Generator Three-Phase Load Balancing Calculator

Three-phase power delivers electricity through three separate alternating current waveforms, each offset by 120 degrees from the other two. Film production generators use three-phase power because it is dramatically more efficient than single-phase at high power levels. A 200-amp three-phase generator at 120/208V delivers 72 kilowatts of available power. To deliver the same 72 kilowatts on single-phase at 120V would require 600 amps and much heavier cable. Three-phase also produces a smoother, more constant power delivery because at least one of the three waveforms is always near its peak, reducing the pulsation that can cause visible flicker in some tungsten and fluorescent fixtures.

Load balancing means distributing the total electrical load as evenly as possible among the three phases of a generator. In a perfectly balanced three-phase system, each phase carries the same current, the neutral conductor carries zero current, and the generator operates at its most efficient. In real film production, you are connecting many different fixtures of different wattages to different circuits, so perfect balance is rarely achieved, but getting close to it matters. The NEMA MG-1 standard recommends keeping load imbalance below 5 percent for optimal generator and motor performance. This calculator shows you your imbalance percentage and suggests the optimal distribution using the auto-balance algorithm.

NEC 210.19(A)(1) requires that the ampacity of branch circuit conductors and the rating of overcurrent protective devices be at least 125 percent of the continuous load they serve. A continuous load is any load that operates for three hours or more, which covers virtually all film set lighting. The 125 percent capacity requirement is equivalent to saying the load cannot exceed 80 percent of the conductor or breaker rating. For a 200-amp generator phase, the maximum continuous lighting load is 160 amps. Running 200 amps of continuous load on a 200-amp generator breaker violates the NEC and risks a thermal trip of the overcurrent device. This calculator flags any phase that exceeds the 80 percent limit in red so you know before you start pulling cable.

An overloaded generator phase will eventually trip the main breaker or the generator’s overcurrent protection for that phase. On a well-maintained production generator, that trip is protective and means no permanent damage. On an older generator with thermally compromised breakers, or when the overload is modest and sustained over many hours, the result can be insulation degradation, overheated connections, and in the worst case, a wiring fire inside the generator or distro. Beyond the safety concern, a phase trip in the middle of a critical take is a production crisis: cameras cut out, monitors go dark, you lose the scene. The time you spend on load balancing during prep is cheap insurance against a full generator trip during the production day.

Neutral current is the return current flowing through the neutral (fourth) conductor of a three-phase four-wire system. In a perfectly balanced three-phase load, the three phase currents cancel each other vectorially and the neutral carries zero current. As the load becomes unbalanced, the neutral carries the vector difference between the phases. This calculator computes neutral current using the phasor sum method: realN = I_A minus 0.5 times I_B minus 0.5 times I_C, and imagN = 0.866 times (I_C minus I_B). The magnitude is the square root of realN squared plus imagN squared. In film production, high neutral current matters because most Bates feeder cable is rated at the same ampacity as the hot legs, but unlike the hot legs, the neutral typically has no overcurrent protection. Overheating of an unprotected neutral conductor under high imbalance is a documented cause of electrical fires on production sets.

In the United States, 120/208V three-phase is the standard for portable production generators and most location shoots. It allows your crew to run 120V single-phase equipment (standard US practicals, consumer-grade lights, monitors, laptops, craft services equipment) directly from the generator’s phase-to-neutral voltage. Most HMI ballasts and LED power supplies in the US are designed to accept 120V or wider voltage ranges. The 277/480V three-phase system is used in permanent studio facilities, large stage complexes, and some 5-ton generator setups where the higher voltage allows more efficient power transmission over long cable runs. If you are tying into a stage or facility power system, you will likely use a step-down transformer to convert 480V to 208V or 240V for your lighting loads unless your fixtures are rated for 277V or 480V operation.

Power factor is the ratio of real power (watts doing work) to apparent power (volts times amps). A load with a power factor of 0.90 draws 10 percent more current than a purely resistive load of the same wattage. For film fixture load calculations, use these typical values: tungsten incandescent bulbs are purely resistive (power factor 1.00), HMI fixtures with electronic ballasts run at approximately 0.90 to 0.92, HMI fixtures with older magnetic ballasts run at approximately 0.85, professional cinema LEDs with active power factor correction run at 0.95 or higher, fluorescent fixtures with electronic ballasts run at approximately 0.95, and motor loads including cooling fans and HVAC compressors run at 0.75 to 0.85 depending on load. Using 1.00 for everything will underestimate your actual current draw. Using the correct power factor for each fixture type gives you a significantly more accurate pre-production load calculation.

Kilowatts (kW) measure real power, which is the actual energy doing useful work such as producing light or heat. Kilovolt-amperes (kVA) measure apparent power, which is the total current demand on the generator including any reactive component. The ratio between them is the power factor. A generator rated at 72 kVA with an average power factor of 0.90 delivers 64.8 kW of real power. The kVA rating is important because it determines the generator’s current capacity per phase, which is what determines whether your feeder cables and breakers are within their rated limits. The kW rating tells you how much real work you can do. For load balancing calculations, kVA and amps per phase are the critical numbers, not kW.

The NEMA MG-1 standard recommends that voltage imbalance across phases not exceed 2 percent at the terminals of motor loads, as greater imbalance causes efficiency losses and accelerated heating. For current imbalance on a generator, the practical target on a film set is below 5 percent for well-balanced loads, with 10 percent considered the maximum acceptable threshold before you begin to see meaningful efficiency losses in the generator and elevated neutral current. This calculator uses three status levels: green for imbalance below 5 percent (well balanced), yellow for 5 to 10 percent (acceptable), and red for above 10 percent (redistribute loads). In practice, a perfectly balanced set is rarely achievable because fixtures come in fixed sizes that do not add up to perfectly equal phase loads. Getting below 5 percent is a reasonable goal on most productions.

NEC Article 530 covers the installation of electrical wiring and equipment in motion picture and television studios, and in the locations where productions are shot. It applies to both permanent studio facilities and temporary production setups at locations. Key provisions include requirements for portable power distribution equipment (which covers your Bates distros and spider boxes), the use of proper cable assemblies for stage and location work, requirements for disconnecting means, and special rules for the high-wattage equipment common to film production. If your production operates in a permanent studio facility, Article 530 governs the facility’s permanent wiring as well as your temporary equipment. On location shoots classified as construction sites, 29 CFR 1926 may also apply. Consult a licensed electrician familiar with film production for jurisdiction-specific guidance.

Bates connectors are the standard locking disconnecting means used for power distribution on US film and television production sets. A standard Bates feeder cable contains four conductors: one for each of the three phases and one neutral. Single-phase Bates cables and connectors tap just one phase and the neutral. Three-phase Bates assemblies carry all three phases plus neutral in one cable. When you connect a Bates distro box to a generator output, the distro splits the three-phase feed into individual single-phase circuits that your fixtures connect to. Each output circuit on the distro is connected to one of the three phases. Knowing which output on your distro corresponds to which generator phase is essential for accurate load balancing. Most professional production distros are labeled by phase, but if yours is not, a licensed electrician can identify the phase for each output with a meter.

A mid-budget feature or episodic production lighting package typically includes two to four HMI 2500W or 4000W fixtures as key and fill sources, plus six to ten smaller fixtures (1K and 2K tungsten fresnels, LED panels, practicals). This package commonly runs between 18 and 35 kilowatts of actual draw depending on the specific fixtures and power factors. Applying the NEC 80% rule, a 200-amp three-phase generator at 120/208V (64.8 kW usable) easily covers this package with significant headroom. For productions that regularly add large generators, crane lights, or HMI 6000-12000W fixtures, a 400-amp three-phase generator (about 103 kW usable) is the standard choice. Use this calculator to enter your specific fixture list and get the exact per-phase loads before you decide on generator size.

Yes, and most film production setups do exactly this. On a 120/208V three-phase wye system, you can run 120V single-phase loads (practicals, LED panels, monitors, craft services) from any phase to neutral. You can also run 208V single-phase loads, such as some HMI ballasts rated for 208V or dual-voltage motors, from phase to phase. And some fixtures accept either 120V or 208V with auto-sensing power supplies. For this calculator’s purposes, always enter the voltage your specific fixture operates at, not the system voltage. A 1200W HMI ballast that runs at 208V line-to-line draws only 6.5 amps at 208V versus 13 amps at 120V for the same power draw. Using the wrong voltage in the calculation would give you significantly wrong amperage results.

The auto-balance algorithm uses a greedy approach commonly called the Longest Processing Time First (LPT) method, adapted from the three-machine job scheduling problem. When you click Auto-Balance, all fixtures in your list are sorted from highest to lowest amperage. Then, starting with the heaviest load, each fixture is assigned to whichever phase currently has the lowest total amperage. This continues down the sorted list. The algorithm reliably produces load imbalance percentages within a few percentage points of the mathematical optimum for typical film production fixture lists of 5 to 20 items. Note that the algorithm changes only the phase assignments in the calculator table. It does not automatically re-run the full calculation. After using auto-balance, click Calculate Phase Loads to see the updated results with the balanced phase assignments.

Yes. The calculator works for any three-phase power source: portable generators, studio utility tie-ins, or shore power on location vehicles. The voltage selector accommodates both the 120/208V standard used on most US generators and the 277/480V system common in studio and industrial facilities. For a studio tie-in, enter the rated ampacity per phase of the studio’s service disconnect or the main breaker serving the stage, and the NEC 80% compliance check will apply accordingly. The underlying load balancing math and neutral current calculation is identical regardless of whether the three-phase source is a portable generator or a permanent utility service. The only meaningful difference is that a utility service can typically accept imbalanced loads more gracefully than a portable generator because it draws from a large power grid rather than a single machine.

This calculator is designed for pre-production planning and decision support, not as a replacement for a licensed electrician’s review of the final electrical installation. The calculations use standard three-phase electrical engineering formulas and the NEC’s published load factors. For productions that require a formal electrical plan (typically any production with generators over 400 amps, studio tie-ins at 480V, or productions subject to permit inspection), a licensed electrical contractor or electrical engineer should review the final design. Use this calculator to arrive at a well-considered starting point for your pre-production electrical planning, to compare different load distribution options quickly, and to demonstrate to your production manager and safety officer that you have thought through the generator loading before the first shooting day.