Example: What size feeder is required for a 100A continuous load and 100A noncontinuous load? ▶Figure 215–4

▶Figure 215–4

Ex 1: If the assembly and the overcurrent protection device are both listed for operation at 100 percent of their rating, the conductors can be sized at 100 percent of the continuous and noncontinuous load.

• Equipment suitable for 100 percent continuous loading is rarely available in ratings under 400A.

• Equipment Terminals Rated 75°C. Feeder circuits must be sized to 125% of the continuous load [215.2(A)(1)(a)] for circuits over 100A. The conductors must be sized to the 75°C ampacity listed on Table 310.15(B)(16) [110.14(C)(1)(b)]. Circuits rated not over 800A can have overcurrent protection sized in accordance with 240.4(B), and for circuits rated over 800A, overcurrent must be in accordance with 240.4(C). The neural conductor is sized to 100 percent of the continuous load [215.2(A)(1) Ex 3] to the 75°C ampacity listed on Table 310.15(B)(16) [110.14(C)(1)(b)], based on the maximum unbalanced load in accordance with 220.61, and not smaller than required by 250.122 for equipment grounding conductor.

▶ 75°C Equipment Terminals

Example: What size feeder conductors are required for a 300A continuous load, having a neutral load of 50A? ▶Figure 215–6

▶Figure 215–6

Answer: Feeder 500 kcmil, rated 380A at 75°C, neutral 3 AWG, rated 100A at 75°C, and equipment grounding conductor of 3 AWG. The feeder conductor must have an ampacity of at least 375A (125 percent of the 300A continuous load) [215.2(A)(1)(a)]; 500 kcmil, rated 380A at 75°C in accordance with Table 310.15(B)(16) is suitable [110.14(C)(1)(b)]. The 500 kcmil conductor, rated 375A at 75°C is permitted to be protected by a 400A protection device [240.4(B)].

The neutral conductor must be sized to carry 50A continuously, which can be 6 AWG, rated 50A [100 Ampacity, 110.14(C)(1)(b), 215.2(A), and 220.61], but it can’t be smaller than 3 AWG based on the 400A overcurrent protection device size in accordance with Table 250.122.

• Equipment Terminals Rated 90°C. Feeder circuits can be sized to 100% of the continuous load based [215.2(A) (1)(a) Ex 1] according to the 90°C ampacity listed on Table 310.15(B)(16) [110.14(C)(2)]. Circuits rated not over 800A can have overcurrent protection sized in accordance with 240.4(B), and for circuits rated over 800A feeder, overcurrent must be in accordance with 240.4(C). The neural conductor is sized to 100% of the continuous load [215.2(A) (1) Ex 3] to the 75°C ampacity listed on Table 310.15(B)(16)[110.14(C)(1)(b)], based on the maximum unbalanced load in accordance with 220.61, and not smaller than required by 250.122 for equipment grounding conductor.

▶ 90°C Equipment Terminals

Example: What size feeder conductor is required for a 300A continuous load? ▶Figure 215–7

▶Figure 215–7

Answer: Feeder 400 kcmil, rated 380A at 90°C, neutral 3 AWG, rated 100A at 75°C, and equipment grounding conductor of 3 AWG. The feeder conductor must have an ampacity of at least 300A (100 percent of the 300A continuous load) [215.2(A)(1)(a) Ex 1]; 400 kcmil, rated 380A at 90°C in accordance with Table 310.15(B)(16) is suitable [110.14(C)(1)(b)]. The 400 kcmil conductor, rated 380A at 90°C is permitted to be protected by a 400A protection device [240.4(B)].

The neutral conductor must be sized to carry 50A continuously, which can be 6 AWG, rated 50A [100 Ampacity, 110.14(C)(1)(b), 215.2(A), and 220.61], but it can’t be smaller than 3 AWG based on the 400A overcurrent protection device size in accordance with Table 250.122.

Ex 3: Neutral conductors must have an ampacity of not less than 100 percent of the continuous and noncontinuous loads.

Example: What size feeder conductors are required for a 200A continuous load if the terminals are rated 75ºC? ▶Figure 215–8

▶Figure 215–8

Answer: 250 kcmil AWG ungrounded conductors and a 3/0 AWG neutral conductor are required. Since the load is 200A continuous, the feeder conductors must have an ampacity of not less than 250A (200A × 1.25). According to the 75ºC column of Table 310.15(B) (16) [110.14(C)(1)(b)], 250 kcmil has an ampacity of 255A.

The neutral conductor is sized to the 200A continuous load (100 percent). According to the 75ºC column of Table 310.15(B)(16) [110.14(C)(1)(b)], 3/0 has an ampacity of 200A.

(b) The feeder conductor must have an ampacity after ampacity correction and adjustment of not less than the load to be served.▶Figure 215–9

• See 215.3 for the feeder overcurrent protection device sizing requirements for continuous and noncontinuous loads.

Note 2: To provide reasonable efficiency of operation of electrical equipment, feeder conductors should be sized to prevent a voltage drop not to exceed 3 percent. In addition, the maximum total voltage drop on both feeders and branch circuits shouldn’t exceed 5 percent.

Note 3: See 210.19(A), Note 4, for voltage drop for branch circuits.

(2) Neutral Conductor Size. The feeder neutral conductor must be sized to carry the maximum unbalanced load in accordance with 220.61, and isn’t permitted to be smaller than listed in 250.122, based on the rating of the feeder overcurrent protection device.

Example: What size neutral conductor is required for a feeder consisting of 250 kcmil ungrounded conductors and one neutral conductor protected by a 250A overcurrent protection device, where the unbalanced load is only 50A, with 75ºC terminals? ▶Figure 215–10

▶Figure 215–10