All questions and answers are based on the 2011 NEC.
Q. What is the Code rule for sizing feeder conductors, both ungrounded and neutral? Please show an example.
A. Section 215.2(A)(1) requires that the minimum feeder conductor ampacity, before the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, must be no less than 125% of the continuous load, plus 100% of the noncontinuous load, based on the terminal temperature rating ampacities as listed in Table 310.15(B)(16) [110.14(C)(1)]. Note: See 215.3 for the feeder overcurrent device sizing requirements for continuous and noncontinuous loads. However, there are two exceptions to this rule:
Exception No. 1: If the assembly and overcurrent device are both listed for operation at 100% of its rating, then the conductors can be sized at 100% of the continuous load.
Equipment suitable for 100% continuous loading is rarely available in ratings under 400A.
Exception No. 2: Neutral conductors can be sized at 100% of the continuous and noncontinuous load (see Figure).
Let’s review a sample calculation to hit this point home. What size feeder conductors are required for a 200A continuous load if the terminals are rated 75°C?
Because the load is 200A continuous, the feeder conductors must have an ampacity of not less than 250A (200A × 1.25). The neutral conductor is sized to the 200A continuous load according to the 75°C column of Table 310.15(B)(16). Therefore, you could use a 250kcmil conductor, which has an ampacity of 255A, and a 3/0 neutral conductor, which has an ampacity of 200A.
The feeder neutral conductor must be sized to carry the maximum unbalanced load in accordance with 220.61 and must not be smaller than the size listed in 250.122, based on the rating of the feeder overcurrent device. The sizing requirements of 250.122(F) for parallel conductors don’t apply [215.2(A)(2)].
Here’s another sample problem that helps show how these rules are applied in the real world.
What size neutral conductor is required for a feeder consisting of 250kcmil ungrounded conductors and one neutral conductor protected by a 250A overcurrent device, where the unbalanced load is only 50A, with 75ºC terminals?
Table 310.15(B)(16) and 220.61 permit an 8 AWG neutral conductor, rated 50A at 75°C 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 device. So, you must use a
4 AWG conductor for this installation. In addition, the feeder conductor ampacity must not be less than that of the service conductors where the feeder conductors carry the total load supplied by service conductors with an ampacity of 55A or less [215.2(A)(3)]. Also, be aware that the feeder conductors for individual dwelling units or mobile homes need not be larger than service conductors sized to 310.15(B)(7) [215.2(A)(4)].
Informational 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%. In addition, the maximum total voltage drop on both feeders and branch circuits shouldn’t exceed 5%.
Informational Note 3: See 210.19(A), Note 4, for voltage drop for branch circuits.
Q. What is the Code rule regarding plugging unused openings in boxes and other enclosures?
A. Openings through which cables or raceways enter must be adequately closed [314.17(A)]. In addition, unused cable or raceway openings in electrical equipment must be effectively closed by fittings that provide protection substantially equivalent to the wall of the equipment [110.12(A)].
