Derating Parallel Conductors

Few topics considered in NECA’s on-line “Code Question of the Day” have kindled more fiery disagreement than that of this month’s article. The question in a nutshell is whether, when Section 310-4 states that “conductors shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor), the expression in parenthesis is considered a definition or an explanation of the words “connected in parallel.” Question: I have installed an 18-inch x 18-inch wireway. In it I have three sets of 4-400 kcmil conductors to feed an 800-ampere disconnect switch and six sets of 4-400 kcmil conductors as a feeder from the utility company. All of the conductors pass through one cross-sectional area of the wireway. The wireway is well within Section 362-5 sizing requirements, but the inspector says the installation does not meet Section 362-5 requirements because there are more than 30 conductors in a cross-sectional area and I have not allowed for proper derating of the conductors. I believe the inspector is wrong, because Section 310-4 states that conductors run in parallel, and when electrically joined at both ends form a single conductor. I believe that the paralleled conductors are permitted to be counted as a single conductor considering Section 362-5 and that the total cross-sectional area of the paralleled conductors should be used to compute the required wireway size. What is the correct interpretation in this circumstance? Answer: Formal interpretations of the National Electrical Code (NEC) may be obtained by following the procedures of the National Fire Protection Association (NFPA). Informal interpretations may be obtained by writing to the NFPA. I can only give you my opinion of what the requirements mean in relation to this question. Simply stated, the question is: “Are paralleled conductors considered as being one conductor when applying the provisions of Section 310-15(b)(2)?” The answer in my opinion is yes. I have never seen an interpretation, either formal or informal, relating to this situation. You make an excellent point here. Section 310-4 recognizes that paralleled conductors, properly joined at both ends, do form a single conductor. Crowding of conductors and heat dissipation should not be problems since the 20 percent fill requirement is met. The only disadvantage to your suggestion is the accessibility of the conductors for making taps and the ease of replacement or addition of conductors in the wireway. It is going to be pretty hard to look at 36 conductors in a wireway and convince the inspector that there are actually only nine. Follow-up question No. 1: I have some concerns with your response to the question concerning derating of paralleled conductors. If you assume the paralleled conductors form a single conductor, then by using 90-degree conductors as permitted in Section 110-14(c) for ampacity adjustment or correction and using Table 310-15(b)(2), the six sets of 400 kcmil conductors would be 1,596 amperes. This calculation is 380 amperes x 6 = 2280 x .70 = 1596A. The three sets of 400 kcmil conductors would be calculated 380 amperes x 3 = 1140 x .70 = 798A. The 800A disconnect would be sized correctly according to Section 240-3(b). I tend to disagree with the assumption that paralleled conductors form a single conductor because of the last sentence in Section 310-4, which states, “Conductors in parallel shall comply with the provisions of Table 310-15(b)(2)(a).” Answer: Your calculations appear to be correct until you derate to 70 percent. Based on the assumption that the paralleled conductors form one conductor, we have seven current-carrying conductors that, according to Table 310-15(b)(2)(a), would require derating to 70 percent based on the last sentence of Section 310-4. But then what about Section 362-5, which does not require derating when the wireway fill is limited to 30 conductors or less? I agree that the last sentence of 310-4 requires that paralleled conductors shall comply with the provisions of 310-15(b)(2)(a). I also agree that 310-4 defines paralleled conductors as forming one conductor. What should we do, choose sides? Follow-up question No. 2: May we please get a correction on your answer about derating paralleled conductors in a wireway? You are not right in your interpretation and you are not reading what the Code says in both Sections 310-4 and 362-5. Section 362-5 clearly shows derating is required if you do not limit the number of conductors to 30. Even though Section 310-4 says that conductors in parallel can be considered as one conductor, the last sentence in Section 310-4 plainly says that conductors in parallel shall comply with the derating rules in Section 310-15(b)(2)(a). The 20 percent fill rate does not apply when you exceed 30 conductors; the same is true in a raceway. If you had 10 No. 12 current-carrying conductors in a 1-inch raceway, you would derate to 50 percent. The same would be true if the 10 No. 12 conductors were in a 6-inch conduit. The heat would fill the space and destroy the insulation if you did not derate in both cases. Answer: And you think I need a course in remedial reading? Maybe I do because I do not understand your statement that “the 20 percent fill rate does not apply when you exceed 30 conductors.” I get the distinct impression from reading Section 362-5 that, “The sum of the cross-sectional areas of all contained conductors at any cross-section of the wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway.” The only exception to this would be as shown in Exception No. 3 for elevators and dumb-waiters. I also believe you had better “rethink” your statement about 10 No. 12 conductors in a 6-inch conduit generating enough heat to melt the insulation if you do not derate to 50 percent. The requirements in Section 310-15(b)(2)(a) would require such derating, but only because Table 310-15(b)(2)(a) is based on realistic standards of installation. However, we have gone off on a tangent. The question is whether or not paralleled conductors, electrically joined at both ends to form a single conductor, must be counted as multiple conductors for derating purposes. Would two or more conductors, each carrying an equal division of the load, create and contain more heat than a single, properly sized conductor carrying the total load? It’s a rather interesting concept, I think. Conclusion: Section 310-4 states that “parallel conductors form a single conductor.” Does it mean having the “form” of a single conductor, or does it really mean “function” as a single conductor? TROUT is a technical consultant for Maron Electric Company of Chicago, and represents NECA as chairman of the National Electrical Code-making panel 12. He is also the principal author of’s online feature, “Code Question of the Day.”

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