# 12V – DC wire gauge | Does length calculation ‘reset’ at socket/ junction?

Sorry in advance that this is a basic question which is pretty obvious. I’ve just been struggling to find a clear answer and wanted to confirm.

I’ve set up a 12 V leisure battery system in my van to to power and charge various bits of kit. The wiring that I’ve done so far has been single length runs which I’ve sized (using online calculators) to reflect the length and max load (amps) expected.

I’m now trying to do a longer run to the back of my van (around 5 meters) where I’d like the option of plugging in another cable (about 4 meters) to take power outside.

My question is, when working out the required wire gauge, is the total length cumulative (i.e. 5m + 4m) or can I work out based on the distance to the socket? Then any extension cable is sized based on its own length and expected load.

I’m pretty sure that the answer is no, you need to size for the cumulative total as the voltage drop only cares about the amount of copper it needs to travel through from the source (if anything, a socket or junction will add more resistance).

If I assume that the answer is, you need to calculate based on the cumulative length from the source, my next question is this: How would I calculate the required gauge if mixing wire sizes? For example, I already have a 4 meter 8.5 mm2 cable which was sized to take an 15 amp load. If I wanted to plug this into the new socket at the rear of my van, how would I work out the gauge of wire needed to supply this socket?

Would I just work the voltage drop for both of these runs (e.g. 4m @8.5 mm2 + 5m @25 mm2 = 3.158% [15 amp]) and change the second wire gauge until I find a voltage drop that is acceptable?

• keep in mind that you can run a thick wire to a point (eg terminal/wago) where two smaller wires break off to separate loads, like a star network topology. The big wire needs to carry the load of both small wires summed. Commented Oct 3, 2023 at 21:44