Mike Dolt Enterprises
Mike Dolt Enterprises
User: Alex_rg15, Location: Texas
Asked on: Monday, March 24, 2025 at 11:50 pm EDT
Answered on: Monday, March 24, 2025 at 11:59 pm EDT
Question: What size inverse time circuit breaker should be installed for a group installation to protect the following 208 V, three-phase motors? One 7.5-HP motor, two 10-HP motors, and one 15-HP.Step 1Find the FLC for each motorFrom NEC Table 430.250:• 7.5 HP motor: 24.2 A• 10 HP motor: 30.8 A• 15 HP motor: 46.2 AStep 2sizeCalculate the maximum allowable circuit breakerApply NEC 430.52:• Maximum breaker size= (2.5 X FLC of largest motor) + sum of FLCs of otl• Maximum breaker size=(2.5×46.2 A) + 24.2 A + 30.8 A + 30.8 A• Maximum breaker size= 115.5 A + 24.2 A + 30.8 A + 30.8 A• Maximum breaker size = 201.3 AStep 3Select the standard circuit breaker sizeSolutionThe size of the inverse time circuit breaker should be 225 A.But in the answer they give me 200 AMay be I’m doing something wrong I just copy and paste the question to google, and google give a 200 A answer, But google give me 42 A for the 15 HP 208V 3-phase motor, I check the table 430.250 and shows 46.2Thank you
You're on the right track! Let's break down where the discrepancy might be.
Your Calculation (Based on NEC Table 430.250)
7.5 HP: 24.2 A 10 HP: 30.8 A 15 HP: 46.2 A Max Breaker = (2.5 46.2) + 24.2 + 30.8 + 30.8 Max Breaker = 115.5 + 24.2 + 30.8 + 30.8 Max Breaker = 201.3 A Rounding up to the next standard breaker size would be 225AThe NEC allows you to round up to the next standard size if the calculation doesn't land directly on a standard breaker size. Standard sizes are important because breakers are manufactured in specific ratings.
The 200A Discrepancy and Code Interpretation
The answer of 200A likely arises from a specific interpretation of the NEC. Here's how:1.
Rounding Down Instead of Up (Permitted, but Less Conservative)
The NEC does not require you to round up to the next standard size. It allows you to. It also permits you to use the next lower standard size provided it can carry the load. But it requires permission from the Electrical Inspector.2.
Applying 430.52(C)(1) Exception 1
NEC 430.52(C)(1) gives you a way to potentially use a smaller breaker if your initial calculation results in an issue. It says that, if the calculated breaker size doesn't correspond to a standard size, you can go up to the next standard size. However, if that next standard size trips on motor starting current, you can increase the breaker, but the code places a cap on how far you can go:For non-time delay fuses, you can't exceed 400% of the motor's FLC.
For Inverse Time Breakers, you can't exceed 250% of the motor's FLC.In our case, using a 225A breaker would be the way to go with the calculations done.
Conclusion and Recommendation
Your Calculation is Correct
Your math and use of NEC Table 430.250 and the basic 250% rule from 430.52 are accurate. A 225A breaker would provide greater protection and safer operation.
200A is Borderline and Requires Scrutiny
A 200A breaker might be permissible under specific circumstances (rounding down, potential application of exceptions), but it's on the edge and requires careful consideration of the motor starting characteristics and potential nuisance tripping.In a real-world installation,
always consult with a qualified electrician and the local electrical inspector. They will have the final say on what is code-compliant and safe for your specific application. They can also assess the actual starting characteristics of the motors being used. Flag for review