Mike Dolt Enterprises
Mike Dolt Enterprises
User: designer82, Location: Boston
Asked on: Sunday, January 5, 2025 at 11:59 pm EST
Answered on: Thursday, March 6, 2025 at 6:58 am EST
Question: Table 450.3(B) says "Maximum" rating or setting of OCPDI have a 45kVA transformer (480/277V). Primary rated current would be 54.126 AmpI have primary and secondary protection in my setup so 250% for primary and 125% for secondary per the table.Question is... does it HAVE to be 250% as in 54.126 * 2.5 = 135.3 A resulting in 150A breaker for the primary?Or can it be less, say 100 Amp?I'm thinking less is acceptable since Table 450.3(B) says "Maximum".What's throwing me off is that the NEC handbook examples are using the maximum for the example problems.Thanks
You are correct. The "Maximum" rating in Table 450.3(B) means you can use a breaker up to that calculated value, but you are not required to. You can definitely use a smaller breaker.Here's why and some considerations: "Maximum" is a Ceiling
The key word is "Maximum." This sets the upper limit for the overcurrent protection device. You are allowed to use a breaker that is lower than the maximum calculated value. The NEC Handbook examples often use the maximum to show how to apply the rule at its limit, but that doesn't mean it's the only valid solution.
Coordination and Protection
The primary goal of overcurrent protection is to protect the transformer from overload and short-circuit conditions. As long as your chosen breaker (in your example, 100A) can: Handle the transformer's inrush current without nuisance tripping during energization. Protect the transformer from sustained overloads exceeding its rating. Provide adequate short-circuit protection.Then, it is acceptable to use a lower-rated breaker.
NEC 240.4(B) "Next Higher Standard Overcurrent Device Rating" If after you calculate the 125% rating for the secondary the calculated value does not match a standard breaker size, you can use the next higher standard breaker size as long as the new overcurrent device is not rated for more than 800 amps. Example
Your calculation yields 54.126A 2.5 = 135.3A. You could use a 150A breaker. However, a 100A breaker is perfectly acceptable if it meets the conditions described above.
In summary
Yes, you can use a 100A breaker in your scenario, as long as it provides adequate protection for the transformer against overloads and short circuits, and handles the inrush current without nuisance tripping. The "Maximum" rating in Table 450.3(B) is an upper limit, not a required value.
Recommendation: Consult with a qualified electrical engineer to perform a short-circuit and coordination study to ensure your selected breaker size is adequate for the entire system and coordinated with the secondary overcurrent protection. This is especially important for larger transformers and critical applications. Flag for review