⚙ Three Phase

Three Phase Calculator

Calculate current draw, power, and circuit sizing for three-phase HVAC equipment. Includes balanced load checks for commercial compressors and motors.

📋 Project Info (optional, appears on printed report)

Project / Client

Address

Prepared By

⚙ Equipment Type

⚡ Circuit Values

Voltage (Line-to-Line)

Load Power

Nameplate Full Load Amps (FLA)

Nameplate Locked Rotor Amps (LRA) i

⚖ Phase Balance Check (Optional)

Enter measured current on each phase from a clamp meter to check for imbalance.

Phase A Current

Phase B Current

Phase C Current

⚙ Three Phase Results

⚡ Running Current per Phase

Enter values and calculate

Circuit Breakdown

Full Calculation Table

Value	Basis	Result

Export:

How to Use the Three Phase Calculator

Select Equipment Type and Enter Voltage

Resistive loads need only a wattage figure. Motors and compressors need nameplate FLA for an accurate current calculation.

Enter Load Values

For motors, add the locked rotor amps if available to see the starting current relationship.

Optionally Check Phase Balance

If you've measured current on each phase with a clamp meter, enter those values to check for imbalance.

Calculate

The calculator returns current, power, an MCA-style estimate, and a phase imbalance check if measured values were entered.

Three-Phase Circuits in Commercial HVAC

Three-phase power is the standard for larger commercial HVAC equipment, including rooftop units, chillers, and large compressors, because it delivers more power through proportionally less current than an equivalent single-phase circuit. Understanding the square root of 3 relationship that governs three-phase calculations is essential for correctly sizing wire, breakers, and disconnects on this equipment.

Why the Square Root of 3 Appears

In a balanced three-phase system, the three voltage waveforms are offset from each other by 120 degrees. This geometric relationship between line-to-line and line-to-neutral voltage produces the square root of 3, approximately 1.732, that shows up throughout three-phase power, current, and voltage drop calculations. It's the mathematical reason three-phase circuits can move the same amount of real power as a single-phase circuit while drawing meaningfully less current per conductor.

Self-Starting Without Extra Components

Unlike single-phase motors, a three-phase motor doesn't need a start capacitor or potential relay to begin rotating, since the three offset voltage waveforms naturally create a rotating magnetic field on their own. This generally gives three-phase motors a lower locked rotor amps to full load amps ratio than equivalent single-phase motors, though starting current is still meaningfully higher than running current and still needs to be accounted for in breaker selection.

Checking Phase Balance

A well-functioning three-phase motor should draw close to the same current on all three phases. Significant imbalance, often caused by voltage imbalance from the utility supply or a wiring issue, generates extra heat in the motor windings even when the average current across all three phases looks acceptable. This calculator's optional phase balance check compares measured current on each phase and flags imbalance worth investigating, which can sometimes catch a developing motor or supply problem before it causes a failure.

Common Three-Phase Voltages in Canada

208V three-phase is common in commercial buildings fed from a 120/208V wye service, while 460V or 600V three-phase is typical for larger industrial and commercial installations with their own dedicated transformer. The voltage selection significantly changes the calculated current for the same power, so always confirm the actual nameplate or system voltage rather than assuming a standard value.

Next Steps

Once you have the calculated current, use the wire size calculator to confirm conductor sizing, and the motor starter calculator if this is feeding a motor through a NEMA or IEC starter rather than a direct connection.

Frequently Asked Questions

The square root of 3, approximately 1.732, comes from the geometric relationship between line-to-line and line-to-neutral voltage in a balanced three-phase system, where the three voltage waveforms are offset by 120 degrees from each other. This factor appears in three-phase power, current, and voltage drop formulas specifically because of that 120-degree phase relationship, and it's the mathematical reason three-phase circuits can deliver more power through proportionally less current than single-phase circuits.

Significant current imbalance across the three phases of a motor, often caused by voltage imbalance from the utility or a wiring issue, generates extra heat in the motor windings and can shorten motor life even when the average current looks acceptable. A common rule of thumb flags voltage imbalance above 1% to 2% as worth investigating, since motor manufacturers often derate a motor's usable horsepower significantly even at fairly small voltage imbalance percentages.

Yes. Three-phase motors and compressors still draw a starting inrush current well above their running FLA, just generally at a lower ratio than equivalent single-phase motors. A HACR-rated breaker is still needed to handle this starting surge reliably without nuisance tripping, the same as for single-phase motor loads.

Not directly. Three-phase equipment requires either a genuine three-phase utility supply or a phase converter device that synthesizes a third phase from a single-phase or split-phase source. Phase converters add cost and complexity, and some types reduce the equipment's usable horsepower rating, so confirming genuine three-phase availability at the site before specifying three-phase equipment is the more straightforward path.