Rule of thumb for calculating watts and amps.

[Robo]

If your fitting say those new driving lights you just purchased, what size wire,relay fuse do you use.
This is fairly easy to calc.

For example
watts divided by volts = amps

EG , 2 x 55w spotties = 110watts

110 watt div by 12 volts = 9.16666 amps. total load amps.

each lamp= 5 amp.
main power= 10amp

and this works in reverse also.
total amp load x volts = watts
9.166666667 amps x 12 volts = 110watts.

so if you know either, watt or amps and volt you can calc what you have. easy.

good idea/practice to allow say 20% increase in cable and relay handling capacity and install correct fuse to protect system.
this is a rule of thumb and only a guide and does not take into account things like distance, voltage drop etc.
heavy load things like a winch or fridge in back, seek out someone qualified.
IF YOU HAVE ANY DOUBT ASK SOMEONE QUALIFIED.
I take no responsibility.

I'm sure someone here knows more about taking into account surge load affect.
And can add more to this info I posted

[Ben-e-boy]

its ohms law........volts(v)=Current (I) / resistance (R) and Power (P) = Volts (V)/Current (I)


For surges and stuff like that you would have to do fault loop impedance and fault current equations. not something I would bother with on 12v system

[Robo]

its ohms law........volts(v)=Current (I) / resistance (R) and Power (P) = Volts (V)/Current (I)


For surges and stuff like that you would have to do fault loop impedance and fault current equations. not something I would bother with on 12v system

thanks for also throwing in, could you explain how ohms law is used/works for us dummies please.
reason I mentioned surge so, say you/someone were able to explain what's needed to take into account for this calculation for surge application like (fridge or winch) as start-up draws a great deal more current and generally drops back, except for the winch scenario.
Cheers