Can I use the 90 degree column to size service conductors?

Only for ambient correction and conductor count adjustment. NEC 110.14(C) caps the final ampacity at the termination temperature rating, which is normally 60C or 75C.

Why is 4/0 aluminum allowed on a 200 amp house service?

NEC 310.12 permits dwelling unit service and main power feeder conductors to be sized at not less than 83 percent of the service rating, and Table 310.12(A) lists 4/0 aluminum for a 200 ampere service.

Which temperature column do you use to size service conductors?

Q: When does the 60 degree column apply?

NEC 110.14(C)(1)(a) applies the 60C column to equipment rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, unless the equipment is listed for higher temperature conductors.

The termination decides the column, not the insulation printed on the jacket. Section 110.14(C) of the 2023 NEC is the controlling rule, and for almost every service you will size out of the 75 degree Celsius column of Table 310.16.

The 110.14(C) rule in plain terms

Section 110.14(C) states that the temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Section 110.14(C)(1)(a) applies to equipment rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors. For that equipment, conductors are sized from the 60 degree Celsius column unless the equipment is listed and identified for use with higher temperature conductors, or the conductor is rated higher and used at the 60 degree ampacity.

Section 110.14(C)(1)(b) applies to equipment rated over 100 amperes, or marked for conductors larger than 1 AWG. For that equipment, the 75 degree Celsius column applies unless the equipment is listed and identified for a higher temperature.

Nearly all modern loadcenters, meter mains, and disconnects are marked 75 degrees Celsius on the lug or on the label inside the door. Some small residential loadcenters are marked 60/75. Read the marking. That marking, not the THHN stamp on the wire, sets the column.

Where the 90 degree column is actually used

The 90 degree Celsius column is not useless. Section 110.14(C) permits conductors with higher temperature ratings to be used for ampacity adjustment, correction, or both. So a 90 degree rated conductor like THHN or XHHW-2 starts its derating math at the 90 degree ampacity for ambient temperature correction under Table 310.15(B)(1) and for conductor count adjustment under Table 310.15(C)(1). After all of that math, the answer is capped by the termination temperature. That is the two step process people skip.

The dwelling service shortcut

Section 310.12 gives dwelling unit services and feeders their own allowance. For a single phase, 120/240 volt service or feeder that serves as the main power feeder to a dwelling unit, the conductors are permitted to have an ampacity not less than 83 percent of the service rating. Table 310.12(A) puts that into a lookup, which is why 4/0 aluminum shows up on a 200 ampere residential service even though the 75 degree ampacity of 4/0 aluminum is 180 amperes. That table already accounts for the diversity of dwelling loads and is not a general purpose shortcut for commercial feeders.

Field takeaway

Check the lug marking on both ends. Use the 60 degree column at 100 amperes and below unless the equipment says otherwise, the 75 degree column above 100 amperes, and use the 90 degree column only as the starting point for correction and adjustment factors. For a straight dwelling service, go to Table 310.12(A) first and confirm the terminations are rated 75 degrees.