J Ask Jack — Tool Finder ›
Full Calculators Directory Index ›

Overcurrent Protection Inputs

Circuit Breaker & Fuse Sizing Calculator

Determine standard circuit breaker and fuse ampere ratings for motor branch circuits (NEC 430.52) and general conductors (NEC 240.4 & 240.6).

⚡ NEC 2023 Code Basis & Electrical Safety Disclaimer

Based on NEC 2023 (same as 2020 & 2017 Articles 240 and 430). Estimates for planning only — electrical installations must comply with local code editions and be performed/verified by a licensed electrician. Not a substitute for engineered design or an AHJ ruling.

🎛️ Overcurrent Protective Device (OCPD) Sizing Schematic
Breaker Sizing DiagramDiagram showing breaker box, OCPD rating, feeder wire, and motor/load termination.

Overcurrent Protection Results

Standard Breaker / OCPD Rating
40 A Breaker
NEC 240.6(A) Standard Size
Calculated Raw Max Rating
38.0 A
250% FLC (15.2 A × 2.50)
Code Rounding Rule Applied
Next Standard Up
NEC 430.52(C)(1) Ex. 1
System Load / FLC Basis
15.2 A
5 HP 3-Phase 230V Motor FLC

Worked Example: 5 HP 3-Phase 230V Motor Breaker Sizing

Step-by-step motor branch protective device calculation:

  • Step 1 (Lookup Table FLC): NEC Table 430.250 for 5 HP 3-phase at 230V $\rightarrow$ 15.2 Amperes FLC.
  • Step 2 (Apply Protection Multiplier): Per NEC Table 430.52, an inverse-time circuit breaker multiplier is 250%. Raw calculation = 15.2 A × 2.50 = 38.0 Amperes.
  • Step 3 (Next Standard Size Up): Because 38.0 A is not listed in NEC 240.6(A) standard ratings (35 A, 40 A, 45 A...), NEC 430.52(C)(1) Exception 1 permits rounding up to the next higher standard rating $\rightarrow$ 40 Ampere Breaker.

Engineering Code & Reference Citations

  • NFPA 70 / NEC Section 240.6(A): Standard Ampere Ratings for Fuses and Fixed-Trip Circuit Breakers. nfpa.org
  • NFPA 70 / NEC Section 430.52: Rating or Setting for Individual Motor Circuit Protection. nfpa.org
  • NFPA 70 / NEC Section 240.4(B): Devices Rated 800 Amperes or Less (Next Higher Standard Rating Rule). nfpa.org

Frequently Asked Questions (FAQ)

What circuit breaker size is required for a 5 HP 3-phase 230V motor?

Per NEC Table 430.250, a 5 HP 230V 3-phase motor has an FLC of 15.2 Amperes. Sizing an inverse-time circuit breaker at 250% (15.2 × 2.50 = 38.0 A) permits rounding up to the next standard rating of 40 Amperes per NEC 430.52(C)(1) Exception 1 and 240.6(A).

What are the standard circuit breaker and fuse sizes in NEC 240.6(A)?

Standard ampere ratings listed in NEC 240.6(A) are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 Amperes.

What is the NEC Next Standard Rating Rule (240.4(B))?

Under NEC 240.4(B), if a calculated conductor ampacity does not correspond to a standard overcurrent device rating in 240.6(A), the next higher standard rating is permitted provided the rating is 800 Amperes or less and the circuit is not a multioutlet branch circuit supplying receptacles.

Why are motor circuit breakers sized larger than the conductor ampacity?

Electric motors draw a heavy locked-rotor inrush current (typically 5 to 6 times FLA) during startup. NEC Article 430 allows larger short-circuit protection (e.g. 250% for breakers) to prevent nuisance tripping during motor starting, while separate thermal overload relays protect the motor and wire against sustained overload.

What percentage applies to dual-element time-delay fuses for motor protection?

Per NEC Table 430.52, dual-element time-delay fuses are sized up to 175% of the motor full-load current (FLC), rounded up to the next standard rating in 240.6(A).

What percentage applies to non-time-delay fuses for motor circuits?

Per NEC Table 430.52, non-time-delay fuses are permitted up to 300% of the motor full-load current (FLC).

How does continuous load affect circuit breaker sizing?

Under NEC 210.20(A) and 215.3, the overcurrent device protecting a continuous load (runs 3 hours or more) must be sized at not less than 125% of the continuous load plus 100% of non-continuous load.

What breaker size is needed for a 10 HP 460V 3-phase motor?

Per NEC Table 430.250, 10 HP at 460V = 14.0 A FLC. An inverse-time breaker at 250% is 14.0 × 2.50 = 35.0 A. Since 35 A is a standard rating in 240.6(A), a 35 Amp breaker is specified.

Can an instantaneous-trip breaker be used for motor branch protection?

Yes, but per NEC 430.52(C)(3), instantaneous-trip circuit breakers (magnetic-only) are permitted only as part of an approved listed combination motor controller assembly and are sized up to 800% (or 1100% for Design B energy efficient motors).

What is the maximum breaker size for a 20 AWG or 14 AWG copper wire?

Per NEC 240.4(D), small conductor overcurrent protection is limited to: 14 AWG Cu = 15 A max; 12 AWG Cu = 20 A max; 10 AWG Cu = 30 A max (unless specifically permitted under Article 430 for motors).

What fuse or breaker size is required for a 1 HP 115V single-phase motor?

1 HP at 115V single-phase = 16.0 A FLC (Table 430.248). Inverse-time breaker max = 16.0 × 2.50 = 40.0 A standard breaker.

Are NEC 240.6 and 430.52 protection rules identical across NEC 2017, 2020, and 2023?

Yes. The standard ampere ratings in NEC 240.6(A) and motor branch protection percentages in Table 430.52 are identical across the 2017, 2020, and 2023 editions of NFPA 70.