Overcurrent devices
Q: I have a lighting panelboard rated 100A, 120/208V, 3-phase. I sized the feeder based on total continuous connected load, which is less than 80A. The feeder size is 4-No. 3 (100A) and about 20 feet from the service panel; the protective device was rated 100A. The state inspector is requiring me to size the protective device using 80A because he is derating the feeder to 80% of its maximum ampacity. My design is based on NEC 2000, Article 215-2 (a) and 215-3. Who is correct?
A: Either of you could be right. You reference Sections 215-2(a) and 215-3. These references give minimum sizes of conductors and overcurrent devices. These sizes are selected “before” the application of correction or adjustment factors. Correction factors are for temperature, and adjustment factors are for more than three conductors in a raceway or cable. Both factors consider the conditions of use and, according to Section 310-15, are selected using the calculated load from Article 220. Your references are outside of this path through the Code. Therefore, while you have complied with the minimum requirements for the feeder conductor and overcurrent sizing, you may not have considered the conditions of use. If you have 80A of continuous load, you must provide a minimum 100A overcurrent device and a minimum No. 3 copper conductor. The selection of the No. 3 assumes that you have 75° C terminals in order to comply with Section 110-14(c).
Now you must consider the conditions of use. Since you are dealing with a three-phase, four-wire wye system, you may have to count the neutral as a current-carrying conductor. You called the panel a lighting panel, so if a major portion of the load (50% or more) is non-linear due to discharge lighting such as fluorescent lighting, then the neutral must be counted as a current-carrying conductor. This rule is found in 310-15(b)(4)(c). When the neutral is counted as the fourth conductor, an adjustment factor of 80% must be applied to the ampacity as required by 310-15(b)(2)(a). This reduces the ampacity of a No. 3 copper conductor from 100A to 80A. But the use of an 80A overcurrent device violates the rules you first referenced. Thus, the conductor ampacity would have to be increased in order to permit the use of a 100A overcurrent device.
How can this be resolved? One possible way is to use the 90° C column of Table 310-16. The ampacity of a No. 3 XHHW-2 conductor for example is 110A. But 80% of this value is only 88A, so a 90A overcurrent device would be permitted, but that still does not permit the REQUIRED 100A overcurrent device.
A No. 2 THWN has an ampacity of 115A in the 75° C column. Eighty percent of 115 is 92, so this conductor could be protected at 100A in accordance with 240-3(b). In short, if the neutral is not to be counted, the terminals are all 75° C and no other condition of use requires a larger conductor, the No. 3 is adequate and may be protected at 100A. However, if the neutral is counted, or any other condition of use, including terminal ratings, requires a larger size, that larger size must be used. Sections 215-2(a) and 215-3 establish only one minimum size to be complied with. Note however that these rules would prohibit the use of the 80A overcurrent device with more than a 64A continuous load.
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