Quick Summary
Before linking fixtures power-to-power, calculate the total current draw using I = P / V. Keep the continuous load to 80% of the circuit's maximum amperage rating:
20A branch circuit: 16A maximum continuous load
15A branch circuit: 12A maximum continuous load
Applies to: [confirm: which fixture ranges this applies to]
Step-by-Step Diagnostic and Resolution Procedure
Work through these steps before connecting fixtures to a circuit. The aim is to confirm that the total load of the linked fixtures stays within the continuous limit for that circuit.
1. Determine the electrical parameters
You need two values to calculate the load: the power each fixture consumes and the voltage the venue supplies.
Find the fixture's power rating, listed as Max Watts, on the electrical rating label (for example, 250W).
Identify the line voltage supplied at the venue. This is 120V (US standard) or 208V/230V (venue power).
2. Calculate the current draw of each fixture
Breakers and cables are rated in amperes, but fixtures are rated in watts. To compare the two, convert the fixture's power rating to current. Current draw depends on both power and voltage. At a higher supply voltage, the same fixture draws less current.
Amperage (A) = Power (Watts) ÷ Line Voltage (Volts)
Example: 250W ÷ 120V = 2.08 amperes per fixture.
3. Apply the NEC 80% Continuous Load Rule
The National Electrical Code (NEC) 80% Continuous Load Rule limits how much of a circuit's rating you can load continuously. Lighting rigs typically stay powered for long periods, so use these limits and not the full breaker rating:
15A circuit: maximum continuous load = 12 amps
20A circuit: maximum continuous load = 16 amps
Check the power connectors in the link chain as well. The current passes through every connector in the chain, so each one must be rated for the load it carries. Neutrik powerCON Blue/Grey connectors are rated up to 20A peak. powerCON TRUE1 TOP connectors are rated to 16A (US) / 16A (EU).
[confirm: which fixtures use which connector type]
4. Determine the maximum number of fixtures to link
Divide the maximum continuous load by the current draw of one fixture. Round the result down to a whole number, because rounding up would put the circuit over its limit.
On a 120V / 20A circuit (16A maximum continuous load):
16A ÷ 2.08A = 7.69, so the maximum is 7 fixtures.
On a 208V / 20A circuit (250W ÷ 208V = 1.20A per fixture):
16A ÷ 1.20A = 13.3, so the maximum is 13 fixtures.
The higher supply voltage allows more fixtures on a circuit with the same breaker rating, because each fixture draws less current.
[confirm: whether the fixture's documentation gives its own maximum number of units per circuit, and whether that limit takes precedence over this calculation]
Technical Background and Prevention
Why breakers trip at power-up
Breakers can trip during power-up because of the high inrush current drawn when the SMPS (switch-mode power supply) bulk capacitors in many fixtures charge at the same time. This surge can be much higher than the steady running current, so a circuit that is within its limits once the fixtures are running can still trip when they all start together.
[confirm: any recommended practice for powering up large numbers of fixtures, such as staggered power-on]
Why the 80% limit helps
Applying the 80% derating rule accounts for thermal variations in circuit breakers. It also prevents nuisance trips when powering on large, daisy-chained light rigs.