Sizing conductors is not a difficult task, but more is involved than referencing just one section, one article or even one chapter in the National Electrical Code (NEC). To select the correct conductor size, it is necessary to reference several sections throughout the Code. Before referencing the NEC, certain information is needed. Gather it by answering some key questions.

What is the lowest temperature rating of any connected termination, conductor or device? What is the connected load, or what is the calculated load in accordance with Article 220? Is the load or any part of the load continuous? What will be the maximum ambient temperature? How many current-carrying conductors will be in the raceway or cable? Will the conductors fall under the small-conductor rules in 240.4(D)? Will the conductors be feeder taps or transformer secondary conductors? Will the conductors supply any motors?

In the first two parts of this series, I covered termination temperature limitations; the third part covered continuous loads. Requirements for termination temperature limitations are in 110.14(C). Continuous load requirements are in 210.19(A)(1), 215.2(A)(1) and 230.42(A).

Conductors that supply continuous loads must also be sized to comply with the termination temperature provisions in 110.14(C). For example, a branch circuit will be installed in a nondwelling occupancy to supply power to a 16-ampere (A) continuous load. The circuit will be supplied from an existing 20A circuit breaker, and the temperature rating is not marked on the breaker. What is the minimum size THHN copper conductor required to supply power to this branch circuit?

Because this branch circuit load is a continuous load, multiply the load by 125 percent [210.19(A)(1)]. The conductors must have an ampacity of at least 20A (16 125% = 20). Table 310.15(B)(16), formerly Table 310.16, provides allowable ampacities for insulated conductors. Although a THHN conductor has a temperature rating of 90°C, the ampacity must not exceed the ampacity in the 60°C column.

Section 110.14(C)(1)(a) covers circuits rated 100A or less or marked for 14 AWG through 1 AWG conductors. While it is permissible to install a conductor with a higher temperature rating, the ampacity shall not exceed the 60°C ampacity of the conductor size used [110.14(C)(1)(a)(2)]. If any termination is either 60°C or unknown, the conductor’s maximum ampacity is listed in the 60°C column, regardless of the insulation rating of the conductor. The ampacity of a 12 AWG conductor in the 60°C column is 20A. Conductors supplying power to this branch circuit must be at least 12 AWG (see Figure 1).

It is not necessary to use the ampacity from the 60°C column if the equipment is listed and identified for use with such conductors [110.14(C)(1)(a)(3)]. This means the ampacity of a 75°C (or a 90°C) conductor can be based on the 75°C column if all the terminations are rated at least 75°C. For example, a 208-volt (V), single-phase branch circuit will be installed in a nondwelling occupancy to supply power to a 40A continuous load. The two-pole circuit breaker has a temperature rating of 75°C, and the termination on the other end also has a rating of 75°C. What are the minimum size THHN copper conductors required to supply power to this branch circuit? Because this branch circuit load is a continuous load, multiply the load by 125 percent [210.19(A)(1)]. The conductors must have an ampacity of at least 50A (40 125% = 50). Because all of the connection points in this example have at least a 75°C rating, the conductor’s ampacity can be based on the 75°C column. The ampacity of an 8 AWG conductor in the 75°C column is 50A. Conductors supplying power to this branch circuit must be at least 8 AWG (see Figure 2).