Circuit Breaker Size Calculator
Calculate the correct circuit breaker (overcurrent protective device) size for any circuit. Enter the load current and load type, and the calculator applies the NEC continuous-load and motor rules to find the next standard breaker rating.
Breaker size
20A
Minimum rating
20A
Use a 20 A breaker (next standard size)
Continuous load (NEC 210.20(A)): OCPD sized at 125 % of the load.
Always confirm the conductor ampacity supports the chosen OCPD; check our Wire Size calculator. Motor circuits also require separate overload protection (NEC 430.32).
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How breaker sizing works
The overcurrent protective device (OCPD) protects the conductor and equipment from overload and short-circuit. The minimum rating depends on the type of load:
- Non-continuous load: OCPD ≥ 100% of the load current.
- Continuous load (3+ hours): OCPD ≥ 125% of the load current (NEC 210.20(A)). The breaker must also be rated for 100% of its continuous rating — most standard breakers are rated at 80% continuous, so in practice the 125% rule and the 80% breaker rating are two ways of expressing the same requirement.
- Motor branch circuit: inverse-time breaker up to 250% of full-load current from NEC Tables 430.248/250 (NEC 430.52), with separate overload protection in the motor starter.
- HVAC / hermetic compressor: up to 175% of rated current, or the MOP listed on the nameplate (NEC 440.22).
The calculated minimum is always rounded up to the next standard size from NEC 240.6(A).
Standard breaker sizes (NEC 240.6(A))
These are the only sizes available for branch circuit protection. Calculated minimums must be rounded up to the next value on this list:
- 15, 20, 25, 30, 35, 40, 45, 50 A — residential and light commercial
- 60, 70, 80, 90, 100 A — appliance circuits and subpanels
- 110, 125, 150, 175, 200 A — service and feeder protection
- 225, 250, 300, 350, 400 A and above — commercial and industrial services
Worked examples
Example 1 — Continuous lighting load:A commercial lighting circuit draws 16 A continuously.
- Minimum OCPD = 16 × 1.25 = 20 A
- Next standard size = 20 A breaker
- Minimum conductor = 12 AWG copper (20 A at 75 °C)
Example 2 — Motor circuit (inverse-time breaker):A 10 HP, 230 V three-phase motor. NEC Table 430.250 gives FLC = 28 A.
- Maximum OCPD = 28 × 2.50 = 70 A
- Select 70 A breaker (standard size)
- Note: separate overload protection in the starter, set to ≤125% of nameplate current
Example 3 — Air conditioner:3-ton split system, compressor rated at 16.7 A, nameplate MOP = 30 A.
- Use the nameplate MOP = 30 A breaker
- This is less than 175% × 16.7 = 29.2 A — nameplate MOP controls
Common mistakes
The most frequent errors are applying 125% to non-continuous loads (only continuous loads require it), forgetting that motor overcurrent protection and motor overload protection are two separate devices with different sizing rules, and using the nameplate current for motor OCPD sizing instead of the NEC table value. For motors, always use the full-load current from NEC Tables 430.247 or 430.250, not the nameplate.
Who is this calculator for?
For electricians and electrical contractors selecting overcurrent protective devices under the NEC. Useful for branch circuit design, panel schedules, and verifying that existing breakers are correctly rated for their loads — particularly important for continuous loads, motor circuits, and HVAC equipment where the sizing rules differ from general branch circuits.
Frequently Asked Questions
- How do I size a breaker for a continuous load?
- NEC 210.20(A) requires the OCPD to be rated at least 125% of a continuous load (one expected to run for three hours or more). A 16 A continuous load needs a 20 A breaker — 16 × 1.25 = 20 A.
- Why are motor breakers sized so much larger than the motor current?
- Motors draw a large inrush current at start-up — typically 5–8× the running current. NEC 430.52 allows an inverse-time breaker up to 250% of the motor full-load current so it doesn't trip on starting. Overload protection is provided separately by the motor starter.
- What are the standard breaker sizes?
- NEC 240.6(A) lists standard ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 A and above. You always round up to the next standard size.
- Does the breaker size depend on the wire size?
- They're linked. The breaker must protect the conductor, so the conductor ampacity must be at least the breaker rating (with the small-conductor limits of 240.4(D)). Always verify both together — a 20 A breaker requires at minimum 12 AWG copper.
- How do I size a breaker for an air conditioner?
- AC units are typically sized per the nameplate Maximum Overcurrent Protection (MOP) rating. NEC 440.22 allows the OCPD up to 175% of the compressor rated current (or higher for hermetic motors). Always check the nameplate MOP first.
- What is the difference between a breaker and a fuse for OCPD purposes?
- Both protect conductors from overcurrent, but they respond differently. Time-delay fuses tolerate brief overloads (useful for motors); instantaneous-trip breakers do not. For motor circuits, the NEC allows inverse-time breakers up to 250% FLC, dual-element fuses up to 175% FLC, and non-time-delay fuses up to 300% FLC.
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