wire size question

rama · April 2012

Hi,
Just curious - I know the awg12 is typically said to have a max ampacity of 20A, but this is based on 110AC system, right? I've read somewhere that on a 12v DC system awg12 stranded cables can actually carry current up to 41 amp or so... is this true? I'd like to know because a solar charger I have is rated for 25A output @ 12V and the terminal size is designed only for up to awg12.

Also is awg12 4.0mm2 or 3.3mm2? I have come across both data in different references.
Thanks in advance.

Windsun · April 2012

Re: wire size question

The max current is based on wire heating and if in free air, conduit, etc, not voltage drop, so while #12 may be fine current-wise for 20 amps @ 12 volts DC, you would have a pretty high voltage drop on longer runs. For low voltage systems you have to go by the voltage drop, not just the max amperage rating.

There is no difference in the current rating between solid and stranded wire.

waynefromnscanada · April 2012

Re: wire size question

A drop of 2.4 volts would not be noticeable on a 120 volt circuit. Have that same 2.4 volt drop in a 12 volt system and there goes 20% of your power!

techntrek · April 2012

Re: wire size question

rama wrote: »

Hi,
...max ampacity of 20A, but this is based on 110AC system ... 12v DC system ... carry current up to 41 amp or so... is this true?

Also is awg12 4.0mm2 or 3.3mm2?

First, here is a link to the wire gauge chart on Wiki: http://en.wikipedia.org/wiki/American_wire_gauge

For wire, when voltage drops, amperage carried must go down too. So while 20 amps is ok at 120 volts (over a short distances), dividing the voltage by ten means you must divide the amps by 10. There are LOTS of variables at play, the best thing you can do is bring up an online voltage drop calculator and play with it for 10 minutes. The best I've seen is on a competitor's web site so I won't link to it, but if you do a search for "voltage drop calculator", look for a web address with "nooutage" in it. You'll see conduit, expected operating temperature, length of wire, AC vs. DC, aluminum vs. copper, all affect the result.

mike95490 · April 2012

Re: wire size question

techntrek wrote: »

First, here is a link to the wire gauge chart on Wiki: http://en.wikipedia.org/wiki/American_wire_gauge

For wire, when voltage drops, amperage carried must go down too. So while 20 amps is ok at 120 volts (over a short distances), dividing the voltage by ten means you must divide the amps by 10. .....

Huh ?? I'm ok if you replace amperage with wattage. Or else I'm lost.

Late, Fuzzy, too much 3-phase brushless motor controllers at work this week.

techntrek · April 2012

Re: wire size question

What I meant was, given the same gauge wire and wire length, when voltage drops, amperage must drop, too. For instance, I just plugged in 120 volts, 50 feet, 12 gauge, 20 amps - which shows a 3.3% voltage drop, acceptable. Change one variable, 120 volts to 12 volts, and now you have a 33% voltage drop. You either have to drop the amperage, or increase the wire size to get back to an acceptable voltage drop.

mike95490 · April 2012

Re: wire size question

techntrek wrote: »

What I meant was, given the same gauge wire and wire length, when voltage drops, amperage must drop, too. .... .

Unless you run a motor, which, with lower voltage, will lessen it's internal impedance, and increase it's amps. That's why electric companies run higher voltage on lines (125 - 130V) till someone complains - it reduces the amp draw.

If you run a passive load, like a resistor, then voltage and amps track as you say. Active loads, well, they are active and can do anything.

techntrek · April 2012

Re: wire size question

Same rules apply, just in your case (motor) the peak amperage is coming from a low power factor, so the VA is more important than the watts. If the PF is corrected at the motor then VA and watts will be about the same. Amps and volts are the driving factors in choosing a wire size.

wire size question

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