Quick easy electrical question

[JB4310]

I think...

I found 2 aux 12 volt truck lights in a dumpster, they don't look like much from the outside, just a plastic housing, but inside they looked pretty nice with twin bulb assemblies in a ceramic inner housing.

Water got in there and messed up the connectors. Fixed that and want to (did) mount one to my canopy to help with night vision when snow plow is raised and blocks headlights.

They are twin 55 watt bulbs, beautiful bright wide pattern way better than the front or rear lights that came with the machine.

Question is how many amp fuse do I need for them?
I figured 110 watts divided by 12 volts comes to 9.2 amps.
I've got a 10 amp circuit available, but it was close enough I figured I better ask.

Is this formula correct? I think it's Ohm's law but not sure if it works the same for dc as ac.

Thanks, JB.

 

[rus_geek]

Your formula is correct. Also given that you're probably running at higher than 12V, you'd be at a lower amperage yet. If you really want to know, you can hook up an amp meter to a single light and see what its "actual" draw will be.

-r-

 

[Redneck in training]

The formula is correct. But you don't need to base the fuse size on the load but on the wire you used to feed the lights. In other words the fuse should break before the wires get hot. Wire size versus amps table is here:
Automotive Wire & Wiring Guide from TESSCO - TESSCO.com

 

[JB4310]

Thanks Rus geek,

I have a multi tester and should start using it for more than just volts and continuity. Does it matter what side of the circuit you test between (+or-).

I had a beautiful professional Fluke brand tester for 1 day last month, the guy from the power company left it at my house when checking for a voltage loss on my UG service, I wasn't gonna call them to tell them they left it since all you get is automated answering system, but when the same guy came 2 days later to follow up on the problem I felt guilty and gave it back to him. then he tells me I embarrassed him in front of another worker, cause he forgot something. Should of just kept it, it was nice with long leads, back lit display, "rush in" feature, and of course the fluke jaws. ~$200. tester.




Thanks Redneck,
Going by that chart 18 gauge wire should be fine, was wondering that also.


JB.

 

[5205deere]

You did right by giving the meter back. Glad you got your answers

 

[biggerten]

Your formula is correct. Also given that you're probably running at higher than 12V, you'd be at a lower amperage yet. If you really want to know, you can hook up an amp meter to a single light and see what its "actual" draw will be.

-r-

Higher voltage will result in increased current and higher wattage. That's why lights get brighter when revving the engine.

 

[JB4310]

Higher voltage will result in increased current and higher wattage. That's why lights get brighter when revving the engine.

I know the lights get brighter but what about the Ohm's law?

110 watts divided by 12 volts = 9.16 amps

110 watts divided by 13.5 volts = 8.14 amps

If this is what Rus Geek is getting at, makes sense.

But your point makes sense to me to, seems brighter light would mean more amps???

now I'm confused
JB.

 

[CurlyDave]

Way back in my my college years, I remember an electrical engineer telling me that a circuit should be sized for about twice the expected current draw.

If you expect 8-9 amps draw, you should size wires and fuse for 20 A.

This has worked well for me for 45 years.

 

[ctgoldwing]

I know the lights get brighter but what about the Ohm's law?

110 watts divided by 12 volts = 9.16 amps

110 watts divided by 13.5 volts = 8.14 amps

If this is what Rus Geek is getting at, makes sense.

But your point makes sense to me to, seems brighter light would mean more amps???

now I'm confused
JB.

You are leaving out one very important component of Ohm's Law - resistance. Let's just say for easy numbers you have a 60 watt bulb (rated 60 watts at 12 volts) The bulb would draw 5 amps AT 12 volts. It's resistance would equal the voltage divided by the current (12/5) or 2.4 ohms. For our discussion lets say that doesn't change (it doesn't change much). At 14 volts the current draw would be 14/2.4 or 5.83 amps. The wattage of the bulb would then be 14*5.83 or 81.6 watts.

In reality I believe auto bulbs are rated (wattage) at 13.6 volts. The formulas don't change just the numbers.

 

[jinman]

Nice job ctgoldwing! It's easy to overlook the obvious.

Dave, I would oversize the fuse to probably 15 amps instead of 20, but 20 is fine if the wire can handle it.