Power arithmetic before the first plug goes in

By Joey Calkins · Section Light

Power is the part of a rig that cannot be negotiated in the room. A circuit either carries the load or opens, a distribution board either has the capacity or does not, and finding out during the technical rehearsal is the expensive way. The arithmetic is simple. What makes it go wrong is that several of the terms are not obvious.

Everything that follows describes work that is regulated. Installing, altering or connecting to fixed electrical supply is a qualified trade with legal duties attached, and it varies by country. This article describes what the calculations are and who does them; it is not a procedure and must not be used as one.

The circuit rating is not the budget

A branch circuit is rated for a maximum, and a load that runs continuously is not permitted to occupy all of it. Electrical codes treat a load lasting three hours or more as continuous and restrict it to a proportion of the circuit rating, conventionally four fifths under United States practice. A performance is a continuous load by that definition.

The reserve is not bureaucratic padding. It accounts for heating in the conductors and in the protective device over a long run, which is exactly the condition a two act show creates. Other countries reach the same end through different rules, and the applicable code is the one for the building, not the one the touring party is used to.

Switching on costs more than running

The load that trips a breaker is frequently not the running load. LED fixtures contain switch mode power supplies whose input capacitors are empty at the moment of switch-on and draw a brief, very high current while they fill. Individually this is trivial. Twenty fixtures switched by one contactor at the same instant is a different event, and the breaker sees a spike that has nothing to do with the modest wattage on the schedule.

The remedy is arrangement rather than capacity: staggering what comes up together, or splitting the rig across more circuits than the running load would suggest. A rig that trips only at the start of the evening, and never during it, is describing this problem precisely.

The neutral carries more than people expect

In a balanced three phase supply feeding ordinary linear loads, the currents in the three legs cancel in the shared neutral, which is why the neutral was historically sized as it was. Switch mode supplies and phase control dimmers do not draw a clean sinusoid. They draw current in pulses, and the distortion is dominated by the third harmonic.

Third harmonics on the three phases arrive in step with each other rather than a third of a cycle apart, so instead of cancelling in the neutral they add. A neutral in a rig full of electronic power supplies can carry more current than any single phase does, and it is not protected by the breakers that protect the phases. This is a question for the electrician specifying the distribution, and it is one of the reasons a modern LED rig is not simply a lower powered version of the tungsten one it replaced.

Balance, and the long run

Two further terms complete the picture. Loads spread unevenly across phases waste capacity, because the supply is limited by its worst leg rather than by its total. And voltage falls along a cable in proportion to its length and the current in it, so a long feeder to a distant position delivers less than the board promised, with fixtures behaving oddly at the far end for reasons that look like faults.

What changed when the rig went electronic

Tungsten rigs were limited by amperes. A hall's supply set the number of lanterns, and every design conversation ran through it. LED rigs draw a fraction of that, and the binding constraint has moved: what runs out now is control, in the sense described in the article on DMX universes, along with data cable, network capacity and the addressing scheme.

Power has not stopped mattering, it has stopped being the thing that says no first. The failure mode has changed shape accordingly, from a supply that cannot carry the rig to a distribution whose neutral, inrush behaviour and phase balance were never recalculated after the equipment changed.

What we cannot verify

Code requirements differ by country and frequently by municipality, and the figures described here reflect one national practice, named above, at a general level. They are not a substitute for the code applicable to a specific building, and they are not advice on which any installation should be based. Power consumption figures on fixture data sheets are published by manufacturers, usually as a maximum at one setting, and inrush behaviour is rarely published at all. The competent person for any of this is the qualified electrician responsible for the installation.