Can 18-20 gauge wire, pass lots of amperes, if for very small distance??

[settra]

hello guys. i am building a board for my car, and i have some problems. i have trouble creating it, becouse, the wires i will use, will have some ampere (5 max), so i have to use big ones. but i wander.
what is the smallest wire i can use, for etc 5 amp, if the total run of the wire is 1cm - 5cm (max 10cm) .
also, how bad is it, if the connections between the cables are done with solder? (no BIG contact from wire to wire)

can anyone clarify this for me? cause i am prety confused... thnx!!

 

[jpanhalt]

The ampere rating for wire is based mostly on the temperature rise to be expected. If running the wire hot is OK, then you can use it for more current than what a table shows. Unfortunately, resistance for plain copper goes up with temperature, which adds to the increase in temperature (i.e., I^2R). There are numerous charts for finding wire size and current capacity. Here is one: **broken link removed**
Here is a tool to calculate track width for PCB traces in which you can enter the allowed temperature rise so you can see the effects on ampere capacity. I do not have a similar tool for regular wire, but I suspect something exists on the Internet that will do that calculation for you.

https://www.desmith.net/NMdS/Electronics/TraceWidth.html

Thus, 18 awg is OK for 3.7A, 16 awg is OK for 5 A, and so forth for coils. Since temperature rise is the limiting factor, remember that a bare wire in the open will be able to carry more current than an insulated wire buried with other things.

John

 

[BeerBelly]

I wouldn't be afraid to use #18 wire for such a small distance unless the circuitry was very voltage sensitive.
Solder connections are very reliable.

 

[Mike odom]

The ampere rating for wire is based mostly on the temperature rise to be expected.

Thus, 18 awg is OK for 3.7A, 16 awg is OK for 5 A, and so forth for coils. Since temperature rise is the limiting factor, remember that a bare wire in the open will be able to carry more current than an insulated wire buried with other things.

John

18AWG better be good for more than 3.7A! The wire tables say 14A with 90°C insulation. I just designed a cable using 18AWG to a board generating 100W @ 12V, or about 8.33A.

You can push 1A through a 30AWG with no ill effects, 'cept voltage loss over (extremely) long distances.

And yes, done properly, solder connections are very reliable.

 

[jpanhalt]

There are different rating criteria. Here's another link that is similar to the one I already posted: https://www.powerstream.com/Wire_Size.htm

However, for automotive use, this source is consistent with what you used: https://www.tessco.com/yts/industry/products/itm/automotive/get_wired.html

I will update my references accordingly.

John

 

[MrAl]

Hi,

Very thin wires are used to connect IC chip dies to package pins. When higher current is required multiple wires will be used, sometimes brought out to separate pins.

So one way to get a higher current rating is to use parallel wires of the same diameter.

Very short runs of 2 inches can be done with much finer wire than say 2 feet. The whole ballgame is how much surface area vs how much power in the wire considering the voltage drop and current, and how much heat is conducted out of the ends and how much heat sinking at the ends.

The fusing current for a #32 gauge (AWG) wire is about 7 amps, and that roughly doubles for every 4 wires sizes lower, so #28 would be about 14 amps. So you'll have to stay under that rating and see if the application can put up with the extra resistance because of the increase in resistance with temperature, and also to make sure that it doesnt get so hot that it starts a fire.

Testing isnt too hard to do however. All you have to do is pump some current through the wire and see how hot it gets This is easy to do with a decent power supply, or with a big battery and a suitable power resistor.

The best test is done right in the application itself, after it is fully assembled and enclosed. After running for a few hours if the wire gets too hot it's a no go of course. This is better than trying to calculate the right size because it's hard to figure out the real amount of cooling that the wire will (or will not) get.
A fan will help too of course, and for very short runs if the ends are connected to large chunks of metal that will help too because they act like heat sinks.

Solder though is not a very good conductor of current. It's about 5 times worse than copper if i rem right. So any joints should have the wires touching before the solder is applied. This usually means twisting the two wires to be joined or using a PC card with a hole drilled into it where the wire is pushed through and bent over on the copper foil side.