Sizing Conductors Based on Temperature Rating

All conductor terminals for devices and equipment have a listed temperature rating. It's important that you size conductors and limit the loads so that you don't exceed these temperature ratings. When selecting a conductor for a circuit, select one to accommodate the temperature termination rating rules outlined in Sec. 110-14(c).

You'll find equipment terminals listed for use with a specific size conductor (given amount of conductor mass). A properly sized conductor acts as a heat sink to move heat away from the terminals. When you use too small of a conductor, the terminals can overheat, which cause fires - particularly at switches, receptacles, and circuit breakers.

Unless marked otherwise, you must size equipment terminals rated 100A or less based on the 60øC temperature rating listed in Table 310-16.

Example No. 1:
Q. What size THHN conductor does the NEC require for a 50A circuit, if the terminals are not marked with a temperature rating?

(a) No. 10
(c) No. 6
(b) No. 8
(d) any of these

A. The answer is (c), No. 6. You can use No. 6 with THHN insulation, but you must select the conductor size based on the 60øC terminal rating of the equipment, not the 90øC rating of the insulation. Using the 60øC column of Table 310-16, this 50A circuit requires a No. 6 conductor.

Q. What size THHN conductor does the NEC require for a 50A circuit, if the terminals are marked for use at 75øC?

(a) No. 10
(c) No. 6
(b) No. 8
(d) any of these

A. The answer is (b), No. 8. Size the conductors to the 75øC terminal rating of the equipment, not the 90øC rating of the conductor insulation. Using the 75øC column of Table 310-16, this installation permits a No. 8 conductor to supply the 50A load.

Circuits over 100A [Sec. 110-14(c)(2)]. Unless the terminals are marked otherwise, you must size equipment terminals rated over 100A according to the 75øC temperature rating listed in Table 310-16.

Example No. 2: Q. What size THHN conductor is required to supply a 225A feeder?

(a) No. 1/0
(c) No. 3/0
(b) No. 2/0
(d) No. 4/0

A. The answer is (d), No. 4/0. You must size the feeder conductors to the 75øC terminal rating of the equipment. Using the 75øC column of Table 310-16, this requires a No. 4/0 to supply the 225A load.

What is the purpose of 90øC wire if we cannot use its higher ampacity? You typically can't use 90øC rated conductor ampacities for sizing circuit conductors. However, this rating comes into play when adjusting conductor ampacity for elevated ambient temperature or when bundling more than three current-carrying conductors together.

The advantage of 90øC wire (THHN) is it can keep you from using a larger wire (when you need ampacity adjustments), which requires larger raceways, greater labor, and increased material costs.

Here's an example. A 4-wire feeder supplies a panelboard loaded to no more than 180A continuously. The panelboard supplies branch circuits for sensitive electronic equipment. This equipment produces reflective harmonic currents (odd triplens) that add onto the neutral conductor, rather than cancel each other out. The neutral conductor acts as a current-carrying conductor. According to Sec. 310-15(b)(2)(a)(4)(c), you must adjust the ampacity of the No. 4/0 THHN conductors by a multiplier of 80% [Table 310-15(b)(2)(a)].

New Ampacity = Original Ampacity x Multiplying Factor

New Ampacity = 260A (at 90øC) x 0.8

New Ampacity = 208A*

*Sec. 240-3(b) permits a 225A protection device to protect a 208A conductor.

You must size the feeder conductor and protection device no less than 125% of the continuous load (i.e. 180A x 1.25 = 225A [Sec. 215-2(a) and 215-3]. No. 4/0 THW (230A) and THHN (260A) meet this requirement. However, No. 4/0 THW (75øC) is only rated 184A after taking the ampacity adjustment factor into account (230A x 0.80 = 184A). Therefore, you can't use the required 225A protection device to protect it.

Some words of caution. When sizing conductors, don't forget to also evaluate voltage drop, ambient temperature, bundled conductor adjustment, overcurrent protection, and continuous load factors. We'll cover ampacity adjustment, overcurrent protection, and continuous load factors in future Code Calculations. Visit http://www.mikeholt.com/studies/conductor.htm to get more information on these topics now.