A Little Necessary Theory 

Time for a little electrical theory now. In order to size your fuses and select thick enough wire, you need a couple of formulas:

Power (Watts) = Volts x Current (Amps) or Current = Power/Volts

Power = Current x Current x Resistance

Volts = Resistance (Ohms) x Current (Amps) or Current = Volts/Resistance

OK, three formulas!

A 12 Volt car system is not actually 12 Volts, its closer to 14 (13.8 officially). So, to work out the fuses add up the power used by all items in the circuit. Say you are doing park lights. You have two tail lights at 6 Watts (written on the lamps), 2 number plate lamps at 3 Watts, 2 front park lamps at 4 Watts, 6 instrument lamps at 2 Watts and 2 interior lights at 6 Watts. This totals 2x6 + 2x3 + 2x4 + 6x2 + 2x6 = 50 Watts. At 14 Volts this equates to 50/14 = approximately 4 Amps, so use a 5 Amp rated fuse and wire.

If you don’t know the power (Watts) a motor uses, say a fan motor, use the resistance (Ohms) function of your multi meter and measure the resistance of the motor. Say it is 3 Ohms, then the current used at 14 Volts is 14/3 = about 5 Amps, so use a 5 Amp fuse. Alternatively if there are a few things in the circuit add up the Watts. To work out the Watts the motor is, Power (Watts) = Current x Current x Resistance = 5 x 5 x 3 =75 Watts. Then you can add it to the others to get a total number of Watts in the circuit, and convert it back to Amps.

Wire Current Ratings 

Conductor		Loomed Wires		Unloomed Wires
Cross Sectional Area (mm2)	Number & Diameter of Wires	Current Rating (Amps)	Volt drop per Amp per Metre (mV)	Current Rating (Amps)	Volt drop per Amp per Metre (mV)
1.0	1@1.13	11	40	13	40
1.5	1@1.38	13	27	16	27
2.5	1@1.78	18	16	23	16
4	7@0.85	24	10	30	10
6	7@1.04	31	6.8	38	6.8
10	7@1.35	42	4.0	51	4.0
16	7@1.70	56	2.6	68	2.6
25	7@2.14	73	1.6	89	1.6
23	19@1.53	90	1.2	109	1.2

The way to interpret this table is as follows - if you are using two 130 Watt head lamps fed from a single wire that is 3 metres long then you will draw (130 x 2)/13.8 = 18.8 Amps (A car is closer to 13.8 Volts than 12). If you use say 2.5 mm2 wire (this rating because the wires are loomed together) then the voltage drop is 16 x 18.8 x 3 = 902.4 millivolts which, since there are 1000 millivolts in a Volt means you are only getting 13.8 - 0.902 = 12.9 Volts to your lights.

As a starter motor is easily rated at 1 kilowatt if the battery is in the boot, say with 4 metres of wire between it and the starter motor, then you have 1000/13.8 = 72 Amps current draw (and more once the motor gets hot, which it will if your car needs a bit of cranking). Say you used 16 mm2 wire ‘cos you bought the closest one to what you needed (and the wire is not loomed up), so now your voltage drop is 2.6 x 72 x 4 = 748.8 mV, so you still get 13 Volts to your starter motor - provided your battery is in perfect health. If you had used 10 mm2 wire you would be down to 12.6 Volts - see how quickly you can get to the point of not being able to start your car?

Basically if you size the wires according to the above table you will be fine. So, if you need a wire to take 7 Amps in a loom then you need only 1 mm2 wire, which is actually the cross sectional area - 1.13 mm is the diameter, which the table also gives. Note that the bigger wires are made up of a number of smaller wires twisted together. The number of strands will often be more in a car - these tables are normally for domestic wiring which has thicker strands. The maximum loadings still apply.

Wire 

5 and 7 core trailer flex is about the best thing around for doing the wiring runs to the front and rear of the car. Standard configuration is rated at 14 Amps and you can get heavy duty rated at 18 Amps. You will rarely need more than 7 wires to the rear of the car, and you can always run two lots of 5 core. Regardless, it prevents having to loom the wires together and can look very neat and tidy. 14 Amps is quite sufficient for 130 watt head lamps as well, provided left and right are separately fused.