08:19 AM

You will have to run a three wire between the two switches and the diagram depends on where the power is. Does it come to the light first or the switch first. Once you understand that, that is the diff., what you find on line is helpful.

JoeD 01-17-2013 09:10 AM

Run a three wire cable to the new switch location.
Take the two wires off the old switch. DO NOT disconnect any of the other wires in the box. I does not matter what colour the wires are. They could be both black or black and white or black and red.
Take one of the wires and connect it to the black from the new cable
Take the other wire and connect it to the COMMON screw of the three way switch in the original box.
Take the two remaining wires (red and white) from the new cable and connect them to the remaining two screws(travellers) of the new switch.
At the new switch location
Connect the black wire to the COMMON screw and the red and white wires to the traveller screws.

This will work no matter what setup you have for your current switching.

It is also not code compliant if you under a code cycle requiring neutrals in switch boxes.

drewdin 01-17-2013 09:33 AM

1 Attachment(s)
Thanks Joe,

I think I understand whats going on, here is a quick sketch. It is correct? Thanks

Wuzzat? 01-17-2013 01:12 PM

This boils down to a series circuit; a power source in series with a lamp in series with a SPST switch made from two SPDT switches.

If you have a problem use an incand. test light rather than a voltmeter because there will be Phantom Voltages in this circuit and cheapie analog meters will be responding to the current rather than the voltage and so you will get different voltage readings on different voltage ranges because a Phantom Voltage acts like a current source rather than a voltage source.

Test your brand new SPDT switches with an ohmmeter before installing them. A wiring mistake can instantly clobber switch contacts. The switch wiper/common contact should have a different colored screw.

When you're wired up, if you want to avoid drama, substitute an incand. bulb in a pigtailed socket that is of equal wattage of the controlled bulb and use it for your closed switch contacts. Both bulbs should light dimly and slowly come up in brightness.
You have four combos to test. Full or no brightness indicates an error.

There are many ways for this to go wrong and only one way for it to go right so work slowly and deliberately because the odds are against the uninformed.
Test each step along the way if you'd like.

To paraphrase a saying having to do with marriage, "Work in haste, repent in leisure."

drewdin 01-17-2013 01:37 PM

Thanks for the response, i should have labeled my drawing better. The offset lug in the bottom left of the 3-way switches in my drawings is the black screw while the top two lugs are the transports.

overall would the current way I have the drawing work properly? Thanks

nealtw 01-17-2013 04:43 PM

1 Attachment(s)
I think this is the one you need.

JoeD 01-17-2013 05:50 PM

You can't use white(even if you remark it) as the switched power back to the light fixture. Code requires it to be either the red or the black. You can't have two whites feeding a fixture.

drewdin 01-17-2013 07:08 PM

Thanks, i appreciate the help!

Wuzzat? 01-18-2013 01:15 PM


Originally Posted by JoeD (Post 82329)
You can't use white(even if you remark it) as the switched power back to the light fixture. Code requires it to be either the red or the black. You can't have two whites feeding a fixture.

You mean neither one of the travelers can be white?

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