Use automotive electrical wire, not house stuff. As long as you don't zip tie it to the exhaust manifolds you will be fine (j/k). Common sense is king here, keep the wires away from really hot stuff ie exhaust manifolds, there is a lot of wire under the hood already. If you want to keep it clean, orderly, and protect it a little put wire loom around it.
Not to be a smartas, but NEMA has a table just for that. As long as you stay under or at 90*C insulation will be ok (for THHN). I list THHN because its the most common.I use Teflon, type EE mil grade; good to over 200C, 400C to burn off.
Just don't pull the wireties too tight, lol.
bowez, I've never known the current carrying capability of wire to decrease with temperature, just the insulation degrades...
Just don't use solid wire, it will break from vibration.
Good, cause that's not what that table says.Not to be a smartas, but NEMA has a table just for that.
I just posted the resistance thing cause it was brought up, I didn't think the engine bay would be hot enough to cause issues, however I am working with mV power, so if there is going to be a slight change in voltage due to heat, even small, it may affect the meters I'm working with.Good, cause that's not what that table says.
You have to 'Derate', or lower, the amount of current you run thru a wire at higher temperatures; it won't lower the current for you, it will catch on fire. (as CrystalPistol said)
The resistance does rise with temperature, but not enough to matter to the copper wire (or the circuit) over any temperature you will run into on a car.
Copper wire in a vacuum can go incandescent and not lose current carrying capability; as long as it doesn't short to something it's fine. I've built equipment that did that, but we weren't using copper wire; Iridium wire lasts longer. The resistance increase actually helps a bit, as it acts as negative feedback; at ~900C copper is a good resistor... but I digress.
The additional temperature rise due to self-heating in combination with the ambient temperature causes the insulation to melt, then a short to form; that's the problem, and the reason for a chassis wiring rating and a bundle wiring rating in the code book. (if a bunch of wires are close together, like a cable or wiring harness, they will get hotter from shared heat and a reduced cooling area, making them melt easier)
If anyone care, here's a link to a decent sizing table for wiring; there's a table at the bottom for teflon based on a 70C ambient; that's a reasonable max temperature under a hood. A search will find a table for other insulators.
There is some good additional info in the notes at the bottom.
Bowez, I guess maybe you were talking about the insulation, but I didn't read it that way. :zdunno:
If you worry about the ambient temp + self heating being too much, use a bigger diameter wire; less self-heating from the current...
The error in the reading is proportional to the resistance of the wires in relation to the resistance of the meter...I just posted the resistance thing cause it was brought up, I didn't think the engine bay would be hot enough to cause issues, however I am working with mV power, so if there is going to be a slight change in voltage due to heat, even small, it may affect the meters I'm working with.
I don't think it should cause an issue though, I just didn't want crumbly wires after 2 years or something.