⚡ Running Current
Circuit Breakdown
Full Calculation Table
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How to Use the Single Phase Calculator
Resistive loads like heat strips need only a wattage figure. Motors and compressors need nameplate FLA, since calculating from horsepower alone is less accurate.
For motors, add the locked rotor amps if it's printed on the nameplate to see an estimate of starting current as well as running current.
The calculator shows running current, power, an MCA-style sizing estimate, and a starting current comparison for motor loads.
Single-Phase Circuits in Residential and Light Commercial HVAC
Single-phase power is the standard for residential HVAC equipment and most light commercial installations in Canada, typically delivered as 120/240V split-phase service. Understanding how single-phase current, power, and starting behavior differ from three-phase equipment matters for correctly sizing wire, breakers, and disconnects on these circuits.
Why Single-Phase Motors Need Extra Starting Help
A three-phase motor starts itself because its three offset voltage waveforms naturally create a rotating magnetic field that gets the rotor moving. A single-phase motor doesn't have this advantage on its own, since a single waveform alone only creates a pulsing, non-rotating field. To solve this, single-phase compressors and motors use a start capacitor, run capacitor, or potential relay to artificially create the phase shift needed for starting torque. This extra complexity is one reason single-phase compressors generally show a higher locked rotor amps to full load amps ratio than equivalent three-phase units.
Locked Rotor Amps and Starting Current
Locked rotor amps (LRA) describes the current a motor draws for a brief moment at the instant of starting, before it begins spinning and its impedance rises. Single-phase HVAC compressors commonly show an LRA figure 4 to 6 times their running FLA, which is why breakers on these circuits need to be HACR-rated and sized with enough headroom to ride through this brief surge without nuisance tripping. This calculator estimates the LRA-to-FLA ratio when you provide the nameplate LRA value, giving a clearer picture of what the circuit needs to tolerate at startup.
Voltage Drop Concerns on Single-Phase Runs
Single-phase voltage drop calculations use a multiplier of 2, accounting for both the outgoing and return conductor carrying the full load current, compared to the lower multiplier used for three-phase circuits. This means single-phase circuits of the same current and distance generally experience more voltage drop than an equivalent three-phase circuit, which matters more on long runs to detached garages, outbuildings, or remote condensing units. The voltage drop calculator covers this calculation in detail.
Next Steps
Once you have the running and starting current figures, use the wire size calculator to confirm conductor sizing, and the breaker size calculator to confirm the breaker handles the locked rotor surge without nuisance tripping.
Frequently Asked Questions
A single-phase motor has no inherent rotating magnetic field to start itself moving the way a three-phase motor does, so it needs an extra component, typically a start capacitor or potential relay, to create enough starting torque to get the compressor moving from a standstill. This is why single-phase compressors generally have a higher locked rotor amps to full load amps ratio than equivalent three-phase compressors, which rely on the natural phase rotation for starting torque instead.
Single-phase HVAC compressors commonly show a locked rotor amps figure 4 to 6 times their full load amps rating, though this varies by compressor design and size. Always use the nameplate LRA value when sizing a breaker's instantaneous trip rating or evaluating voltage drop during startup, rather than assuming a ratio, since actual values vary between manufacturers and models.
Use the nameplate-listed voltage for the specific equipment. Residential window units and small appliances often run at 120V, while residential central HVAC equipment and light commercial units commonly run at 240V or sometimes 208V when fed from a three-phase building service. Using the wrong voltage in this calculator will produce an inaccurate current figure, since current and voltage are inversely related for a fixed power.
Not necessarily in terms of energy efficiency, but single-phase motors are generally less efficient at converting electrical input into mechanical output compared to equivalent three-phase motors, particularly at larger horsepower ratings. This is one reason larger commercial HVAC equipment almost always specifies three-phase power when it's available at the site, reserving single-phase for residential and smaller light commercial applications.
Related HVAC Tools
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