When dealing with DC voltage figuring out which cable to use is critical. With DC voltage the cables have a resistance to the electricity flowing through the wire so the amount of electricity that comes out may well be significantly different from the amount that went in.

In a car the cable lengths are so short the thickness of the cable is less critical than when you are talking about running a 30m cable from one side of a house to another.

Let’s look at the numbers.

A 4mm (11awg) wire at 12v would be rated to take a maximum of 40A but if you pull 40A through it the voltage will drop by 4% over a length of only 6 feet, over 20 meters (66 feet) this would be 50%, enough to stop devices from working properly.

I am looking to only run mobile phone or at the most a laptop charger on the longest lengths of cable. These lengths would be no more than 22m(72 feet).

Because of costs I’ve decided to use cable rated for 25A, this cable has a Cross Surface Ares (CSA) of 2.5mm which is AWG13.

I’ve used this Voltage Drop Calculator to calculate the losses.

I estimate the load to be no more than 5A for the laptop charger, over the 72feet using AWG13 cable I will lose approx 9.8% of the voltage. This is not ideal but probably within the limits for charging the laptop because as the laptop charges the amount of power is needs will drop and therefore so will the losses. Mostly these lengths would be used for charging phones with a power draw of less than 0.5A

Because the two long lengths are going to be on either side of the same wall I will join the two lengths together at both ends to create a loop or ‘Ring Main’ this will increase the total CSA and will reduce the loses in the cable to an acceptable level.

This table shows how to convert the size of your wire from CSA/mm to AWG: Original Article

 AWG Diameter Square (mm) (mm2) 40 0.08 0.005 39 0.09 0.0064 38 0.1 0.0078 37 0.11 0.0095 36 0.13 0.013 35 0.14 0.015 34 0.16 0.02 33 0.18 0.026 32 0.2 0.031 30 0.25 0.049 28 0.33 0.08 27 0.36 0.096 26 0.41 0.13 25 0.45 0.16 24 0.51 0.2 22 0.64 0.33 20 0.81 0.5 18 1.02 0.82 16 1.29 1.3 14 1.63 2 13 1.8 2.6 12 2.05 3.3 10 2.59 5.26 8 3.25 8.3 6 4.115 13.3 4 5.189 21.15 2 6.543 33.62 1 7.348 42.41 0 8.252 53.49

The only other thing to consider is the possibility of connecting a 12v kettle up in the kitchen. I have some off cuts of 4mm (AWG11) cable rated at 40A. The run would be no more than 10 meters but with a load of about 150w which at 12v is about 12A. I can expect to lose 11% of the voltage on this which will be more noticeable on a device like a kettle as with a lower voltage it will take longer to boil.

This chart is useful for working out what power you can pull through a size of cable over what distance: Original Article

 Length Current (amps) (feet) 5 10 15 20 25 30 40 50 60 70 15 16 12 10 10 8 8 6 6 4 4 20 14 12 10 8 8 6 6 4 4 4 25 14 10 8 8 6 6 4 4 2 2 30 12 10 8 6 6 4 4 2 2 2 40 12 8 6 6 4 4 2 2 1 1/0 50 10 8 6 4 4 2 2 1 1/0 1/0 60 10 6 6 4 2 2 1 1/0 2/0 2/0 70 10 6 4 2 2 2 1/0 2/0 2/0 3/0 80 8 6 4 2 2 1 1/0 2/0 3/0 3/0 90 8 4 4 2 1 1/0 2/0 3/0 3/0 4/0