From: MV (MV@dc9.tzo.com)
Date: Sat Jun 18 2005 - 14:27:52 PDT
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
J. Forster wrote:
> As I remember the NEC wire sizing is based on allowable voltage drop in 120 Volt
> systems. The allowable voltage drop in a 12 Volt system, at a given current,
> would be much less, hence the requirement for a larger wire size.
>
> e.g.: A 12 V = 10% of 12 V but 1% 0f 120 V
>
> -John
>
>
> MV wrote:
>
>
>>The chassis amps numbers in that chart looks reasonable. The power
>>delivery amps is way way off. Even the NEC is much less conservative
>>than those numbers.
>>
>>Dave
>
>
>
>
>
This archive was generated by hypermail 2.1.4 : Fri Oct 28 2005 - 22:47:29 PDT