From: J. Forster (jfor@quik.com)
Date: Sat Jun 18 2005 - 16:15:46 PDT
Actually, we may both be right. I can't find my NEC book right off, but branch
circuit the fusing is certainly to protect the wires and devices. It is possible the
ampacity of wires is heating limited, rather than voltage drop limited.
That is NOT the case for low voltage circuits. See":
http://www.poweriseverything.com/impt-info/v-a-tablespage.html
-John
MV wrote:
> The NEC is actually driven by the NFPA, the National Fire Protection
> Association. The NEC centers on preventing fires from faulty wiring.
> So they really come from a standpoint of how hot will wiring joints and
> wire get when XX amps run through it under ___ conditions. Hence their
> deratings for running wire in bunches, or conduits - the heat doesn't
> dissapate and can start a fire if the wire gets too hot. On the
> otherhand overhead wires you can push the heck out of since they are in
> open air.
>
> Interesting enough, per NEC code, you can run 40 or 50 amps through a
> 220 single phase circuit with 10 gauge wire, if it is dedicated to a
> welder with a 20% duty cycle. The welder only draws a lot of power for
> short periods of time, so the supply wires have time to cool off between
> welding sessions. The NEC is that centered on heat/overheat/and fire
> prevention. And that makes sense to me.
>
> If this chart was concerned about voltage drops, then they wouldn't be
> suggesting 40 amps in a 12 gauge wire for chassis wiring in open air.
>
> FWIW,
>
> Dave
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