Small AC motors and Inverters
Disappointed that my ancient Mod Sine Wave inverter (140W cont, 200W surge) shuts down with a 120W coffee grinder. Works ok with the 400W (cont) MSW inverter built-into the truck, though a little ...
Forum Discussion
BFL wrote:
“I don't understand ampacity amounts for different gauge wires since they don't say (or I missed that) how long the wire is. AFAIK if the wire is shorter it can do more amps at the same gauge. “
“Thanks. I still don't understand when they say #4 is good for 100 amps if that means 3 feet of #4 or 30 feet of #4. “
I'll try to confuse you even more!
Ampacity is a rating to do with heat (power). A current flowing in a wire will heat it up. The amount of heat is determined by the current squared times the resistance of the wire per unit length. If the wire is 1 foot long and has 1 Amp flowing it will reach a set temperature. (Temperature will vary depending if it is in free air or in an enclosure like conduit.) If the same size wire is 10 feet long and has the same 1 Amp flowing in it the wire will reach the same temperature along the length of the wire as the 1 foot long wire. The amount of power dissipated (heat) is the same for every unit length of wire so the length of the wire has nothing to do with how hot it gets. The total power dissipated in 10 feet will be 10 time that of 1 foot but the power per unit length is the same. The ampacity it a rating of how hot the wire is allowed to get.
This is completely different than voltage drop across the wire which is a function of length and current.
“I don't understand ampacity amounts for different gauge wires since they don't say (or I missed that) how long the wire is. AFAIK if the wire is shorter it can do more amps at the same gauge. “
“Thanks. I still don't understand when they say #4 is good for 100 amps if that means 3 feet of #4 or 30 feet of #4. “
I'll try to confuse you even more!
Ampacity is a rating to do with heat (power). A current flowing in a wire will heat it up. The amount of heat is determined by the current squared times the resistance of the wire per unit length. If the wire is 1 foot long and has 1 Amp flowing it will reach a set temperature. (Temperature will vary depending if it is in free air or in an enclosure like conduit.) If the same size wire is 10 feet long and has the same 1 Amp flowing in it the wire will reach the same temperature along the length of the wire as the 1 foot long wire. The amount of power dissipated (heat) is the same for every unit length of wire so the length of the wire has nothing to do with how hot it gets. The total power dissipated in 10 feet will be 10 time that of 1 foot but the power per unit length is the same. The ampacity it a rating of how hot the wire is allowed to get.
This is completely different than voltage drop across the wire which is a function of length and current.
