Choosing Wire Gauge

[mjosbesh]

If I need to run 12VDC 20A through a 20 foot cable how do I choose my wire? Is there a calculator to help me decide the gauge and/or other properties? If i find I need 14 Gauge wire does it matter if it is 14 Gauge "speaker" wire or "Power" wire?

Finally, any recommendations for good wire and good prices?

 

[Harald Kapp]

Is there a calculator to help me decide the gauge and/or other properties?

Yes there is.

If i find I need 14 Gauge wire does it matter if it is 14 Gauge "speaker" wire or "Power" wire?

A 'power' wire is probably better insulated than a speaker wire.

 

[hevans1944]

Maybe this table will help or visit this Google page:

It also depends on how much voltage drop you can tolerate. The ampacity table just tells what is a safe amount of current a given wire gauge will carry for a given distance based on the temperature rise in the wire. The longer the wire, the hotter it will become the more voltage drop for any given current. You may also need to consider flexibility and resistance to oil and abrasion, depending on how you are using the wire.

Some people say 14 AWG "speaker wire" will safely carry 20A and the length doesn't matter, but the voltage drop along that wire as a result of the wire resistance increases with distance, so you won't have 12 V DC at the other end of a 20-foot length of wire if you pass 20 A through it. Look up ohms per thousand feet and calculate the resistance of 20 feet. Multiply by the current to find the voltage drop in one wire. Multiply by two to find the voltage drop in both wires (supply and return).

 

[duke37]

Hop
"The longer the wire, the hotter it will become for any given current."

I think not, the longer wire will have a bigger cooling area, the temperature will depend on the current but not the length.

Trevor

 

[hevans1944]

Hop
"The longer the wire, the hotter it will become for any given current."

I think not, the longer wire will have a bigger cooling area, the temperature will depend on the current but not the length.

Trevor

I believe you are right! I just copied the first pretty picture I found on the Google image page. Turns out this was from an automobile forum where the length in the table takes into account the voltage drop. Here is a better table more suited to electronics work:

This still doesn't take into account insulation type, but there is plenty of information available on allowable temperature rise for a given type of insulation. The OP will probably be okay with 14 AWG "speaker wire" for 20 A at 20 feet if the voltage drop of about two volts (both wires) at the far end is acceptable. I have no idea what voltage drop criteria were used to produce the original table I posted, but it "feels" pretty conservative.

Thanks, Trevor, for catching my mistake.

Hop

 

[BobK]

Yes, that table grossly overestimates the wire gauge needs. If it were true, I would need 4 gauge wire for some of the runs in my house wiring, when 14 gauge works perfectly well.

Bob

 

[hevans1944]

14 gauge works perfectly well.

Good thing your house wiring doesn't run on 12 V DC.

 

[BobK]

Good thing your house wiring doesn't run on 12 V DC.

Yes, because then I would need 150A or 200A in each circuit instead of 15 or 20, requiring probably 4 gauge wire. Ever try to bend 4 gauge wire around a screw terminal?

I calculate that 50 ft of 14 ga at 15A drops 1.89 V. At 150A it would be 18.9V, a little high for a 12V system.

Bob

 

[hevans1944]

Ever try to bend 4 gauge wire around a screw terminal?

No. I have a tough time trying to crimp a ring terminal onto 4 gauge wire. I do like fine-stranded 4 gauge welding wire for high current applications when I can find it at the right price (cheap) at Mendelson's. I purchased out of my own pocket about a hundred feet of it and took it to work one day, so they could temporarily hook up a large welder power supply to a three-phase feed some distance away. When they were all done the wire disappeared, probably sold for its copper. Haven't looked to buy any more of it.

 

[Kiwi]

I use these formulas for selecting wire size for automotive applications.

Volt drop = (0.0183 x L x A) / CSA

CSA = (0.0183 x L x A) / Volt drop

L - length of wire in metres(return length)
A - Amps
CSA - Cross Sectional Area of wire in mm2
0.0183 - copper resistivity constant. (Often see 0.0172 used instead)

Don't use AWG for wire size and feet are the things we stand on