why does 17th ed allow so much power per wire size than NEC?

[1capybara]

here's one example. table 4d4a column 3 for 240mm2 shows 409amps/3phase.
at 380Y/220v thats = 155KW.
NEC table 310.16 for 75 degree C wire, 500MCM [thats 253 mm2] allows 380 amps/3phase.
at 208Y/120v thats = 79KW.
Why the big difference - almost 100% difference ???

 

[telectrix]

the current capacity seems similar. is the lower KW value due to the lower voltage?

 

[Knobhead]

Your calculations are wrong and your not comparing like for like: 409A @ 380V = 269KW. 380A @ 208V = 136KW. If we make the voltage equal:409A @ 380V = 269KW. 280A @ 380V = 250KW.