Mike Dolt Enterprises
Mike Dolt Enterprises
User: ammklq143, Location: Iowa
Asked on: Tuesday, June 10, 2025 at 8:28 pm EDT
Answered on: Tuesday, June 10, 2025 at 8:28 pm EDT
Question: I saw a post about a motor OCPD and wondering what is correct. 430.248 days FLC was 30.8 amps. Inverse time breaker so 250% is 77 amps. OCPD would be a 70a or 80a breaker. He says 240.4(b) applies and the next size up breaker could be used and an 80a could be installed. Is that correct? 240.4(b) says it allows you to upsize the breaker in relation to the actual ampacity of the conductor if a standard breaker doesn’t match the ampacity but most likely the conductors would be #8’s. The ampacity of the conductor isn’t 77 amps though, so the way I read it is you couldn’t use the one size up to go to 80 amps. Am I missing something?
You're absolutely right to question the application of 240.4(B) in this scenario. Your understanding is correct; it's not applicable here. Let's break down why
Understanding the Code Sections:
430.52 (Motor Overcurrent Protection)
This section is the primary guidance for sizing the overcurrent protective device (OCPD) for a motor circuit. In your example, you correctly calculated the 250% based on the motor's Full Load Current (FLC).
240.4(B) (Overcurrent Protection Based on Calculated Load)
This section generally applies when protecting conductors and allows you to go to the next standard OCPD size only if the conductor's ampacity doesn't match a standard breaker size.
Why 240.4(B) Doesn't Apply Here
The key is that
430.52 takes precedence over 240.4(B) when sizing motor OCPD. 430.52 provides specific rules for motor protection, which are different from general conductor protection. Your motor OCPD sizing calculation has nothing to do with the conductor ampacity. You are using the 250% number based on the FLC of the motor for the breaker size and it has nothing to do with the conductor ampacity.
Correct Procedure
Calculate the Maximum OCPD Size
Based on 430.52 (using an inverse time breaker), the maximum OCPD is 30.8 amps x 2.5 = 77 amps.
2.
Select the Standard OCPD Size
Now, look at the standard breaker sizes (240.6). A 70 amp breaker is the correct selection. You can not go to the next size up like the original person said.
The Issue with Using 240.4(B)
Applying 240.4(B) based on the calculated OCPD size (77 amps) is incorrect. You're not trying to protect the conductors from overload in the same way you would in a general branch circuit. The motor OCPD is primarily intended to protect the motor and its controller from short circuits and ground faults, and to a lesser extent, overloads.
Conductor Sizing
Don't forget to size the conductors according to 430.22, which requires a minimum ampacity of 125% of the motor's FLC. In this case, that would be 30.8 amps x 1.25 = 38.5 amps. So, #8 THHN would be acceptable.
In Summary:
You are correct in your assessment. The original post incorrectly applied 240.4(B). The correct procedure is to:1. Calculate the maximum OCPD size based on 430.
52.2. Choose a standard* breaker size (240.6) that does not exceed the calculated maximum.
3. Size the conductors according to 430.22 (minimum 125% of the FLC).Therefore, a 70 amp breaker is the correct OCPD. Flag for review