Electricity 101

Happy New Year!

I have a basic question.

I am looking at a toggle switch that is rated 20A at 125VAC or 10A at 277VAC. What would the rating be for 12VDC??

Would it be basically P=IE? Where Watts = Amps x volts, etc.?

Regards, Twobit

 

According to McMaster-Carr, (a wholesale warehouse), a switch that was rated 15 amps at 120 VAC, would be rated 20 amps at 28 VDC, so there is some increase in amperage, but apparently not what you would ***ume using the standard P=IXE formula.

 

With switches, the current rating for DC voltage is always lower than for AC.
Switching DC voltage causes arcing inside the switch, like your basic spark plug does.
You have to get the manufacturer's suggested DC rating and cannot use ohms law to find the DC rating.
If it's not known, use a switch with a very high AC current rating.

I use a switch with the AC rating you describe for the heater in my 12V wiring without any problems for years.


You could also use a relay rated for the load you have, but if you know how to do this, you wouldn't have posted your question.

 

Thanks;

It would be helpful if the manufacturers would put the DC rating on their switches, also.

I shall use the 20A 125VAC switch on my 12volt, within reason of course.

Regards, Twobit

 

...for what it's worth, it was p***ed down to me to not use parts if they were not rated for the application. There may be a good reason that the switch is not rated DC. I do know in some instances that DC current can do damage to non rated parts. My experience is with industrial applications, not so much cars, but I always go with what I know. If you can find the manufacturers data on the part you could see if it did indeed have a DC rating. If not, then I wouldn't use it outside its known application. I'm a firm believer in do it once, do it right, especially when it comes to electrical.

Just my two cents worth...

 

anything that has a high amperage draw ( like a fan , fuel pump , heater fan , etc ) i suggest you use a relay and use that switch to trigger it

 

Alternating voltage switches are rated in RMS values that are similar to DC ratings. Switch ratings are relative to internal resistance for current capability and dielectric or insulated value for voltage value. Switch power calculations are not simple ohms law. It is best to use the properly rated switch for each application. I also suggest a switch relay combo for high power applications as it keeps high current wiring away from the dash and can shorten heavy wire runs to the device.

 

...I got to think about it and I remembered the specific problem using AC switches in DC applications. It has to do with breaking current.

A DC rated switch is designed to withstand a much higher breaking (arc) current when the switch is disabled. As you open the switch the voltage tries to maintain itself by increasing the current, and it will arc across the opening points until the arc current is overwhelmed by the dielectric value of the air.

AC switches are rated much lower because the voltage changes direction 60 times a second. When you flip the AC switch, the voltage has less time to build breaking current because the voltage is always alternating back to zero as it changes direction. RMS is the value ***igned to AC voltage, think of it as an average. 120VAC actually peaks at (I'm not looking this up, just putting you in the ballpark) 165VAC. Electrical meters are calibrated to look at the RMS value rather than the peak value. DO NOT ASK ME TO EXPLAIN HOW TO FIGURE OUT RMS...I'd rather beat myself in the head with a hammer.

So, AC switch may burn up under use as a DC device. If it's not DC rated, or at least dual rated, don't use it in your car! And like has already been said, use a relay on any high current circuits; it keeps that high current under the hood and out from under the dash, just a plain good idea...

 

I concure mdbomstad and electrodude on this, always use a relay on high load conditions it will save you in the long run. I also concure about explaining RMS I have a hard enough time training my guys on my ship on the theory behind it.

 

This is the way to go as you can get relays that are rated for the amps that you need for your application...

 

[QUOTE DO NOT ASK ME TO EXPLAIN HOW TO FIGURE OUT RMS...I'd rather beat myself in the head with a hammer.

QUOTE]

http://en.wikipedia.org/wiki/Root_mean_square

 

use the switch. but just use a relay to take the load off the switch. there less then 2amp draw on most "bosch" style relays. take the load outa the switch put it in the relay you will be good to go. there very easy to wire up

 

Boy, thats' some advice. It would have been easier on everybody if you had mentioned the intended load(s) and maybe a relay or even a solenoid would be handy. The AC switches are best left for commercial use, use a DC rated switch and i get mine from flea market MIL spec and never have a problem. When it comes to AMP ratings - as you have probably figured out by now - there are too many ways to calculate the amperes (like the advertised wattage of a sterio), the actual switch ratings are generous esp with AC because those don't tell whether it is an inductive or resistive load current rating and those loads don't apply to DC voltage because of the flat signwave.
There, i've done it now and just thoroughly confused a simple question.