Adding on/off switch to 12v 3-bulb "chandelier"

[KTManiac]

Need advice from a bona fide electrician.

I want to add an on/off push button switch to my 12volt 3-bulb "chandelier" so I can switch it off and still have the kitchen (1 or both) ceiling lights on to conserve the battery when boon-docking. I want to put it in the the lamp's ceiling cowling to avoid fishing wires down the wall to a remote switch, so it has to be kind of small.

I found a suitably sized switch, but it is rated at 3amp 125vac.

Using an online calculator I came up with some figures:
A type 912 12v 13w bulb @ 12v comes out to 1.08333amps, times 3 bulbs = 3.25amps. Running at 14v, it comes out to 2.79amps for 3 bulbs. Are these figures correct, or am I totally wrong? More importantly, can I use this switch for the DC application, or am I going to fry the switch (and maybe the Montana) when the batteries voltage goes lower than 12 volts? Does the switch need to be a DC rated switch, and if so, why?

Thanks!

[ARJ]

There are no "DC rated" switches to my knowledge. The switch you found will handle 375 watts. Your load is 39 watts. When boondocking, your battery voltage will start out slightly above 12v for a fully charged battery and then slowly decrease as you use appliances.

Your switch will be fine!

[NCFischers]

I concur. At 12 volts, your load is 1.87 amps. The switch will work fine.

[KTManiac]

Cool! I went ahead and installed the switch and it all works like I had envisioned. I'm just not positive that my textbook calculations truely apply, or even if they are remotely close. I know just enough about electricity to be dangerous!


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NCFischers,

Could you explain your 1.87 amps calculation to me? I still get 3.25 amps for 3x13 watt bulbs. re: watts/volts=amps, ergo: 39 watts/12 volts=3.25 amps.

Now I am confused.
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ARJ,

I also calculate 375 watts at 12 volts, but using a further conversion of 375 watts/12 volts get an amperage rating of 31.25 amps. I can't fathom that a switch with contacts that small could carry that load without frying to a crisp. But, from what I gather, the three bulbs are running at about 1/10th of that.
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Thanks for the help, everyone. I guess I need to take some classes on all of this stuff.

[Art-n-Marge]

LOL KTmaniac, don't be so hard on yourself. If we took classes on everything we needed to know about RVs and everything in them, we wouldn't need the forum, and probably would learn too much that we would be too frustrated to want to own an RV.

Things to learn - DC & AC electrical, plumbing, structural, physics, mechanical, carpentry, framing, metal work, brakes, suspensions, sewage, chemistry, and I can go on, but this tired me out.

ARJ - I have seen switches rated for 12v, 120v and 240v, so I know they are out there.

NCFishers is an electrician so I would believe him. He has had great advice for many other related electrical issues. For me I try not to mix 12v stuff with 120v stuff but I'm not an electrician. To answer your DC versus AC question, that would require a short course in electrical 101.

That's what this forum is for. If you got it working then it's good. You were smart enough to ask and the answer came... YES!!!

[ARJ]

You are mixing apples & oranges. First, watts are a result of voltage x amperage (or current). So, 125vac @ 3a = 375 watts. That same 375 watts divided by 12v = 31.25 amps. Now you can see that the switch will carry 3 amps @ 120vac or 31.25 amps @ 12vdc. Ac~Dc who cares-the wattage is what matters.

[blarkman]

Maybe LED bulbs?

[KTManiac]

I kinda thought that is what I tried to explain in my last post, but maybe not so clearly. So, indeed, I guess the textbook stuff is all true with electricity.

I grew up and worked 39 years in the machine shop world, where machine shop theory and book learning does not always transfer over to real world usage very well, ...too many variables.

Thanks for your help and advice!

BTW, on kind of a different topic, you say "the wattage is what matters", but, I have always heard that it's the amperage that kills you!

[ARJ]

Depends on whether you are trying to light a light bulb or standing in a tub of water and grabbing the hot wire. Yes, current is what kills but it has to have voltage driving it!

[KTManiac]

quote:Originally posted by blarkman

Maybe LED bulbs?

Those will definitely be in the works as well, but I still want a mix of 12v incandescent, 115v incandescent, and LED lighting. I don't think I want to go 100% LED because it just doesn't put out the same kind of light, although it is very advantageous for boondocking.

...and we all know that any type of light is good light when it's dark & you are all alone in the woods. [wolves eerily howling]

[NCFischers]

KTManiac,
I used my handy dandy ElectriCalc Pro electric calculator. It has the formulas built into it. I plugged in 12 volts and 39 watts and hit the amp button. It came back with 1.87 amps. If I do it with just one bulb at 13 watts, it gives me .625 amps.

[ARJ]

quote:Originally posted by NCFischers

KTManiac,
I used my handy dandy ElectriCalc Pro electric calculator. It has the formulas built into it. I plugged in 12 volts and 39 watts and hit the amp button. It came back with 1.87 amps. If I do it with just one bulb at 13 watts, it gives me .625 amps.

I think you may need a new "Handy Dandy " calculator. Here is how it is done.

http://www.powerstream.com/Amps-Watts.htm

[KTManiac]

Oops, maybe it needs new batteries, ....or see if you can find a miniature Hughes Autoformer to boost up the voltage.

[SlickWillie]

Maybe made in China!

[NCFischers]

Oops, cockpit error. The calculator was set on 3 phase instead of single phase. I didn't see the tiny little symbol in the corner of the display. As I get older, they make the displays smaller. I believe it's a plot against seniors.

[ARJ]

quote:Originally posted by NCFischers

Oops, cockpit error. The calculator was set on 3 phase instead of single phase. I didn't see the tiny little symbol in the corner of the display. As I get older, they make the displays smaller. I believe it's a plot against seniors.

Also it seems that the cockpit errors get more frequent. BTW-Walmart has some pretty good MAGNIFYING glasses! I know.............

[NCFischers]

ARJ,
I wear magnifiers. I get them at Sams Club by the 4 pack. That little symbol is still too small. I'm telling you, it's a plot.