Calculators

Electrical calculators for HVAC work

The circuit for a condenser, heat pump or mini split is sized from the nameplate, not the tonnage. The breaker calculator starts from the compressor RLA and the fan motor currents, or the heat strip rating. It works out the MCA under NEC 2023 Article 440, or Article 424 for heat strips, and the largest breaker by the manufacturer method, with the 225 percent limit in 440.22 as the ceiling. A nameplate MOCP, when you enter it, governs the breaker. The wire and voltage drop calculators carry the circuit on to the conductor and the run. The amps and watts calculator gives running watts and amps from the capacity and EER2 or from a current reading, and starting kVA from the LRA.

They are for checking a quote and talking to the electrician on the job. A licensed electrician installs the circuit, and the nameplate and local amendments govern over any number here.

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Which one you need

Every tool in this section

  • Electrical for HVAC

    AC Breaker Size Calculator

    Minimum circuit ampacity and the largest breaker for an AC, heat pump or mini split, plus heat strip circuits.

    Worked from the compressor and fan currents on the nameplate. The nameplate MCA and MOCP govern, and no values are given by tonnage or model. Heat strip mode is single phase, and a licensed electrician installs the circuit.

    Based on NEC 2023 Article 440 and Article 424

  • Electrical for HVAC

    AC Wire Size Calculator

    Copper or aluminum conductor for the nameplate MCA, with the voltage drop over your run.

    Applies the 60°C (140°F) or 75°C (167°F) terminal rule, optional heat and conductor-count corrections, and the 3 percent voltage drop guideline from the Informational Note, which is not a requirement.

    Based on NEC 2023 Table 310.16, 110.14(C) and 310.15

  • Electrical for HVAC

    AC Amps and Watts Calculator

    Running watts and amps of an AC or heat pump from its capacity and EER2, watts from a current reading, and starting kVA from the LRA.

    Works at the 95°F EER2 rating point, on single or three phase. No power factor is guessed, no generator size is given, and the breaker and wire come from the nameplate.

    Based on 10 CFR 430 Appendix M1 (EER2), DOE power equations

  • Electrical

    Voltage Drop Calculator

    Volts and percent dropped over a run, the smallest wire for a target drop, and the longest run for a size.

    Single phase or balanced three phase, 120 to 480 V, copper or aluminum from 14 AWG to 4/0. Shows the Table 310.16 ampacity next to each size. The 3 percent figure is an Informational Note, not a requirement.

    Based on K method (Mike Holt), NEC 2023 Chapter 9 Table 8

  • Electrical

    Baseboard Heater Breaker Size Calculator

    Breaker, wire and circuits for electric baseboard heaters, or how many heaters fit on a 15, 20, 25 or 30 amp circuit.

    Single phase at 120, 208 or 240 V, up to two heater sizes. Conductors at 125 percent of the load, the breaker at the next standard size, and 3,840 W on a 20 A circuit at 240 V. A 240 V heater on 208 V makes 75 percent of its watts. A circuit with more than one heater is held to 30 A.

    Based on NEC 2023 Article 424 (424.4), Table 240.6(A)

Other calculators

Every calculator here lists the standard it works from and what it leaves out. The numbers are for sales-phase sizing and field checks, not a permit submittal. Methodology lists the standard behind each one.