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Monday, February 6, 2023
Question:
Hello CQD, I'm an apprentice that's been really enjoying the CQD for about a year now, and we had a problem come up in class that sparked disagreement. The scenario is this:
With three 208V, 3-Phase, Design-B, CONT Duty, Squirrel Cage motors rated at 10 HP, 25 HP, and 30 HP, and all on a single branch circuit, which code section do you use to determine the maximum size of the Inverse-Time circuit breaker being used as the OCPD for the circuit?
It seems to me that 430.53(C)(4) indicates that Table 430.52 should be applied to the largest motor (30 HP, FLC determined by Table 430.250), and then the FLCs (per Table 430.250) of the other two motors should be added -- without a multiplier -- to that product. Finally, the next breaker size down per Table 240.6(A) should be chosen. Thanks, Ira Wahlmeier
A
Answer:
Hi Ira, glad to hear you are enjoying the CQD, and kudos to you for taking such an interest in the NEC®. It sounds like you are on the right track. Section 430.53 provides general guidance on installations of multiple motors and/or loads on one branch circuit.
Sections 430.53(A)-(D) are applied based on the installation applications.
To apply Section 430.53(C), you must meet the criteria listed in the charging text requiring factory assembly or field installed:
- Controller(s)
- Overload Devices
- Branch-circuit, short-circuit and ground-fault devices
To answer the question yes, Section 430.53(C)(4) would be used to determine the rating of Fuses or Inverse Time CB. The branch-circuit OCPD shall have a rating not exceeding that specified in Section 430.52 for the largest motor (30HP) plus the SUM of the full-load current rating of the other motors connected to the branch circuit (10HP & 25HP) found in Table 430.250. If the calculated value is less than the ampacity of the branch circuit conductors, the OCPD can be increased in accordance with Section 240.4(B).
Keep in mind that the other conditions: (1), (2), (3), (5) and (6) would all need to be met as well to utilize Section 430.53(C).