NEC Conductor de-rating 1

[danfo]

Here's an NE Code question. It's based on a real world case. Invite your learned opinion or interpretation:

Ladder type tray system. Mix of sizes, 600V 3 conductor 90 dC rated type TC cables. Tray fill ruled by Art. 392.9 (A) which refers to ampacity Table 310.16

Conductors terminate on 75 dC rated lugs (both ends). Max ambient temp at terminations is spec'd 40 dC max. 70% of cable length (about 300 ft) is exposed to 50 dC max.

Must I derate 75 dC column of table 210.16 to 0.75 (172.5 A for 4/0) or may I derate the 75 dC column of this table to 0.82, the 90 dC insulation factor for this 50 dC maximum ambient? (188.6 A for 4/0)

310.10 FPN 1 seems to allow this latitude

I understand that 240.4 (B) allows same overcurrent protection in either case.

Ladies and Gentlemen; your thoughts are appreciated.

 

[davidbeach]

If your installation is such that 90 C insulation is appropriate, you can use the 90 C column for all of your derating. Once the derate calculations are completed, you use the lower of your derate number or the 75 C number.

 

[saladhawks]

See NEC 2002 Article 110.14(C)(1)(b) for circuits rated over 100 Amperes.