Mike Dolt Enterprises
Mike Dolt Enterprises
User: hardworkingstiff, Location: Wilmington, NC
Asked on: Friday, March 19, 2010 at 9:49 am EDT
Answered on: Monday, May 13, 2024 at 10:18 am EDT
Question: I sent this to the AHJ, and I thought I would put it out here to see what comments I might get.Jimmy,Please find 2 PDF files attached. This is a job I'm bidding (without engineered drawings at this time) and I'd like to see if you agree or disagree with my understanding of the NEC.All but 4 of the pedestals will have (2) 30-amp receptacles with circuit breakers in the pedestals. The 4 single loaded pedestals are the ones for the slips nearest the bulkhead on circuits A1 and B1 (see drawing). Each pedestal will have a KWH meter for the individual slip so we will be taking a 10% reduction in feeder calculations per 555.12 note 2.The circuit that I specifically would like you to comment on is circuit B2. This circuit consists of 4 pedestal each with (2) 30-amp receptacles and individual KWH meters for the slips. The 30-amp receptacles are 120-volts and the feeder is a MWBC consisting of A-phase, B-phase, N, and Gnd from a single-phase 120/240-volt service. In each pedestal, the (2) 30-amps circuits are wired to opposite phases of the feeder. Each phase of the MWBC (from the 1st pedestal in the circuit to the panel) will have a calculated load of 30*4 = 120-amps * 90% (for meter) = 108-amps per phase. 110.14(C) requires us to use table 310.16 for the ampacities at our termination. Table 310.16 lists a #2 copper wire at 75c (our equipment terminations have a minimum of a 75c rating) as having an ampacity of 115-amps. We are thereby OK with this installation at this point.Here is where I need to know I am understanding the code correctly. The load calculation after the 1st pedestal becomes 30*3 = 90-amps * 90% (for meter) = 81-amps per phase. A #4 copper conductor in table 310.16 has a 75c rating of 85-amps which is greater than the 81-amp load calculation, so use of a #4 type W cable will be in compliance with 110.14(C). Table 400.5(B) has a 75c rating for a #4 W cable of 115-amps (column E) so it is adequately protected by the 110-amp breaker.Please let me know if you see a fallacy in my thinking.
Your understanding of the NEC is generally correct. Here are a few comments:1. Feeder Calculations
You are correct in applying the 10% reduction in feeder calculations per 555.12 Note 2 due to the individual KWH meters.
2.
Circuit B2 Feeder
Your calculations for the MWBC feeder are correct. The feeder conductors (A-phase, B-phase, N, and Gnd) must be sized for an ampacity of 108 amps per phase (120 amps x 90%). As you mentioned, a #2 copper wire has an ampacity of 115 amps at 75°C, which is sufficient for this application.3.
Branch Circuit Conductors
Your calculations for the branch circuit conductors are also correct. The load calculation after the first pedestal becomes 90 amps x 90% = 81 amps per phase. A #4 copper conductor has an ampacity of 85 amps at 75°C, which is greater than the 81-amp load calculation. Therefore, a #4 type W cable can be used for the branch circuits.4.
Overcurrent Protection: As you noted, the 110-amp breaker is adequate protection for the #4 W cable, which has an ampacity of 115 amps at 75°C.Overall, your understanding of the NEC requirements for this installation appears to be sound. However, it is always advisable to consult with the AHJ to confirm your interpretations and to ensure compliance with local codes and regulations. Flag for review