switching 125VDC with 120VAC device

[dstampfli]

Does anyone know of any guidelines for switching 125VDC backup incandescent lights using a standard 120VAC wall switch or timer? Most toggle wall switches and/or timers are not UL rated 125VDC. Is there a safe derating factor? i.e. using a switch rated at 30A @ 120VAC at no more than 20A @ 125VDC. Will derating and using a switch at 125VDC violate the UL rating or break any NEC codes?
Thanks much!

 

[tom baker]

Re: switching 125VDC with 120VAC device

The requirement can be found in NEC section 110.3(B). The switch has to be DC rated. Check the mfgs instructions and catalogs

 

[templdl]

Re: switching 125VDC with 120VAC device

dstampfli,
Derating factors relate to voltage. Because a switching device, such as a simple switch, breaker, contactor, relay, etc., has to open and clear an arc it is much easier to clear AC than DC because AC goes through zero volts 120 times a second. The devices that I'm familiar with have AC voltage ratings that are higher than DC but the current ratings are the same.
So, is you take a switch rated 120vac it most likely will never pass a test for 125vdc. Remember, current is not an issue.
You may have to go it on your own. BUT, to give you a tip that you will say "Gee, I should have thought of that" is to use a 2p switch which doubles the contact gap to clear a DC arc. You can either just loop through the contacts of have one on the + and - side, doesn't matter.
Breakers are used like that all of the time but they do have a DC rating when wired in configuration to 3 poles in series. (Something I be your mother never taught you. )
But you will still have no
"Official" dc rating if it is not labeled as such.
Dave

 

[jim dungar]

Re: switching 125VDC with 120VAC device

First, you can not violate a listing. When UL lists a device they are confirming that it has been manufactured to specific standards. Only a manufacturer can void a listing. You can however misapply a device, by using/installing it in a manner that has not been tested or goes against the manufacturers instructions.

Second, slow make/quite switches do not carry DC ratings because they rely on the zero crossing of the AC waveform to actually "stop" the current flow. DC devices rely on different methods (primarily distance and speed) to break the current, which is why many snap action switches have DC ratings.

In general, switch will have different voltage and current ratings for AC and DC.