|
 Keeping up with the requirements of the Code should be the goal of everyone involved in electrical safety. Here is this month's newsletter with what I feel is an important rule in the 2017 NEC®, complete with graphics and a video. I encourage you to use it as a training resource for your organization, and share it with your colleagues.
The content below is extracted from Mike Holt's Understanding the 2017 National Electrical Code®, Volume 1.
Any underlined text denotes a change to the Code for the 2017 NEC.
(A) Feeder Conductor Size.
(1) General. Feeder conductors must be sized to carry the larger of
(a) or (b):
(a) The feeder conductor must have an ampacity before ampacity correction
and adjustment of not less than 125 percent of the continuous
load, plus 100 percent of the noncontinuous load, based on the
terminal temperature rating ampacities as listed in Table 310.15(B)
(16) [110.14(C)(1)]. ▶Figure 215“3
|
|
▶Figure 215“3 |
|
Example: What size feeder is required for a 100A continuous load
and 100A noncontinuous load? ▶Figure 215“4
|
|
▶Figure 215“4 |
Answer: 4/0 AWG rated 230A [Table 310.15(B)(16)]. The feeder
conductor must have an ampacity of 225A [(100A × 1.25) + 100A]. |
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. |
| Author's Comment: |
- Equipment suitable for 100 percent continuous loading is
rarely available in ratings under 400A.
|
Ex 2: A section of feeder conductors that terminates in a junction box at both ends to 90°C terminals in accordance with 110.14(C)(2) are permitted to have an ampacity of not less than 100 percent of the continuous and 100 percent of the noncontinuous load based on the 90°C column of Table 310.15(B)(16) for 90°C conductor insulation. The 100 percent at 90°C feeder conductors aren't permitted to extend into the supply or the load terminations to the feeder circuit. ▶Figure 215“5
|
|
▶Figure 215“5 |
| Author's Comment: |
-
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. |
| |
| Author's Comment: |
-
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 |
|
|
▶Figure 215“9 |
| Author's Comment: |
-
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 |
Answer: A 4 AWG neutral conductor is required [Table 250.122]. Table 310.15(B)(16) and 220.61 permit an 8 AWG neutral conductor rated 50A at 75ºC to carry the 50A unbalanced load, but the neutral conductor isn't permitted to be smaller than 4 AWG, as listed in Table 250.122, based on the 250A overcurrent protection device. |
We'd love to hear from you about this series, and the ways you're using it. Send us your comments and feedback by clicking on Post a Comment below. Look out for the next part in this series a month from now, and please share with your colleagues.
This content is extracted from Mike Holt's Illustrated Guide to Understanding the 2017 National Electrical Code, Volume 1 textbook.
To review or catch up on previous newsletters on the National Electrical Code click here.
 |
Learn More
Understanding the NEC Complete Library
(3 books | 12 DVDs)
For further information about this or other products please call us at 888.632.2633
We look forward to helping you
|
|
|
|
|
