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Circuit Load Calculator

Watts divided by volts gives amps, and the useful answer is not the current but the headroom left over. The figure worth knowing is the eighty per cent rule: a breaker held at its full rating for hours is a breaker about to trip, so a sustained load should stay under four fifths of it. That is the difference between a rig that runs all evening and one that goes dark during the second half.

What this generator does

Adds up the wattage of the equipment, divides by the supply voltage to get current, and compares that against both the breaker rating and the eighty per cent figure a continuous load should stay within.

How to use this tool

  1. Choose a fixture and how many of them are on the circuit.
  2. Set the breaker rating.
  3. Read the current drawn and the headroom left.
  4. Aim to stay under the sustained limit, not merely under the rating.

Understanding the controls

Fixture
Chosen from typical event equipment with realistic wattages, from a 60 W wash light to a 900 W haze machine.
How many
Units of that fixture on the same circuit. Wattage adds up directly; the current follows.
Breaker rating
The circuit's protective device in amps. A 13 A plug in the UK, or a 16 or 32 A distribution outlet.

Common use cases

  • Checking how many fixtures a circuit will take
  • Planning power distribution for an event or a studio
  • Understanding what the eighty per cent rule means and why it exists
  • Converting equipment wattage into current
  • Sanity-checking a load before plugging things in

How this generator works

Current is power over voltage, and headroom is the rating minus that current — which the check enforces as an exact identity rather than a recomputation. The verdict is derived from where the draw falls relative to both the rating and the eighty per cent figure, and is checked against those thresholds separately.

Randomness and fairness

Nothing here is random. The load follows entirely from the equipment and rating you choose.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

  • This is an arithmetic aid, not an electrical design, and it does not replace a qualified electrician.
  • Startup surge is not modelled, and it is what actually trips breakers — motors and some lamps draw several times their running current momentarily.
  • 230 V single phase is assumed. Three-phase distribution and other supply voltages work differently.
  • Power factor is ignored, so equipment with a poor one draws more current than its wattage suggests.
  • Cable rating, run length and voltage drop all matter for a real installation and none is considered.

Privacy and your data

The calculation runs in your browser. Nothing about your equipment or circuit is transmitted or stored.

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