Three Phase Balance Calculator
Distribute single-phase loads across three phases for optimal balance
Reference Info & Formulas
- Reduces neutral current
- Prevents cable overheating
- Improves system efficiency
- Reduces voltage imbalance
Max imbalance: 15%
Target: <10%
Based on EREC G5/5
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Data: General power engineering practice
For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.
How This Calculator Works
Three-phase load balancing distributes circuits across L1, L2, and L3 to minimise neutral current and voltage imbalance.
The calculator sums the current drawn on each phase and determines the percentage imbalance and resulting neutral current. In a perfectly balanced system, the three phase currents cancel in the neutral and the neutral current is zero. Any imbalance produces a residual current that the neutral conductor must carry.
I_N = √(I_A² + I_B² + I_C² − I_A×I_B − I_B×I_C − I_C×I_A)- I_N
- = Neutral current in amps
- I_A
- = Phase A (L1) current in amps
- I_B
- = Phase B (L2) current in amps
- I_C
- = Phase C (L3) current in amps
BS 7671 Appendix 4
Quick Reference — Three-Phase Relationships
Fundamental three-phase voltage and power relationships
BS 7671 Appendix 4| Parameter | Formula | UK Standard Value |
|---|---|---|
| Line voltage (V_L) | √3 × V_P | 400V |
| Phase voltage (V_P) | V_L / √3 | 230V |
| √3 factor | 1.732 | — |
| Three-phase power (kW) | √3 × V_L × I_L × cos(φ) / 1000 | Depends on load |
| Line current from kW | P / (√3 × V_L × cos(φ)) | Depends on load |
UK three-phase supply is 400V line-to-line, 230V line-to-neutral at 50Hz.
Understanding Phase Imbalance
Imbalance percentage is calculated as the deviation of the most loaded or least loaded phase from the average phase current, divided by the average. For example, if L1 carries 40A, L2 carries 35A, and L3 carries 45A, the average is 40A and the imbalance is (45 - 40) / 40 = 12.5%.
High imbalance causes the neutral conductor to carry current even when it should ideally carry none. It also creates voltage asymmetry between phases, which can reduce motor efficiency and increase losses in three-phase equipment.
Practical Notes
Triplen Harmonics in the Neutral
Aim for Less Than 10% Imbalance
Phase Rotation Matters
Frequently Asked Questions
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