Posted by John T on July 19, 2014 at 07:37:12 from (216.249.72.121):
In Reply to: Re: 240 VAC Wiring posted by David G on July 19, 2014 at 07:05:48:
David, Good point, and the two (wire size PLUS insulation) are related as I was taught years ago. Here's my more complete take on it now that you brought it up as I best recall, but that was yearssssssssss ago lol so again NO WARRANTY.
You could rate a stand alone bare wires ampacity considering how much current it can carry BEFORE it might heat to the point it looses mechanical strength or integrity or a certain degree of conductivity.
NEXT when you encase it with insulation, the insulating materials (such as a THHN thermoplastic) design and quality has a certain rating such as how much current the enclosed wire can carry BEFORE the insulation degrades or deteriorates and at what current is there enough heat generated which could degrade or overheat or melt the insulation.
I see it takes BOTH wire size and wire type PLUS the physical and chemical characteristics of the insulation all coupled together to achieve a final "ampacity" rating. If you have No 6 bare copper wire in air it may have a higher ampacity rating then if it were covered with a thermoplastic insulation which heats up the more current the inner wire carries, therefore if insulated you may not be able to conduct quite as much current because you have to consider the chemical property of the insulation and how much heat before it degrades.
If you look at NEC ampacity charts, there are columns based on ambient temp,,,,,,,,, type of insulation (like THHN),,,,,,,,,,,,if enclosed,,,,,,,,,,,,if in free air or if buried or how may conductors inside a conduit etc etc SO I SEE IT AS WIRE SIZE PLUS INSULATION PLUS SURROUNDINGS ALLLLLLLLLLLLLL WHICH DETERMINE AMPACITY
That all has to do with current. The max voltage rating has to do with the insulation such as the type and thickness and wire spacing etc etc.
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