AC breaker size calculator

How the circuit is worked

Two things set the circuit for a compressor. The wire has to carry the minimum circuit ampacity, and the breaker has to be small enough to protect the circuit yet big enough to let the compressor start. Article 440 of the NEC treats them separately, which is why the breaker can be larger than the wire would suggest.

Minimum circuit ampacity (MCA)

MCA = RLA of every compressor + full-load amps of the other motors + 25 percent of the highest of themNEC 440.33 adds 25 percent of the highest motor-compressor or motor current in the group to the sum of all the loads. Usually that is the compressor, and the MCA is 1.25 x its RLA plus the rest. Here the fan and other motors are one field, so when their total is the highest the page takes it as one motor, which can only raise the MCA. The worked example, a 17.6 A compressor with a 1.2 A fan, comes to 23.2 A.

Maximum breaker or fuse (MOP)

limit = 2.25 x RLA of the largest compressor + a second compressor + other motors, rounded down to a standard size, then raised to the size of the MCA and to 15 A if lowerThe method one manufacturer publishes for UL listing (Titus, for terminal units), and a compressor maker's own figure may differ. 225 percent is also the top of what NEC 440.22(A) allows. In the worked example 2.25 x 17.6 A + 1.2 A is 40.8 A, so 40 A. It is worked only when nothing from the nameplate is entered and a compressor is the largest load.

NEC basic limit, a reference line

1.75 x RLA of the largest compressor + a second compressor + other motorsThe basic limit NEC 440.22(A) starts from, without an allowance for starting the compressor. The code allows more, up to 225 percent, when the compressor will not start on the smaller breaker. In the worked example it is 32 A. A breaker at 175 percent can trip at start, and then the nameplate value is the one to use.

Standard sizes

NEC Table 240.6(A), from 10 A up440.22(A) also says the device is not required to be smaller than 15 A, and the manufacturer method prints no less, so a worked maximum is never below 15 A. A nameplate MOP below 15 A is shown as the standard breaker it allows, with a warning to check the plate.

Heat strips

I = kW x 1,000 x V / Vrated squared; conductor at 1.25 x I; elements in groups of 48 A or lessNEC 424.4(B) for the 125 percent on the conductors, and 424.22(B) for the heating elements, which are protected at no more than 60 A and subdivided into groups of 48 A or less when the equipment is rated above 48 A. A 10 kW strip at 240 V draws 41.7 A, so its circuit needs 52.1 A of conductor and a 60 A breaker. A heater circuit is also held to the small-conductor limits of 240.4(D), which the result shows.

Seven circuits worked the same way

The currents below are illustrations and published worked examples, not typical values for any equipment. Each row runs through the same function the form uses.

For the 20 A compressor with a 2.5 A fan, 225 percent comes to 47.5 A, so the maximum is 45 A. That follows the method one manufacturer publishes for UL listing, and a compressor maker's own figure may differ. The NEC basic limit of 175 percent, 37.5 A, is a reference line. NEC 440.22(A) allows more, up to 225 percent, when the compressor will not start on the smaller breaker, and a breaker sized at 175 percent can trip at start. In that case the nameplate value is the one to use. On heat strips, the same 10 kW element draws 41.7 A on 240 V and 36.1 A on 208 V, because it makes 7.51 kW there.

Standards and data sources

• NEC 2023, Article 440, Air-Conditioning and Refrigerating Equipment - 440.22(A) for the breaker or fuse limit, 440.32 and 440.33 for the conductor ampacity, 440.4(B) for the nameplate markings, and 440.34 where other loads share the circuit.
• NEC 2023, Article 424, Fixed Electric Space-Heating Equipment - 424.4(B) for the 125 percent conductor sizing and 424.22(B) for dividing the resistance elements of a heater rated above 48 A into groups of 48 A or less, protected at no more than 60 A.
• NEC 2023, 110.3(B) and Table 240.6(A) - 110.3(B) for installing listed equipment according to its instructions and markings, and Table 240.6(A) for the standard ampere ratings of breakers and fuses.

Limitations of this calculator

• The nameplate comes first. When the unit prints a minimum circuit ampacity (MCA) and a maximum overcurrent protection (MOP or MOCP, sometimes called max fuse), those two printed numbers govern the installation. Without them the page works both figures from the currents; once you enter them, it shows the printed values, checks them against the currents and does not print a worked maximum beside them. NEC 110.3(B) has listed equipment installed according to its listing and labeling, and 440.4(B) has multimotor equipment marked with these values.
• This page gives preliminary numbers for estimating and for checking a quote. A licensed electrician sizes, installs and signs off the circuit under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
• No typical RLA, MCA or MOCP is given for any tonnage or model. They differ between manufacturers and between two units of the same tonnage, so you type in the numbers from the data plate. The default example is a published worked example, not a typical unit.
• The calculation follows the 2023 edition of the NEC. Your state or city may have adopted another edition or added amendments. NFPA committee documents show that the 2023 cycle added an exception to 440.22(A) about the next higher standard size. The final 2023 text was not read, and this page does not use it. Check the edition your AHJ enforces.
• The maximum follows the method one manufacturer publishes for UL listing (Titus, for terminal units): 225 percent of the largest motor plus the other loads, rounded down, never below the MCA's size and never below 15 A. A compressor maker's own figure may differ, which is one more reason the printed MOP governs. The 175 percent line is the NEC basic limit, shown for reference. The worked maximum appears only when nothing from the nameplate is entered and a compressor is the largest load. When the fan and other motors total more, NEC 440.22(B)(2) sets the maximum, which the page does not model, so it gives none. A nameplate MOP is shown as the breaker it allows, even below 15 A, and as per nameplate when it is above what 225 percent of the currents gives.
• The smallest breaker shown, the smallest standard size at or above the MCA, is this calculator's convention for a sensible lower end. The code sets a maximum for the compressor device, and it does not name a minimum here.
• The standard sizes are the ones in Table 240.6(A) from 10 A up. The extra fuse-only sizes of 1, 3, 6 and 601 A are not offered.
• Compressor mode takes one or two compressors and works in amps, so it does not depend on the number of phases. Heat strip mode is single phase only. A three-phase heater needs a different current formula that this page does not carry.
• A heater kit inside an air handler has its own MCA and MOCP on the unit's nameplate, with the blower motor counted in. Use those numbers. The heat strip mode is for a stand-alone resistance heater or a kit you are checking by itself.
• Above 48 A the heating elements are divided into groups of 48 A or less, and the page splits them evenly, which real kits often do not, for example 15 kW as 10 kW + 5 kW. The groups, and how many supply circuits feed them, are on the heater's nameplate. The sizes shown are for one branch circuit per group, and a kit with a single supply circuit uses its own nameplate numbers.
• Cord-and-plug room air conditioners (Article 440, Part VII), motor circuits that are not hermetic compressors (Article 430), disconnects, ground-fault and arc-fault protection, and the equipment grounding conductor are not covered.

Common mistakes sizing an AC breaker

1. Sizing the breaker from the wire. For a compressor circuit the breaker is not chosen from the conductor's ampacity. Article 440 sets its own limit from the compressor current, and the nameplate prints the result as MOCP. A breaker larger than the wire's ampacity can be correct there. A heater circuit is different, because it is not an Article 440 circuit and the small-conductor limits of 240.4(D) hold the breaker down on 14, 12 and 10 AWG.
2. Using the MCA as the breaker size. The MCA is a floor for the wire. The breaker has a ceiling, the MOCP. A breaker equal to the MCA is the least that carries the load, and one above the MOP is never allowed. Read the plate.
3. Using the motor tables of Article 430. A hermetic compressor is worked from its rated-load current under Article 440, with different factors from an ordinary motor. Use Article 440 for the compressor and Article 430 only for motors that are not hermetic compressors.
4. Stopping at 175 percent. The NEC's 175 percent figure is where 440.22(A) starts, and the code allows up to 225 percent when the compressor will not start on the smaller breaker. A breaker at 175 percent can trip at start. In the worked example the page shows 40 A against a 32 A basic limit. Where the plate prints an MOP, install to it.
5. A rule of thumb per ton. Two 3 ton units can print different MCA and MOCP, because the compressor, the fan motors and the efficiency differ. Read the plate on the unit you are wiring.
6. Treating a large heat strip as one load. Above 48 A the heating elements are divided into groups of 48 A or less. A 15 kW strip at 240 V draws 62.5 A, which is two groups of 31.3 A each. Real kits often divide unevenly, for example 15 kW as 10 kW + 5 kW, and the 10 kW group needs a 60 A breaker. The heater's nameplate has the groups and the supply.
7. Forgetting that 240 V ratings fall on 208 V. A heater rated in kW at 240 V makes about three quarters of that on 208 V. The current drops too, so work the circuit at the voltage that will really be there.

Common questions about AC breaker sizing

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