Watt Guage Wire Confusion?

Thread Starter


Joined Oct 15, 2013
Hi all, I'm trying to make a battery pack and connect it to a 2000w inverter for my van. My confusion is this, many sites such as THIS state that for a 2000w inverter I need to use 4 AWG wire? I'm in the UK so everything is generally mm2 but not to worry. What I don't understand is this, a typical 4 gang extension cable from the mains can handle 13amp at 240v. This would mean its carrying 3120watts? The cable on this extension cable is 1.25mm2, equivalent to 16awg? Is it really because the wire from the battery pack needs to be far greater as it is sending DC at that point whereas the comparison from the wall is obviously AC?


Joined Jan 18, 2008
It's the current, not the wattage that determines wire size. 2000 W at 24V = 83 A (DC). 2000 W at 240 V (DC)= 8.3 A. Thus thinner wire is allowed for the higher voltage. AC calculations will generally use RMS values rather than DC values, but the principle is the same..

There are lots of tables for the ampere carrying "capacity" of wire. Here is one: https://www.pupman.com/listarchives/1998/April/msg00222.html The issue is heat (W = I^2R). Thus, superconductors can use considerably finer wire for the same current.


Joined Aug 7, 2020
It's about Prospective Fault Current. If there is a short circuit, enough current must flow to blow your fuse (you do have a fuse, don't you?). If the wire is too thin (or too long) then the short circuit current is lower and the fuse may not blow. If the fuse doesn't blow, then you have a fire.
Whilst we're on the subject of safety, don't forget to connect the Neutral to Earth before your residual current device. Otherwise you will have no effective shock prevention. Just because you've made your 230V from a 12V battery it doesn't make it any less dangerous than the 230V from the mains!

sparky 1

Joined Nov 3, 2018
In 1857 American Wire Guage system standardized wire based on mathematics used logarithmic steps to compute wire size.
Elites in the UK who also manufactured wire used a non metric system based on traditional machinery methods of drawing out wire.
Later in 1875 The Royal Society of London finally let James Watt residing in Glasgow become a member.
James Watt was able to compute whatever system in spite of big business involvement. That is why we use the conversion chart.

Mod: Political comment deleted.
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Joined Sep 24, 2015
Watts is the amount of power in a DC circuit. You can have 10 volts and 1 amp and you calculate the wattage to be 10 watts. OR you could have 1 volt and 10 amps. You still have 10 watts. Voltage times amperage equals wattage. Wire gauge needs to be in terms of gauge and amount of amps running through the circuit.

Those ultra high tension power lines that stretch across the country are in the tens of thousands of volts (if not hundreds). Yet the wire gauge is relatively small. Why? Because the voltage is so high, the amperage is quite low. Like the 1 volt / 10 volt thing I mentioned. At 1 volt and 10 amps - you need a heavier wire than with 10 volts at 1 amp.