I'm missing something from this question and answer in the latest Mike Holt Newsletter:
Q5. We recently sized a circuit to a commercial oven. The manufacturer says the total load per phase is 58 amps single phase 240V and it?s not capable of being used continuously. What size conductor and protection can I use?
A5. The overcurrent rule is as follows: For nonmotor operated appliances, the appliance overcurrent device must not exceed 150 percent of the appliance rated current if the overcurrent device rating isn?t marked, and the appliance is rated over 13.30A. Where 150 percent of the appliance rating doesn?t correspond to a standard overcurrent device ampere rating listed in 240.6(A), the next higher standard rating is permitted.
Protection Size: 58A x 150% = 87A, 90A next size up [240.6(A)].
Conductor size: 4AWG, rated 85A at 75?C [Table 310.16].
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While the answer is compliant, it seems kind of strange. This states the maximum size breaker you can use, and the wire that must be used with a breaker that size. But wouldn't a 60A breaker also be acceptable on #6 copper? Since the question asked what size wire and breaker can be used, there are many answers. I see #6 copper as the minimum size wire if terminations are good for 75C. Number 4 copper as the minimum if 60C is the limit. Breaker size would be 60A or 70A on #6 copper (if 75C allowed), and 60A, 70A, 80A, or 90A on #4 copper.
Am I missing something?
Q5. We recently sized a circuit to a commercial oven. The manufacturer says the total load per phase is 58 amps single phase 240V and it?s not capable of being used continuously. What size conductor and protection can I use?
A5. The overcurrent rule is as follows: For nonmotor operated appliances, the appliance overcurrent device must not exceed 150 percent of the appliance rated current if the overcurrent device rating isn?t marked, and the appliance is rated over 13.30A. Where 150 percent of the appliance rating doesn?t correspond to a standard overcurrent device ampere rating listed in 240.6(A), the next higher standard rating is permitted.
Protection Size: 58A x 150% = 87A, 90A next size up [240.6(A)].
Conductor size: 4AWG, rated 85A at 75?C [Table 310.16].
- - - - - - - - - - - - - - -
While the answer is compliant, it seems kind of strange. This states the maximum size breaker you can use, and the wire that must be used with a breaker that size. But wouldn't a 60A breaker also be acceptable on #6 copper? Since the question asked what size wire and breaker can be used, there are many answers. I see #6 copper as the minimum size wire if terminations are good for 75C. Number 4 copper as the minimum if 60C is the limit. Breaker size would be 60A or 70A on #6 copper (if 75C allowed), and 60A, 70A, 80A, or 90A on #4 copper.
Am I missing something?