# What wire do I need to carry 30 A for 18"?

I have an 11.1v lithium polymer battery pack which has 1300 mAh capacity and has a 20 C rating. Therefore I figure the maximum amperage the battery can supply is 26 A. (C * mAh / 10001) To be safe, I figure the wiring I use with it should be able to handle 30 A in case a higher capacity battery is used later. I figure that I can safely use an ~11.1 V, 1500 mAh, 20 C battery at that current.

The circuit this will be used in has a momentary switch (trigger) and a DC motor. (Airsoft gun wiring diagram) There will be about a foot and a half of wire from battery terminal to battery terminal. The circuit under nominal conditions would be under load for no more than half a second every few minutes. However it could potentially be under continuous load for up to 5 seconds with an average of those cases being 3 seconds.

The problem is, the wiring has to fit through small passages. And I need to use the smallest possible wire gauge that will handle the current. I currently have some stranded copper 18 AWG wire for automotive use. I'm pretty sure that this will not handle the current this battery is likely to supply3. From memory, (without measuring the clearance of the passages) 18 gauge wire seems to be an ideal size, though I think I really need 14 gauge wire to handle the current if it's just stranded copper.

I have trouble finding any easily understood documentation on the subject and there are so many variables to consider. Such as the length of the wire, the diameter of the strands (if stranded), the number of strands, the coating (if any), the ambient temperature, etc.

My research leads me to believe that if I were to get some 18 or 16 gauge wire with silver coating and a thin insulator (teflon?) that it would fit the bill. What is the proper gauge (solid or stranded), coating, and/or insulation for this application?

Update: I added "duty cycle" information to paragraph 2.

• What's the duty cycle of your application? If you're just pulsing the trigger for 100ms every 5 minutes, you don't need to worry much about wire heating, which will be an issue with higher (or continuous) duty cycles. – Kevin Vermeer Feb 8 '11 at 19:48
• There's a possibility for continuous operation for probably no more than 3 seconds at a time. The user could potentially hold the trigger down indefinitely. (they should know better) I added this information to paragraph 2 in the OP. – sholsinger Feb 8 '11 at 20:15
• Your assumption of 30 amps may not be correct. Just because the battery can supply that current, does not mean that it WILL do so - that is dependent on the load it sees. A simple test to see how much current the motor draws is needed here. You may find that the motor only draws a couple of amps, meaning you can use 18 gauge with no problem. – Norm Feb 28 '18 at 16:03

Over 18 inches you would drop about 0.675 volts with 18 gauge wire. 12 gauge would drop about 0.1 volts. You would need to use 6 or 7 gauge to handle 30 A over an extended period.

If you are not using continuous current, but pulses, you could get away with using a smaller gauge wire. However an undersized wire would heat up significantly in just a few seconds. Unless you are sending pulses of less then 1 sec, you will need a large diameter wire.

AS for the insulation, it depends on how the wire is being used and how it is routed. 30 A is a lot of current. If some one can come in contact with it, you need heavy insulation for safety. Especially if it flexes. If it is total enclosed in a non conductive housing, you could use thinner insulation.

• @Jim - He specified an 18" run. That might be doubled (out and back) to the battery, but it probably wasn't going to be 6'. – Kevin Vermeer Feb 8 '11 at 20:59
• I specified 18" from + to - on the battery. That is all-inclusive of the entire circuit. Granted, there's a motor in the middle adding resistance. The trigger switch is a sliding plastic plate with copper on the end which makes contact with another copper plate. I'm not sure if that would add significant resistance or not. (I assume motors add significant resistance) – sholsinger Feb 8 '11 at 21:22
• I read that you can use a smaller wire if it is plated with silver as it has a slightly lower resistance than copper, is that true? – sholsinger Feb 8 '11 at 21:25
• Why does current matter with respect to safety? Isn't it just voltage (electrical risk, not thermal)? – tyblu Feb 8 '11 at 21:40
• Silver plating won't help noticeably at DC as it's too thin - it's only useful at RF where more of the current flows near the surface. – mikeselectricstuff Feb 9 '11 at 12:40

If space is tight check out Alpha's Ecowire, which has thin but very tough insulation, so the overall diameter is less than PVC wire of the same copper cross-section.

• Is there anywhere that you know of that I can get it in lower quantities than 100ft spools? – sholsinger Feb 9 '11 at 14:54