trying to figure out this 110.14(c) and why it lets you use the 90 degree column for derating when more than 3 current carrying conductors applies... is this for everything feeders, branch circuits? I always thought if you land in a lug (disco, panel) you had to calculate using the 75 degree column.
thanks,
H2o

H2O:

Basically, in a short, plain English statement; you can use the 90 degree col. for derating calculations, but you must not exceed the 60 or 75 degree maximums after your calculations.

The 75c is for the lug. With a simple 3 wires in a pipe, that will usually be the limiting factor.
As soon as you get more wire in the pipe and start running it through hostile areas where you have "adders" on, you are happy you can start from 90 and the wire is still going to be the limiting factor.

Also; you can only start in the 90 degree column if the insulation is rated at 90 degrees...like THHN is.

90c is becoming the norm these days since the introduction of THWN-2 and the dual rated THHN/THWN-2 conductor.
You still need to be sure what you have but what you need is usually available.

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.

............................................................

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)