Load calculation

A load calculation adds up what every piece of equipment draws, converts it to current, and distributes it across the phases of a service. It is the step that decides what size generator or company switch the show needs, and it is done on paper long before anything is plugged in.

The method is simple. Take the rated input power of each item — the figure on the data plate, not a lamp equivalent and not a marketing number. Divide by the voltage to get amps. Assign each item to a phase and total each phase separately.

The part that is almost universally stated backwards is the 80 percent rule. There is no 80 percent rule in the electrical code. What the code says is that the breaker must be rated for at least 125 percent of a continuous load, and 80 percent is simply the inverse of that: a 20 amp circuit may carry 16 amps continuously, which at 120 volts is 1,920 watts rather than 2,400. Stage lighting held on through a session is a continuous load. Getting this the wrong way round makes every number downstream of it wrong.

Three other things get left out and all of them are real. Equipment that is not lighting — video walls, motors, audio, catering, the client’s own kit. Inrush when everything switches on at once. And the worst phase rather than the average, because that is the leg that trips.

Scope your event — owned inventory, our own crew, and a site survey before anything is specified.

• Three-phase power The service nearly every event runs on — three live conductors and a neutral, giving 120 volts to neutral and 208 between phases, which is not 240 and not an arithmetic sum.
• Power distro The portable box that takes one large feed and breaks it into protected circuits — the thing between a building’s power and every piece of equipment on the show.
• Generator (event) Portable power for a site that has none — specified by the load it will actually carry, sited for noise and exhaust, and hurt more by running too light than too heavy.