Rule of thumb for calculating watts and amps.

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

:icon_driving:

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

Quote:

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Originally Posted by 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

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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