Lighting Circuit Load Calculator

A lighting circuit load calculator converts connected fixture wattage into the adjusted circuit load and circuit current used for preliminary branch-circuit planning. The result supports an early review of breaker loading, conductor ampacity, voltage-drop exposure, panelboard capacity, and the practical distribution of luminaires across branch circuits.

The calculator begins with the connected lighting load, applies the selected Continuous multiplier, and converts the resulting VA load to amperes using the selected System voltage and Phase. The Entered breaker is a comparison value only; it does not select a breaker, conductor AWG or kcmil size, insulation type, terminal rating, or overcurrent protective device.

Connected Lighting Watts

Fixture watts is the wattage assigned to one luminaire or lighting load. Fixture count is the number of those loads placed on the circuit screen.

\(\displaystyle \text{Total fixture watts} = \text{Fixture watts} \times \text{Fixture count}\)

For the entered values:

\(\displaystyle 60\text{ W} \times 24\text{ fixtures} = 1{,}440\text{ W}\)

The calculator reports:

Result	Value
Total fixture watts	1,440 W

This connected-watt figure is the starting point for distributing lighting loads among branch circuits. It is not, by itself, the conductor ampacity requirement or the final circuit load used for installation approval.

Adjusted Circuit Load

The Continuous multiplier applies an adjustment to the connected fixture load. With a 125% multiplier, each 60 W fixture becomes 75 VA for the calculator’s adjusted-load calculation.

\(\displaystyle \text{Adjusted VA per fixture} = \text{Fixture watts} \times \left(\frac{\text{Continuous multiplier}}{100}\right)\)

\(\displaystyle 60\text{ W} \times 1.25 = 75\text{ VA per fixture}\)

The total adjusted load is:

\(\displaystyle \text{Adjusted circuit load} = \text{Total fixture watts} \times \left(\frac{\text{Continuous multiplier}}{100}\right)\)

\(\displaystyle 1{,}440\text{ W} \times 1.25 = 1{,}800\text{ VA}\)

Result	Value
Adjusted VA per fixture	75 VA
Adjusted circuit load	1,800 VA

The adjusted VA value is the load used for current conversion in this worksheet. It gives a practical early load number for checking how many fixtures can be assigned to a proposed lighting branch circuit.

Circuit Current

System voltage converts the adjusted circuit load from VA to amperes. Phase determines whether the calculator uses single-phase current math or balanced three-phase current math.

For single-phase circuits:

\(\displaystyle \text{Circuit current} = \frac{\text{Adjusted circuit load}}{\text{System voltage}}\)

For balanced three-phase circuits:

\(\displaystyle \text{Circuit current} = \frac{\text{Adjusted circuit load}} {\sqrt{3} \times \text{System voltage}}\)

With Single-phase selected and a 120 V system:

\(\displaystyle \frac{1{,}800\text{ VA}}{120\text{ V}} = 15\text{ A}\)

Result	Value
Circuit current	15 A

A 15 A calculated lighting load can then be carried into branch-circuit design review. That review may include conductor ampacity, the selected OCPD, terminal temperature limitations, voltage drop over the actual circuit length, and the combined loading of circuits sharing a raceway.

Breaker Comparison

The calculator uses Entered breaker only to compare the calculated circuit current against the entered ampere value. For a 20 A entered breaker:

\(\displaystyle \text{Entered breaker margin} = \text{Entered breaker} - \text{Circuit current}\)

\(\displaystyle 20\text{ A} - 15\text{ A} = 5\text{ A}\)

The maximum fixture count is based on the entered breaker capacity divided by the adjusted VA per fixture:

\(\displaystyle \text{Max fixtures by entered breaker} = \frac{\text{Entered breaker} \times \text{System voltage}} {\text{Adjusted VA per fixture}}\)

\(\displaystyle \frac{20\text{ A} \times 120\text{ V}} {75\text{ VA per fixture}} = 32\text{ fixtures}\)

Breaker comparison result	Value
Max fixtures by entered breaker	32 fixtures
Entered breaker margin	5 A

Under the entered assumptions, 24 fixtures produce 15 A of calculated current, leaving 5 A between that result and the 20 A comparison breaker. The same assumptions indicate a maximum of 32 identical 60 W fixtures at 125% on a 120 V single-phase circuit with a 20 A entered breaker.

Branch-Circuit Design Limits

The calculator performs load arithmetic; final electrical design requires installation-specific verification. Confirm the actual luminaire input rating and any driver, ballast, control, emergency, or inrush characteristics rather than relying on a nominal fixture wattage alone.

Verify the final branch-circuit conductor AWG or kcmil size against applicable ampacity requirements, conductor insulation temperature rating, terminal rating, ambient-temperature correction factor, and adjustment factors for current-carrying conductors. Raceway fill and conduit layout are separate calculations; conductor count affects both physical fill and, where applicable, ampacity adjustment.

The calculated amperes also do not replace a voltage-drop review. Long lighting runs, remote fixtures, low-voltage driver locations, and high-resistance conductor paths may require a separate voltage-drop calculation even when the entered breaker comparison shows available margin.

Use the actual system arrangement when selecting Phase and System voltage. Balanced three-phase current math does not establish that a real lighting load is balanced across phases. Panel schedule allocation, shared-neutral arrangements, switching zones, and field wiring must be verified separately. Final equipment selection and installation remain subject to the adopted electrical code, manufacturer instructions, project documents, and AHJ requirements.

Related calculations: LED Driver Load Calculator, Lighting Load Calculator, Lumen Calculator.