this kinda went full circle. we are at the point where the #'s as-is are known to be overly conservative.
:lol: In a way it did. When it comes to exposed conduit or wire run in walls without insulation without a doubt the NEC is highly conservative. In fact if one adjusts 2.08mm2 (14 gauge) to 2.5mm2 wire clipped direct, #14 would be legally allowed to carry 24 amps continuously at 70*C.
trying to figure out how it got like that. NEC doesnt seem to do any real-world testing on their own, its a voting committee that reads proposals, looks at submitted test data, and then decides what should be ratified into the next version of NEC.
Looks to be so. What also I am wondering (and I ask anyone reading if they can help) is what drove the NEC to go from 60*C insulation to 90*C insulation in MN wire.
if the current method is to use the N-M calc method then show me where the N-M #'s get adjusted to arrive at new conservative #'s. this is the piece i cant find.
To be honest at this point I think we may never know, especially considering that the tables when originally derived were based around old rubber insulation which is where (guessing) the safety factor may have come from. Rubber insulation is pure trash when compared to THHN-THWN2. This is not to say its not a valid question, but to be honest I just dont know.
Personally, I think table 310.14 needs to be revisited and studied in depth more then ever. With wire insulation becoming better, building thermal insulation evolving, green sustainability now a concern, coupled with the volatile prices of copper; re-evaluating wire ampacity is a smart idea.
I think the NEC should go for a table based on the wire's installed ability to dissipate heat, adjusted for bundling/ambient temps, then checked against voltage drop. This imo would result in the best utilization of copper.
I also think a few new wire gauges should be created for common use applications were the actual wire current capacity tends to be higher or lower then what the typical application calls for. An example is 30 amps. In the IEC there is a bridge between 4 and 6 mm2 wire. 4mm2 is rated 26 amps in contact with thermal insulation, yet 6mm2 at 34 amps. Thus a bridge exists. In the CEC, a 30amp electric heater would also see that same gap with #12 (3.31mm2) rated 25 amps and #10 (5.26mm2) rated 35 amps. Thus, in both cases creating a new wire gauge may be a feasible investment. Something like like 4.62mm2 would be just right, allowing copper saving for a very typical current carrying requirement. This also extends to what the link mentions. If the AWG system was put in the IEC (and visa versa) electricians would in theory have more wire gauges to choose from to best suite ampacity and voltage drop.
This, of course, is meaningless without testing. This where you come in
