How the calculation works
Full-load currents are from NEC Table 430.248 (single-phase) and 430.250 (three-phase). Branch-circuit conductors are sized at 125% of FLC (430.22) using the 75°C column. Short-circuit/ground-fault protection has specific percentage limits and exceptions under the adopted code; this tool no longer selects breaker or fuse sizes. Overload protection (430.32) is separate, typically 115–125% of nameplate current. Use nameplate current for overloads; this tool does not size starting or high-efficiency motors beyond the tables.
Worked example
10 HP, 3-phase, 460 V: table FLC = 14 A; 125% conductor-current screen = 17.5 A. These figures do not select a final conductor, breaker, fuse or overload. Apply installation derating, terminal limits, nameplate overload rules and the applicable code exceptions.
Common mistakes to avoid
- Sizing conductors from nameplate current instead of the table value.
- Skipping the 125% conductor factor.
- Treating the maximum breaker rating as a required size.
Frequently asked questions
Do I use nameplate or table FLC?
Use the table FLC for conductor and short-circuit sizing; use the nameplate for overload protection.
When can maximum sizes be exceeded?
If the motor will not start, 430.52(C)(1) Exceptions allow higher ratings (up to 400% for breakers in some cases).
How is motor overload protection sized?
Per 430.32, typically 125% of nameplate current for motors with a 1.15 service factor or a 40°C rise or less, and 115% for others.
What about motors on variable frequency drives?
Use the drive's input current ratings and the Article 430 Part X rules, not these tables alone.
Read the guide
Motor Circuit Sizing Under NEC Article 430: A Worked Example
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For informational and educational purposes only. Always consult a licensed electrician and local code authority. See our Electrical & Legal Disclaimer. Found an error? Tell us.