Three-Phase Load Balance Calculator

A three-phase load balance calculator compares the entered Phase A load, Phase B load, and Phase C load to identify the heaviest phase, lightest phase, and the maximum departure from the average phase load. The primary output is the Imbalance estimate, expressed as a percentage.

Phase balancing is commonly reviewed when assigning single-phase branch circuits across a three-phase panelboard, evaluating a feeder load distribution, planning equipment additions, or investigating unequal voltage drop on phase conductors. A balanced phase loading arrangement generally helps avoid placing unnecessary current, voltage-drop burden, and thermal loading on one phase conductor while other phase conductors remain comparatively lightly loaded.

The calculation uses the three entered load values only. It does not create a panel schedule or determine feeder ampacity, conductor AWG or kcmil size, raceway fill, breaker size, or required adjustment and correction factors.

Phase Load Comparison

Enter the connected or calculated load assigned to each phase:

Input	Electrical use
Phase A load	Load assigned to phase A
Phase B load	Load assigned to phase B
Phase C load	Load assigned to phase C

All three entries should represent the same type of current value. For example, compare calculated feeder demand current on all phases, or compare actual measured amperes under the same operating condition. Do not compare a measured operating current on one phase with nameplate current or calculated demand load on another.

The calculator produces:

Result	Meaning
Total phase load	Sum of the three entered phase loads
Average phase load	Arithmetic average of Phase A, B, and C loads
Maximum phase load	Highest entered phase load
Minimum phase load	Lowest entered phase load
Maximum deviation	Largest difference between any phase load and the average phase load
Imbalance estimate	Maximum deviation expressed as a percentage of the average phase load

Imbalance Estimate Formula

The calculator first determines the average phase load:

\(\text{Average phase load} = \frac{\text{Phase A load} + \text{Phase B load} + \text{Phase C load}}{3}\)

It then finds the largest absolute difference between an entered phase load and that average:

\(\text{Maximum deviation} = \max \left( |A-\text{Average}|, |B-\text{Average}|, |C-\text{Average}| \right)\)

The Imbalance estimate is:

\(\text{Imbalance estimate} = \frac{\text{Maximum deviation}}{\text{Average phase load}} \times 100\%\)

This method shows how far the most uneven phase is from the three-phase average. It does not use a highest-phase-to-lowest-phase percentage formula, motor voltage-unbalance formula, or neutral-current calculation.

Calculation Example

For the following phase assignments:

Input	Entered load
Phase A load	100 A
Phase B load	80 A
Phase C load	90 A

The calculator returns:

Result	Value
Total phase load	270 A
Average phase load	90 A
Maximum phase load	100 A
Minimum phase load	80 A
Maximum deviation	10 A
Imbalance estimate	11.1111%

The average is:

\(\frac{100 + 80 + 90}{3} = 90\text{ A}\)

Phase A is 10 A above the average, Phase B is 10 A below the average, and Phase C matches the average. The largest departure is therefore 10 A:

\(\frac{10\text{ A}}{90\text{ A}} \times 100\% = 11.1111\%\)

For a panel-load review, this result indicates that moving a suitable single-phase branch circuit from Phase A to Phase B may reduce the phase spread, provided the circuit arrangement, equipment listing, and load characteristics support the change.

Feeder and Panel Load Review

Phase-load comparison supports preliminary decisions in several electrical workflows:

• Distributing 120 V branch circuits across phases in a three-phase, four-wire panelboard
• Reviewing phase loading before adding a branch circuit or equipment load
• Identifying the phase likely to experience the greatest conductor loading and voltage drop
• Comparing calculated demand currents during feeder or service load review
• Reviewing measured phase currents during troubleshooting or maintenance
• Planning circuit redistribution while maintaining proper breaker arrangement and system identification

The highest phase load may guide further conductor and feeder review, but it does not independently establish ampacity. Final conductor sizing must account for the applicable load calculation, continuous-load treatment, terminal rating, insulation temperature rating, ambient-temperature correction factor, adjustment factor for current-carrying conductors, overcurrent protection, and installation conditions.

A lower calculated phase imbalance also does not eliminate the need to evaluate voltage drop. Voltage drop depends on conductor length, AWG or kcmil size, conductor material, impedance, circuit current, and system configuration. A heavily loaded phase on a long feeder can require attention even when the three phase loads appear relatively close.

Field Verification Limits

This calculator evaluates only the three entered phase-load values. It does not model neutral current, nonlinear loads, harmonic current, phase angle, power factor, motor starting current, demand diversity, simultaneous operation, or a complete panel schedule.

For field measurements, compare currents taken under comparable operating conditions. Intermittent equipment, cycling HVAC loads, variable-frequency drives, and loads with changing duty cycles can produce a phase snapshot that differs from the calculated design load.

For code and installation decisions, separately verify the applicable NEC requirements, equipment manufacturer instructions, conductor ampacity, terminal limitations, raceway fill, overcurrent protection, available fault current, and AHJ requirements.