To elaborate:
If the Insulation is rated for 90ºC, the derating may begin at the Ampacity in that column, per Table 310.16; however the actual Termination Ampacity is based on the 60ºC Column (for Conductors size #2 and smaller), or the 75ºC Column (for Conductors size #1 and larger).
Examples:
Ex. 1:
(2) 20 Amp 3 Phase 4 Wire multiwire Circuits run in single Raceway.
All Conductors are #12 THHN CU., and are Current Carrying; total of (8) Current Carrying Conductors.
Derating value for 7-9 Conductors = 0.7
Table 310.16 Ampacity for #12 THHN CU. at 90ºC = 30 Amps.
30 Amps X 0.7 = 21 Amps
Maximum OCPD is 20 Amps (for general circuits)
The derated value of 21 Amps is greater than the Maximum OCPD rating of 20 Amps, therefore the Circuits Terminated to 20 Amp OCPD's are compliant.
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Ex. 2:
(3) 30 Amp 3 Phase 4 Wire multiwire Circuits run in single Raceway.
All Conductors are #10 THHN CU., and are Current Carrying; total of (12) Current Carrying Conductors.
Derating value for 10 - 20 Conductors = 0.5
Table 310.16 Ampacity for #10 THHN CU. at 90ºC = 40 Amps.
40 Amps X 0.5 = 20 Amps
Maximum OCPD for #10 CU is 30 Amps (for general circuits).
Derated value of 20 Amps is less than the maximum OCPD value of 30 Amps; therefore, to be compliant, the Circuits will be Terminated to 20 Amp OCPD's.
That is a basic version of the derating methods.
Feel free to respond.
--Scott (EE)
