Air Conditioner Breaker Size Calculator

Related Calculators

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

MOCP or MOP: Maximum overcurrent protection. On an AC nameplate, it is the highest breaker or fuse size the equipment label allows.

MCA: Minimum circuit ampacity. It is the minimum wire amp rating to check, not the breaker size.

Compressor RLA or FLA: The compressor running amp rating. RLA means rated-load amps, and FLA means full-load amps.

Fan and other loads: The outdoor fan motor and any other load on the same AC equipment circuit.

Starting allowance: A multiplier used in estimate mode so the protective device can handle motor starting current.

Standard breaker size: A commonly listed breaker amp rating, such as 30, 35, 40, or 45 amps.

Headroom or margin: The number of amps left between a limit and the value being checked. A negative margin means the checked value is over the limit.

Mistake: Using MCA on nameplate as the breaker size.
Fix: Use MOCP or max breaker on nameplate for the breaker limit, and use MCA on nameplate only for Minimum wire amp rating to check.

Mistake: Rounding MOCP or max breaker on nameplate up to the next common breaker.
Fix: Use Largest standard breaker at or below the limit, which rounds down when needed.

Mistake: Entering locked-rotor amps in Compressor RLA or FLA.
Fix: Enter the running amp value labeled RLA or FLA, then add Fan and other loads separately.

Mistake: Leaving Fan and other loads blank in estimate mode.
Fix: Enter the fan and other load amps, or enter 0 if there are no other loads on the same AC equipment circuit.

Mistake: Treating Existing breaker size as proof that the setup is correct.
Fix: Check Existing breaker room below the limit; a negative number means the entered breaker is above the calculator limit.

Mistake: Entering wire gauge instead of ampacity in Entered wire amp rating.
Fix: Enter the conductor ampacity in amps after wire type, temperature rating, conduit fill, and local code rules are considered.

• The nameplate should control when MOCP or MOP is available. Estimate mode is only a fallback when the breaker limit is missing or unreadable.
• This calculator does not use BTU, tons, EER, or SEER for final breaker sizing because those values do not replace MCA and MOCP on the equipment label.
• Estimate mode is a simplified single-compressor calculation using compressor amps plus other loads. Multi-compressor equipment, heat pumps with extra equipment, and manufacturer-specific instructions can differ.
• The standard breaker list is limited to 15 through 200 amps. If the limit is below 15 amps or above 200 amps, use the exact manufacturer instructions and get a professional design.
• Entered wire amp rating assumes you already adjusted conductor ampacity for wire material, insulation temperature rating, terminal temperature rating, conduit fill, ambient temperature, and local code.
• The disconnect check is only a quick screen using Rated load for disconnect check. It does not replace the disconnect rating, fuse, or installation instructions on the nameplate.
• Electrical work can create fire, shock, and equipment-damage risks. A licensed electrician or local inspector should verify the final breaker, wire, disconnect, and installation.

Nameplate-first method

When Use nameplate MOCP is selected, the calculator uses the nameplate breaker or fuse limit directly. AC equipment branch-circuit ampacity and protection are normally based on the marked equipment values when they are provided [3].

mocp_limit_amps = nameplate_mocp_amps

The calculator also carries MCA on nameplate to the wire check output. MCA is treated as the conductor ampacity value to check, not as the breaker size.

Estimate method

When Estimate from compressor amps is selected, the calculator estimates MCA from compressor running amps and other loads. The conductor factor is 1.25 for the compressor in this simplified single-compressor method [1].

mca_required_amps = 1.25 * compressor_rla_amps + other_load_amps

The estimated breaker or fuse limit uses the selected starting allowance. Normal start allowance uses 1.75. Larger allowed limit for hard starting uses 2.25.

mocp_limit_amps = start_factor * compressor_rla_amps + other_load_amps

Standard breaker selection

The calculator selects from this loaded standard-size list: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, and 200 amps [2]. It picks the largest listed size that is not above the MOCP limit.

recommended_breaker_amps = largest standard_breaker_size where standard_breaker_size <= mocp_limit_amps

breaker_headroom_amps = mocp_limit_amps - recommended_breaker_amps

Optional checks

If Existing breaker size is entered, the calculator compares it with the MOCP limit and keeps negative values so an oversized breaker is visible.

existing_breaker_margin_amps = mocp_limit_amps - existing_breaker_amps

If Entered wire amp rating is entered, the calculator compares it with the MCA requirement and keeps negative values so undersized conductor ampacity is visible.

conductor_ampacity_margin_amps = conductor_ampacity_amps - mca_required_amps

If Rated load for disconnect check is entered, the quick disconnect screen uses 115 percent of that value.

disconnect_min_rating_amps = 1.15 * rated_load_for_disconnect_amps

Worked mini-example

Suppose the nameplate says MOCP 43 amps and MCA 31.8 amps. The standard breaker list has 40 amps and 45 amps, but 45 amps is above the 43 amp limit. The calculator returns 40 amps for Largest standard breaker at or below the limit, 43 amps for Breaker or fuse limit used, 31.8 amps for Minimum wire amp rating to check, and 3 amps for Room below the breaker limit.