How this calculator sizes a show
Every item in the list carries a nameplate figure — the number on the fixture's label, drawn from current touring and festival gear — and a starting factor for anything with a motor or compressor in it. LED fixtures and dimmed tungsten also carry a duty factor, because nameplate is what a fixture pulls with every cell at full white, and no show looks like that. The math runs in six steps.
1. Nameplate connected load
Quantity times nameplate watts, summed across every department. This is the everything-switched-on-at-once number, and it is what the copper gets scoped against. Anything the list does not cover — a carnival ride, a pump, a client trailer — goes in Custom Loads with its own watts and starting factor. Every active line lands in the load schedule, which is what you send the power vendor.
2. Connected running load
The same list with the duty factor applied to lighting and video. An all-LED concert rig averages a little over half its nameplate across a show: colors instead of full white, fixtures out of the look, intensities down. A corporate or theatrical gig runs lower, a hard rock show with heavy white and strobe runs higher. This is what the generator actually carries once the show is up and the doors are open.
3. Starting and switching
Two different things happen when load arrives, and they size two different pieces of the package.
Motor starting sizes the generator. A compressor, HVAC unit, pump or chain hoist holds locked-rotor current for seconds, which is long enough to pull the engine down and is exactly what a genset's transient rating is about. The calculator adds the single largest individual start to the running load to get the design load. Quantity does not multiply it: ten dressing room trailers still means one compressor start, because they do not agree to start together.
Switch-mode inrush normally does not size engine kW, but it is not irrelevant to the generator either. LED walls, moving-head rigs and media servers draw a very high current spike when energized, and it lasts milliseconds — far shorter than a governor can respond to, so adding it to the design load like a motor start would buy an engine two sizes too big for a problem it was never going to have. A Chroma-Q Color Force II 72 publishes 80 A of inrush against 5.5 A of running current. That spike is primarily a magnetic-trip and sequencing matter, so it belongs to branch breaker selection and power-up order. Use manufacturer inrush figures and the published maximum fixtures per circuit when laying out branches.
Where the same gear does reach back to the generator is on the alternator side rather than the engine side. Non-linear loads draw harmonic currents that do no useful work but add heating in the windings and distort the voltage waveform, and Cummins notes that oversizing may be necessary to limit harmonic voltage distortion. The usual fix is a larger or better-specified alternator, or a PMG, rather than a bigger engine — sizing tools let you compare alternators within the same set for exactly this reason. On a heavily electronic rig, ask the vendor about alternator specification and THD, not just about kW.
What grouped loads do ask of the generator is load acceptance. Energizing a block of gear puts its whole steady-state load on the set in a single step. Note the difference from capacity: the set can carry that load all night, the question is whether it can take it all at once. The calculator reports the largest block your power-up staging creates, as kW and as a percentage of the set.
The 40% and 60% marks it flags against are a conservative field heuristic, not a rating. Real load-step capability is generator-specific — ISO 8528-5 bases the load steps on the engine's BMEP, and the applicable performance class matters — and it varies enormously. Many current sets are built and marketed specifically to accept 100% block load in one step, from small standby packages up through multi-megawatt units. So treat a flagged block as a prompt to ask the vendor for the published transient figure rather than as a verdict on the set. Where you cannot get the figure, sequencing the power-up is the fix, and it costs nothing.
4. Genset rating
A generator has two separate limits and the set has to clear both. The engine sets the kW limit; the alternator sets the kVA limit. Those are fixed by the machine, not by your load. A three-phase rental set is rated at 0.80 power factor — a Cat XQ125 is 100 kW / 125 kVA at 480/277V and the same 100 kW / 125 kVA at 208/120V — and connecting a 0.95 power factor LED rig to it does not turn it into a 100 kW / 105 kVA machine. So the calculator keeps load power factor and generator rated power factor as separate numbers. Load p.f. sets the current and the kVA your show demands. Rated p.f. sets what the machine can deliver. The design load is divided by your target loading and the smallest set that satisfies both the kW and the kVA requirement wins. When your load p.f. is worse than the set's rated p.f., the alternator runs out first and governs the size even though the kW figure looks comfortable.
Running a diesel set between roughly 60% and 85% of its rating is the sweet spot: below about 30% it wet-stacks and glazes cylinders, and above 85% you have nothing left for the guest artist who shows up with an extra rack. This is where the LED transition bites hardest. A rig that would have been 150 kW of tungsten is 30 kW of LED now, and a set ordered on the old rules of thumb spends the night wet-stacking.
5. Phase and voltage
Most reconnectable rental sets hold the same three-phase kW across 208V, 480V and 600V, with only the current changing — a Cat XQ125 is 100 kW / 125 kVA at both 480/277V and 208/120V. That is the common case, not a guarantee: winding and configuration-specific derates do occur, so read the rating table on the set you are actually getting.
Single phase is a different matter, and there is no general derate rule. It varies by machine and by winding, and the spread is wide enough to change the genset you order:
- Cat XQ125 — 100 kW three-phase, 65 kW single-phase. A 65% derate.
- Cat XQ60 — 47.5 kW three-phase, 39 kW single-phase. About 82%.
- Cat XQ35 — 27 kW three-phase, 27 kW single-phase. No kW derate at all; only the kVA drops, from 35 to 27, because the single-phase rating is published at 1.0 power factor.
So the calculator does not assume a figure. The 1ø Rating field takes the percentage from the vendor's rating table, and defaults to a conservative 65% for planning before you have that number. Enter the real one as soon as you know it: at the XQ35 end of the range, 65% would under-rate the machine by a third and buy a generator you did not need.
One limitation to keep in mind. This percentage is applied across the calculator's generic planning catalog, so the ratio you take off one model gets used for whatever size the tool then selects — and the whole point of the three examples above is that the ratio varies by model. Derated capacities are rounded down rather than to the nearest step, so the figure never overstates what the assumption supports, but that does not make it model-specific. Confirm the actual single-phase kW/kVA rating of the specific set ultimately quoted by the vendor.
6. Distribution
The tool reports three different currents per leg, and they are not interchangeable. Running current is what actually flows once the show is up. Design current is the running load plus the largest motor start, and it is the figure shown in the Current Per Leg panel. Nameplate current is everything switched on at once. Service connection and feeder gauge come off the nameplate figure, so the copper covers the worst case even when the genset is sized for the realistic one. Generator output current comes off the set's rated kVA, not off your load at all. Feeder gauge is picked against the ampacity basis you select: Type SC portable entertainment feeder run as single conductors in free air, or building wire in a raceway. Those are very different numbers for the same conductor, and picking the wrong basis is how a cable order ends up two sizes off. The same 200 kW set is a 555 A service at 208V and a 240 A service at 480V, which is a completely different cable package.
About the 125% margin
For preliminary planning, the calculator applies a flat 125% margin to the calculated connected current and sizes the service and feeder against that figure. Treat it as a TSE estimating margin, not as a code calculation. The NEC does not size feeders this way: it distinguishes continuous from noncontinuous load, generally requiring 125% of the continuous portion plus 100% of the noncontinuous portion, subject to specific conditions and exceptions, with further limits from termination temperature ratings and any applicable derating. Final conductor and overcurrent-device sizing must be established by the electrical contractor using the applicable NEC load rules, cable type, installation method and termination ratings. The number here is for scoping and budgeting a cable package, not for specifying one.
What this does not cover
Altitude and ambient derating, the actual harmonic distortion figure your rig will produce, soft starts and VFDs, grounding and bonding, and the utility tie-in if there is one. Those belong to the licensed electrician and the power vendor who signs off on the site. The tool flags neutral sizing and harmonics as a site note when there is an LED rig on the plot, but it does not calculate THD. Voltage drop is not folded into the sizing either, though there is a calculator for it under Power Conversions so you can check a run before you order cable.
