I have a question for you. The Code allows us to use THHN conductors rated 90°C in dry environment. 90°C conductors have a higher conductor ampacity, which should allow us to downsize larger feeders and bring down the cost of construction. But the engineers who I'm working with, always size the larger feeder conductors based on the 75°C conductors rating listed in Table 310.16. What are the requirements for use of 90°C conductors for feeder work?
Thank you in advance,
Sam Mazo, New York, NY
Mike's Response: Sam the following is extracted from my textbook Understanding the 2005 National Electrical Code.
(C) Temperature Limitations (Conductor Size). Conductors are to be sized to the lowest temperature rating of any terminal, device, or conductor of the circuit in accordance with (1) for terminals of equipment and (2) for independent pressure connectors on bus.
Conductors with insulation temperature ratings higher than terminations temperature rating can be used for ampacity adjustment, correction, or both.
Author's Comment: This means that conductor ampacity must be based on the conductor's insulation temperature ratings listed in Table 310.16, as adjusted for ambient temperature correction factors, conductor bundling adjustment factors or both. This means that conductor ampacity, when required to be adjusted, is based on the conductor insulation temperature rating in accordance with Table 310.16. For example, the ampacity of each 12 THHN is 30A, based on the values listed in the 90°*C column of Table 310.16.
If we bundle nine
current-carrying 12 THHN conductors in the same raceway or cable, the ampacity for each
conductor (30A at 90°*C, Table 310.16) needs to be adjusted by a 70 percent adjustment
factor [Table 310.15(B)(2)(a)].
See necdigest magazine, winter 2003 issues, page 32 and NEC Handbook, 310.15(B)(2)(a) Ex 5 for examples of 90°*C ampacity for conductor ampacity adjustment.
(1) Equipment Provisions.
Unless the equipment is listed and marked otherwise, conductor sizing for equipment termination
must be based on Table 310.16 in accordance with (a) or (b):
Author's Comment: Conductors are sized to prevent the overheating of terminals, in accordance with listing standards. For example, a 50A circuit with 60°*C terminals requires the circuit conductors to be sized not smaller than 6 AWG, in accordance with the 60°*C ampacity listed in Table 310.16. However, an 8 THHN insulated conductor has a 90°*C ampacity of 50A, but 8 AWG cannot be used for this circuit because the conductor's operating temperature at full load ampacity (50A) will be near 90°*C, which is well in excess of the 60°*C terminal rating. Figure 110-14
(2) Conductors with insulation temperature rating greater than 60°*C, such as THHN, which is rated 90°*C, can be used on terminals that are rated 60°*C, but the conductor must be sized based on the 60°*C temperature column of Table 310.16. Figure 110-25
(3) If the terminals are listed and identified as suitable for 75°*C, then conductors rated not less than 75°*C can be sized to the 75°*C temperature column of Table 310.16. See Figure 110-25
(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°*C or higher can be used provided the ampacity of such conductors doesn't exceed the 75°*C ampacity.
(b) Equipment Rated
(2) Separate Connector Provisions. Conductors terminating on independent pressure connectors on a bus, are permitted to be sized to the temperature rating of the connector. For example, if 90°*C pressure connectors are located on busbars, feeder conductors have a THHN insulation can be sized to the 90°*C temperature column of Table 310.16.
Mike Holt's Comment: Sam, larger
feeder conductors (over 100A) must be sized to the 75°C column of Table 310.16, unless
they terminate on independent pressure connectors on a bus [110.14(C)(2)].