Wire size contradiction

[Rawls007]

Mike Holt has a commercial and industrial wire sizing chart that seems to show each wire being one size greater than necessary.http://muscoengineering.com/documents/Formulas.pdf For example, it shows using a minimum 4/0 THHN copper for 225 amp breakers when NEC shows 3/0 having an ampacity of 225 amps.

 

[infinity]

You reading the 90 degree C column. There are really no terminations rated for 90 degrees C so you must use the 75 degree column which means #4/0 Cu for 225 amps.

 

[Rawls007]

I see. Just out of curiousity, why does table 310.16 show all the different types of wire and temperature ratings when equipment terminals are universally rated for 75 degrees, therefore reducing the allowable ampacities to such?

 

[emahler]

primarily for derating purposes here....

 

[Dennis Alwon]

I see. Just out of curiousity, why does table 310.16 show all the different types of wire and temperature ratings when equipment terminals are universally rated for 75 degrees, therefore reducing the allowable ampacities to such?

You may use the 90C to derate as long as the final OCPD is not larger than the 75C column will allow. This is helpful when pulling more than 3 current carrying conductors in a conduit. See T.310.15(B)(2)(a)