Assign circuits to Phase A / B / C · Live imbalance · NEC 220 panel schedule
This simulator assigns circuits to phases A, B, and C of a three-phase panelboard and calculates the resulting current imbalance in real time. Electrical engineers and panel designers use it to distribute loads evenly across phases, minimizing neutral current, reducing losses, and complying with NEMA MG-1 voltage unbalance limits for motors.
In a three-phase, four-wire system (208Y/120V or 480Y/277V), single-phase branch circuits are connected between one phase and neutral. The goal is to distribute circuit loads as evenly as possible across Phase A, Phase B, and Phase C. The percentage voltage unbalance is calculated as: Unbalance% = (Maximum Phase Current Deviation from Average / Average Phase Current) × 100.
For example, if Phase A = 120A, Phase B = 80A, Phase C = 100A: Average = 100A, Maximum deviation = 20A (Phase A), Unbalance = 20/100 × 100 = 20%. NEMA MG-1 limits motor operating conditions to 1% voltage unbalance for rated performance, with derating required above 1% and hard limits at 5% (motor must not operate above 5% unbalance to prevent overheating).
Neutral current in a balanced three-phase system is theoretically zero. As imbalance increases, neutral current rises — an unbalanced neutral can carry substantial current if significant single-phase load is present. Excessive neutral current causes overheating in shared neutral conductors and increased losses.
NEC Article 408 governs panelboard installation, busbar ratings, and circuit arrangement. NEC 408.3(F) requires panelboards to be installed so that the busbars and internal wiring are accessible. NEMA MG-1 Part 12 specifies that motors must be derated when voltage unbalance exceeds 1%, and must not operate above 5% unbalance. ANSI C84.1 defines acceptable service and utilization voltage ranges (Range A: ±5% of nominal). IEEE 141 (Red Book) provides recommended practice for load balancing and voltage regulation in industrial power systems. IEEE 519 governs harmonic current limits which also affect neutral sizing.
Panel schedule layout should alternate circuit assignment: phases are typically arranged A-B-C-A-B-C down the breaker column. Large single-phase loads (water heaters, HVAC, EV chargers) should be assigned first to the most lightly loaded phase. The auto-balance algorithm uses a greedy approach — sorting loads largest to smallest, then assigning each load to the phase with the lowest current total at that moment.
For panels with significant motor loads, imbalance above 1% causes increased motor slip, higher winding temperature, and reduced motor life. A 3.5% voltage unbalance can increase motor winding temperature by as much as 25°C above nameplate. Where automatic balancing is not possible due to fixed single-phase loads, consider using a closed-delta transformer bank to rephase selected circuits.
Enter your circuit names and ampere loads, then assign each circuit to Phase A, B, or C using the phase selector buttons. The phase bar chart updates in real time, and the imbalance percentage is shown color-coded (green ≤5%, yellow ≤10%, red >10%). Use the Auto-Balance button to let the greedy algorithm find an optimal assignment. The apparent power display shows total kVA at the selected system voltage. Add new circuits with the form above the circuit list.
For motor loads, NEMA MG-1 requires derating above 1% voltage unbalance. For general commercial lighting and receptacle loads, most engineers target ≤10% current imbalance as a practical goal. IEEE 141 recommends ≤5% for industrial systems. The imbalance displayed here is current imbalance — voltage unbalance at the motor terminals is typically 1/3 to 1/4 of the current imbalance due to source impedance effects.
In a balanced three-phase system, phase currents cancel and neutral current is zero. Load imbalance causes residual neutral current. Additionally, non-linear loads (computers, VFDs, LED drivers) generate third-harmonic current, which does not cancel in the neutral — instead, all three phases' third harmonics add together. In a system with heavy computer loads, the neutral can carry 173% of the phase current.
Current unbalance causes negative-sequence current, which circulates in the transformer delta winding and creates additional copper losses (I²R). For a 10% current imbalance, transformer losses increase approximately 1–3%. Additionally, unbalanced currents can cause higher-than-expected neutral-to-ground voltages on the secondary side.
For panels supplying mixed loads at different voltages (e.g., some circuits at 120V single-phase and some at 208V single-phase in a 208Y/120V system), kVA is the more meaningful balancing metric. For a simple 120V single-phase circuit panel where all circuits operate at the same voltage, current (amps) and kVA are directly proportional, so either method gives the same result.
A multi-wire branch circuit uses two or three phase conductors sharing a single neutral. NEC 210.4 permits this when each ungrounded conductor is connected to a different phase leg. In a balanced multi-wire circuit, the neutral carries only the difference in phase currents. However, if circuits are on the same phase or if non-linear loads are present, the neutral can be overloaded. NEC 210.4(D) requires a handle-tie to disconnect all ungrounded conductors simultaneously.
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