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voltage drop

Andy in Co.

NAXJA Forum User
Location
Vail, Co.
At what point would you go to the next bigger guage wire, for voltage drop? .5, .75 ?? Still trying to get my lights set up. 4x100 watt. I want to make sure if I run 130watt lights that I already am set on the wire. :confused:
 
While there has to be some practical limit... if that 100W is rated for 12v at the lamp and you're running from a 13.nn V system, then you could probably stand to run at 1.nn drop -- if you're OK with the 100W (8.33A draw across a lighted filament resistance of something like 1.44 ohms) --

If you're looking to get 100W starting with a 12.0v system (and those are rated 100W@12V as above) then you should get to the no-drop situation ...

Let's guess that you prolly have like 13.5v available to work with...
If you end-up with a 1.5 volt drop total (for 12v at the lamp for the rated 100w output drawing 8.33A) then your wire will be dissipating some of the power that could'a been available to your lamps (about 12.5 watts (1.5 volts dropped at 8.33A) --

Go for what is most practical for you and your mounting situation - no need to run #00 welding cables for the lamps, but use some common sense as well -- watch the power dissipation of the wire. While the ~12.5W above seems large (afer all a curling iron, or small soldering iron only dissipates 15W or so), it's reasonable for a long run of wire and does give you 12V to these lamps so you'll get rated output...

A little bigger wire would allow you to provide a hotter line and produce more light, but who's to say how 13.5v on a 12v rated lamp (OK so not a HUGE overdriving, but), a 12.5% overvoltage condition , would shorte filament life -- we all know it'll be brighter, just for how long and how much extraordinary effort...?

IMHO, don't loose qty=2 volts and things should be fine (of course that implies a perfect ground as well! - the same current will have to move there)
 
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Okay so my goal would be to not go under 12v by time I reach my lights. Would 12g from batt to relays, about 6'. 14g for the rest of the power, relays to lights lets just say 10'. Wires for my switchs to relays 16g about 6'. If I move to 130 watt lights this should be okay. I plan on only 2 lights per circuit.
 
You're going to be using one relay for every 2 lights ? Right?

If you consider 260w(2x130W) per circuit that's about 22 Amps.

For 22 Amps, to get a 1 Volt drop requires 0.045 Ohms.

With 10 AWG, 0.045 Ohms is 45'.

With 12 AWG, 0.045 Ohms is 28'.

16 AWG is more than enough from switch to relay.


Consider that the connections themselves(and relay contacts) probably introduce more resistance than the wire itself. Given a choice -- solder -- don't crimp.


Handy Dandy wire resistance chart
 
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If you go to the local library, try and find an "IEEE Handbook". This book will have the recommended wire sizes for both DC and AC circuits (cars are DC, of course).

They will have charts that will show you the recommended wire size for a given length to achieve a give voltage drop.

This would be the best solution to your problem, or you could just guess, as I will now.

If you are running (4) 100w lights, I would use no less than 12 guage to feed both the lights and the relay. That's almost double of a regular auto headlamp, you know......
 
14 gauge would be okya for 2 lights -- obviously, 12 gauge would be better.

Remember that your total run is only the distance from the battery to the relay, and the relay to the light(s). The feed from a power tap to the switch to the relay is technically a separate circuit from the lights, and has very low draw. That can be 16 gauge, because it doesn't carry any lighting load. All it does is turn on the relay.
 
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