Three-Phase Balancing Calculator

Unbalanced three-phase loads cause excessive neutral current, increased losses, voltage imbalance between phases, overheating of the neutral conductor, and potential nuisance tripping of protective devices. The IET On-Site Guide advises distributing single-phase loads as evenly as practicable, with the current in any phase ideally differing from the average by no more than about 15% — though your DNO’s connection agreement may set its own balancing requirement, so check it. Severe imbalance can cause motors to overheat and fail, and can affect sensitive electronic equipment. Check our Maximum Demand Calculator to assess per-phase loading.

For a three-phase four-wire system with unbalanced resistive loads, the neutral current is: In = √(Ia² + Ib² + Ic² - Ia·Ib - Ib·Ic - Ia·Ic). For a perfectly balanced load, this gives zero neutral current. The neutral current increases as the imbalance grows. Note: this formula assumes resistive loads (unity power factor). For loads with significant harmonic content (e.g., LED lighting, VFDs), the neutral current can be higher due to triplen harmonics.

The IET On-Site Guide suggests that the current in any phase should not differ from the average by more than about 15% — which is how this calculator measures imbalance (maximum deviation from the average, the same method NEMA MG-1 and IEEE 1159 define). Some specifications require 10% or less for sensitive installations, and DNO connection agreements may impose their own figure. BS 7671 itself sets no single numeric balancing limit. For motor loads, voltage imbalance should be kept below 2% to prevent overheating — even 3.5% voltage imbalance can cause a 25% increase in motor heating (NEMA MG-1 rule of thumb: temperature-rise increase ≈ 2 × imbalance%²).

Start by listing all circuits and their design currents. Assign the largest loads first, distributing them across phases. Single-phase circuits on a three-phase board typically connect to phases in sequence: way 1 = L1, way 2 = L2, way 3 = L3, way 4 = L1, etc. Group related circuits (e.g., all lighting on one phase) can cause imbalance — spread them across phases instead. Use this calculator to check the balance before finalising the distribution schedule.

Yes — on a TN-C-S (PME) supply, the neutral carries the unbalanced current back to the transformer. If the neutral impedance is significant, the voltage on the more heavily loaded phase drops while the lighter phases rise. In extreme cases (broken neutral), voltages can become dangerously unbalanced — the light phase may rise to near 400V while the heavy phase drops. This is one reason PME earthing has special requirements.