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Minimum Circuit Ampacity Calculator

Enter compressor and motor loads to get the MCA, the maximum overcurrent protection, or check a circuit from a nameplate.

Assumes NEC 2023 Article 440 (MCA 125 % of the largest compressor, MOCP 175 % up to 225 %) and NEC 310.16 copper or aluminum in the terminal column you pick; the equipment nameplate MCA and MOCP govern. Free, runs in your browser. Verify with your local code and a licensed electrician before you install.

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Minimum circuit ampacity (MCA) is the lowest ampacity the supply wires of an air conditioner, heat pump or other motor-driven unit may have, and this calculator works it out together with the largest breaker or fuse allowed (MOCP). Enter the amps of each compressor, fan and other load to get the MCA from NEC 440.33, or type the MCA and MOCP printed on a nameplate to get the smallest wire and check the breaker you plan to fit. A third mode answers questions such as “can I run 16 A on a 20 A circuit?” with the 80 % rule for continuous loads. Every answer lists its calculation steps with NEC article numbers, plus any warnings. Voltage drop is not part of the result, so check long runs separately.
Minimum circuit ampacity example

How to use the Minimum Circuit Ampacity Calculator

  1. Under “What do you want to check?”, pick “Work out MCA and MOCP from motor amps”, “Wire and breaker from the unit’s nameplate” or “Can I run this load on this circuit?”.
  2. For motor amps, type one load per line in “Loads in the unit (one per line: name, amps, type)”. The type is c for a compressor (RLA), m for a fan or other motor (FLA) or l for another load such as a heater, for example: compressor, 24, c.
  3. If heaters or other non-motor loads run 3 hours or more, open “More options” and tick “Other (non-motor) loads run 3 hours or more: take them at 125 %”.
  4. For a nameplate, enter “MCA on the nameplate (A)” and “MOCP / max fuse or breaker on the nameplate (A)”. Add “Breaker you plan to use (A, optional)” for a check.
  5. For a circuit check, enter “Load current (A)” and “Circuit breaker rating (A)”, and tick “Continuous load (runs 3 hours or more)” when it applies.
  6. Set “Conductor” to Copper or Aluminum. Under “More options”, the “Ampacity column (terminal / insulation rating)” defaults to 75 °C; pick 60 °C if the terminals are not marked 75 °C.
  7. Read the result rows, steps and warnings.

Example buttons such as “AC condenser, 24 A compressor” load sample inputs. “Copy result”, “Copy link” and “Print / save as PDF” keep or share the answer, and Reset clears the form.

How do you calculate minimum circuit ampacity?

Minimum circuit ampacity is 125 % of the largest motor or compressor current plus 100 % of every other motor and load in the unit (NEC 440.33, 430.24). Use the compressor’s rated-load amps, or its branch-circuit selection current when that is larger, and the full-load amps of each fan. The supply wire needs at least this ampacity.

  1. List each load with its amps and type.
  2. Multiply the largest motor current by 1.25.
  3. Add the other motors and loads at 100 %. Non-motor loads that run 3 hours or more count at 125 % (NEC 210.19(A)(1)).
  4. Pick the smallest conductor whose NEC 310.16 ampacity in the chosen column is at least the MCA.

With the default inputs, a 24 A compressor and a 1.2 A condenser fan, the tool returns:

ResultCalculationValue
MCA125 % × 24 A + 1.2 A31.2 A
Max breaker or fuse (175 %)175 % × 24 A + 1.2 A = 43.2 A, rounded down40 A
Max if it will not start (225 %)225 % × 24 A + 1.2 A = 55.2 A, rounded down50 A
Copper wire, 75 °C columnat least 31.2 A10 AWG (35 A)
Copper wire, 60 °C columnat least 31.2 A8 AWG (40 A)

How is MOCP (maximum overcurrent protection) calculated?

MOCP is 175 % of the largest compressor’s current plus the sum of all other loads, rounded down to a standard breaker or fuse rating because the rule sets a ceiling (NEC 440.22(A), (B)(1), 240.6(A)). If a device of that size cannot carry the starting current, the limit may be raised to 225 %.

For a rooftop unit with two 16.3 A compressors, two 3.5 A condenser fans, a 10.9 A supply fan and a 5.5 A power exhaust, the tool gives an MCA of 60.075 A (6 AWG copper, 65 A at 75 °C). The 175 % sum is 68.225 A, rounded down to 60 A; at 225 % it is 76.375 A, rounded down to 70 A. Because 60 A is below the MCA, the tool warns you to use the MOCP marked by the manufacturer.

The tool calculates no MOCP when no compressor is entered, or when a fan or other motor is larger than the largest compressor (NEC 440.22(B)(2)). Use the nameplate in those cases.

What is the difference between MCA and MOP?

MCA is the minimum ampacity of the wire that feeds the unit, while MOP (also written MOCP) is the largest fuse or breaker allowed to protect it. The wire must be rated at least the MCA, and the breaker must not be above the MOP. A standard breaker between the two values passes the tool’s check.

Nameplate MCA / MOCPBreaker you plan to useTool’s answer
15 A / 20 A15 AYes, 14 AWG copper
28.5 A / 45 A40 AYes, 10 AWG copper (NEC 240.4(G) allows it)
28.5 A / 45 A25 AAllowed, but below the MCA, so it may trip in normal use
28.5 A / 45 A50 ANo: above the 45 A marked on the unit

Worked examples

InputResult
2 loads from the unitMCA 31.2 A, max breaker 40 A
6 loads from the unitMCA 60.08 A, max breaker 60 A
2 loads from the unitMCA 23.4 A, max breaker 30 A
Nameplate MCA 28.5 A, MOCP 45 A10 AWG Copper (35 A at 75 °C, NEC 310.16)
16 A load on a 20 A circuitYes: 16 A is within the 16 A that a 20 A circuit may carry.

Calculated by the tool above; open it with the same values to see each step.

Questions, briefly answered

What does MCA mean on an AC unit?

MCA stands for minimum circuit ampacity: the smallest ampacity the branch-circuit wires feeding the unit may have. Nameplates print it as “Min. circuit ampacity” or “Minimum circuit amps”.

Can I run 12 amps on a 15 amp circuit?

Yes. Even for a continuous load, a 15 A breaker may carry 80 %, which is 12 A, and the tool suggests 14 AWG copper. A non-continuous load may use the full 15 A.

Can I use a breaker smaller than the MOP?

Yes, the MOP is a maximum, not a target. A breaker below the MCA, though, may trip in normal use; the nameplate mode flags this when you fill in “Breaker you plan to use (A, optional)”.

Should I use the nameplate values or calculate them?

Use the nameplate. Listed equipment is marked with its own MCA and MOCP, and NEC 110.3(B) requires following those markings. Calculate from motor amps only when the marked values are missing or unreadable.

Why does the 60 °C column give a bigger wire?

Circuits of 100 A or less use the 60 °C column unless the equipment terminals are marked 75 °C (NEC 110.14(C)(1)). For a 31.2 A MCA that means 8 AWG copper instead of 10 AWG.

Why can the breaker be larger than the wire’s ampacity?

Air-conditioning and refrigeration conductors protected under Article 440 are exempt from the usual small-conductor limits (NEC 240.4(G)). A 40 A breaker on 10 AWG copper is therefore accepted when the nameplate MOCP allows 40 A.