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Monday, January 16, 2023
Question:
Hello CQD, can you please provide correct interpretation of code section 310.15 (B) (5) (b)? If 225A Bus panel with 225A Main CB is fed from upstream 3Ph, 4W panelboard, typically we would use 3#4/0 + 1#4/0 Neutral and 1#4 CU GND. Does this section mean that instead I need to now use 3#300KCMIL + 1#300 KCMIL Neutral due to 80% Derating factor per Table 310.15(B)(3)(a)? I haven't seen this type of installation as it will drive up the costs. Please advise. Thank you, Abhi P.
A
Answer:
Abhi, thank you for the question. Section 310.15(B)(5) provides guidance on when a Neutral/Grounded Conductor is considered current carrying for the purposes of calculating ampactiy adjustments and applying Table 310.15(B)(3)(a).
Looking at Section 310.15(B)(5)(a)-(b) there are (3) three scenarios addressed:
(a) Addresses a typical 3-wire, 1-phase or 4-wire, 3-phase, shared neutral installations, as the neutral will only carry the unbalanced current from the other conductors. These neutral conductors would not be counted as current carrying. In an installation consisting of a 1-phase, 2-wire(1 ungrounded, 1 grounded/neutral) conductors, the neutral is not just carrying the unbalanced current and therefore must be counted as current carrying.
(b) Addresses when a neutral conductor is shared by only two (2) ungrounded conductors from a 3 phase/4-wire, wye-connected system. In this type of installation, the shared neutral is carrying approximately the same amount of current as the ungrounded conductors, so it shall be counted as a current carrying conductor.
(c) Addresses a 3-phase, 4-wire circuit where the major portion of the load consists of nonlinear loads (i.e., computers, printers, servers, rectifiers, electronic ballasts, etc.). In these instances, harmonic currents are present on the neutral conductor, so the neutral conductor shall be counted as a current carrying conductor.
With the information and example, you provided in the question, and assuming the loads are linear, the 3-phase, 4-wire installation, the neutral is only carrying the unbalanced current, and would not be counted as a current carrying conductor. This means you only have 3 current carrying conductors and the and the adjustment factors would not apply.