# Choosing the right type of wire?

I am going to have a something thats runs on 120vac pulling 1-2amps is there a chart or something for choosing the right gauge of wire?

The wire will run from an inverter in the front of my car(by the battery) to the back (near the taillights).

I need the smallest diameter wire possible.

• How long a run of cable are we talking here? – Majenko Jul 16 '11 at 21:29
• The smallest diameter depends on the temperature rise you accept. – stevenvh Jul 17 '11 at 5:53
• @Matt - Most likely irrelevant. For thin wire temperature rise becomes more relevant than voltage drop on 120V. – stevenvh Jul 17 '11 at 5:55
• @stevenvh true, but if we're talking 3 miles then it does become an issue... – Majenko Jul 17 '11 at 9:55
• The wire will run from an inverter in the front of my car(by the battery) to the back (near the taillights). – AndrewFerrara Jul 17 '11 at 11:58

There are charts that give properties for different gauges of wires. However, you haven't said what your criterion is so there is no way to give a single answer.

One spec might be how much voltage you are willing to let the wire drop. At your 2A current you can compute a maximum resistance from that. With the Ω/foot from a chart, you can compute the maximum length of each guage of wire your system can tolerate.

Or, your criterion might be maximum temperature rise of the wire. There are charts for temperature rise as a function of current for various wires.

For house wiring there are legal requirements too. These are usually based on temperature rise. Each gauge is specified for the maximum fuse or circuit breaker that must be on that circuit.

• The wire will run from an inverter in the front of my car(by the battery) to the back (near the taillights). – AndrewFerrara Jul 17 '11 at 11:57
• It may not be a good idea to put an inverter in the engine compartment. Automotive engine electronics has a higher temperature requirement (150 C max) than consumer electronics (70 C max). Strictly looking at your current requirements, 18 AWG could do just fine, but most 18 AWG wire won't be rated for that voltage. In a house, you'd be required to use 14/2 wire (14 AWG, 2 conductors plus ground), but I have no idea if THHN is rated for auto temperatures. – Mike DeSimone Jul 17 '11 at 12:09

The maximum current a wire can carry depends on several factor:

1. wire diameter, obviously (Kortuk may take exception on me using the word obvious again, but this time it really is; anyone older than 10 will understand that thinner wires will allow less current through them)
2. wire material. Often copper, but in some applications where weight is paramount aluminium may be used. Think overhead high-voltage power lines.
3. insulation. The energy dissipated in the wire will cause a temperature rise, but the heat will also be given to the environment. The type of insulation determines the amount of thermal isolation. It also determines the maximum temperature allowable. Think melting of the insulation. edit: insulation will also determine maximum voltage. The insulation may breakdown when too thin in combination with a high dielectrical constant.

This document gives the maximum current for copper wire as a function of diameter and temperature rise in free air at 30°C ambient temperature.

• The wire will run from an inverter in the front of my car(by the battery) to the back (near the taillights). – AndrewFerrara Jul 17 '11 at 11:58
• The insulation also determines the voltage rating. You'll probably want at least 250 VAC. – Mike DeSimone Jul 17 '11 at 12:10
• @Mike - It's an automotive application, so voltage is probably less than 250V. But I agree it's a factor for the insulation. I'll add it to my answer. – stevenvh Jul 17 '11 at 12:13
• He's running the 120V output from an inverter on this wire. 250V is only a FOS of 2. – Mike DeSimone Jul 17 '11 at 12:19
• @Mike - OK, I missed the 120V, I was looking at the car :-) – stevenvh Jul 17 '11 at 12:22

As Olin notes, if this is for house wiring it will need to meet regulatory limits.

If safety is a concern you may be concerned with eg temperature rise.

"Smallest diameter possible" may be to fit in a duct etc - you need to say WHY for this to be able to be answered well.

Here is a table that will both help and confuse. Note that at lower current levels there are two standards that vary widely. See also the note re contacting your local electrician re legality.

http://www.powerstream.com/Wire_Size.htm

All that said, if smallest diameter is really your aim, then something like 26 gauge AWG is liable to be "fairly safe".