Electricity Explained

Stolen from the comments on an AvE video…

(Dan Burch, EvilGTV & Snoggatog, if any of you ever see this page & have a problem with my posting it here… Give me a valid reason to take it down.)

Dan Burch

You sound like you might be an engineer. Maybe I don’t understand as much as I thought I do about electricity. Isn’t 12ga wire for 20 amp circuits at 120v. 120v @ 20amps = 2400 watts. The charger puts out 12v @ 80amps = 960 watts. So the 12ga battery cable wire is plenty big enough. My 180amp welder also has 12ga wire powering the thing. 24v welding voltage @ 180 amps = 4320 watts. 240v @ 20amps = 4800 watts. I’m thinking that 12ga wire is rated for 20amps on 120vac circuits but not for 240v circuits. 240v circuits would be limited to 10amps with 12ga wire. In other words, 12ga wire should not be used to carry more than 2400 watts (minus 20% safety factor).


You’re coming at it from the wrong end, unfortunately. Watts are a calculation of total electrical energy expended, including heat.

Current is a measure of how many pixies are running in the tunnel, and voltage is the measure of how pissed off they are, and how much they want to escape. In cable sizing, voltage is only relevant to the insulation class – to stop the pixies chewing their way out before they get to the end. Resistance is the one you’re missing, and it’s the opposition to current flow – in other words, how big the tunnel is the pixies are running through. Small cables have high resistance per metre, and resistance drops as cable size increases. If the pixies are running in a small tunnel, they’re going to crash into each other, trip over, and generally start brawls – in a large tunnel, they all have their own space.

P= I^2 x R (If you’re not familiar with this version, P=VI, and V=IR. thus P=(IR)xI,). So if we say that our length of 12ga has a resistance of 0.01 Ohm (according to wikipedia, that would be a little over 6 foot long). P= 80 A x 80 A x 0.01 Ohm = 64W being consumed just by the cable. Have you ever made the mistake of trying to unscrew a 60W lightglobe after its been on for a while? By comparison, at 20 amps, that 6 foot length of cable is only drawing 4W.

Notice we haven’t got to voltage yet… That 64W requires a voltage to exist, doesn’t it. So, P=VI, or rather V=P/I. 64 W / 80 A = 0.8 V used in the cable. So if the battery charger is outputting a regulated 13.5V, you’re only getting 12.7V at the battery. Or in your 120V circuit (pretending a 6 foot cable run is useful in a house), 4 W / 120 V = 0.033 V lost from your supply voltage. In a 240V circuit at the same 20 amps, 0.016V drop.

This is the reason why the electricity distributors cables look so small for how much they feed – they use transformers to bring their output up to a very high transmission voltage (apparently 500kV for long haul in the US, and 66 or 115kV as it gets into local substations). Voltage goes way up, current goes way down – thus smaller cables required, and as an added bonus, much less voltage drop.?


EvilGTV i hope to be as knowledgeable and well articulated as you one day?