Lighting Circuit Voltage Drop Calculator

A lighting circuit voltage drop calculator determines the voltage lost in the circuit conductors between the source and the lighting load. The primary result is Voltage drop in volts and percent, followed by Estimated end voltage at the load and Approximate circuit loss in watts.

Use the calculation when the circuit current, one-way length, conductor resistance, system voltage, and phase model are known. It provides a direct resistance-based check for long lighting branch circuits, lighting feeders, site-lighting runs, and other circuits where reduced voltage at luminaires, drivers, ballasts, controls, or contactors may affect operation.

The calculation can support conductor-sizing review after an initial ampacity selection. A conductor may have sufficient ampacity for the calculated load yet still produce excessive voltage drop because of distance, conductor resistance, or system voltage. Increasing conductor size generally lowers resistance per 1,000 ft and reduces voltage drop, although raceway fill, conductor termination space, lug range, pulling tension, equipment listing, and cost must also be reviewed separately.

Circuit Inputs and Electrical Meaning

Calculator field	Electrical use
Circuit current (A)	The load current expected to flow through the lighting circuit. The voltage drop increases in direct proportion to current.
One-way length (ft)	The distance from the source to the load. The calculator applies the phase multiplier to account for the electrical path used by the selected system.
Conductor resistance (ohm/1000 ft)	The resistance value for the selected conductor and temperature basis. Lower-resistance conductors produce less voltage drop.
System voltage (V)	The circuit voltage used to calculate voltage-drop percentage and estimated voltage at the load.
Phase model	Selects the voltage-drop multiplier: 2 for a single-phase two-conductor circuit or sqrt{3} for a balanced three-phase circuit.

For a 120V lighting branch circuit, a given number of lost volts represents a larger percentage of available voltage than it would on a 277V lighting circuit. A 6V drop is 5% of 120V but about 2.2% of 277V.

The entered Conductor resistance (ohm/1000 ft) must match the actual conductor selection and temperature basis used for the screening calculation. Do not substitute an ampacity value, AWG designation, kcmil area, impedance value, or resistance value taken from a different conductor material or temperature condition.

Voltage-Drop Formula

The calculator first determines resistance for the one-way run:

\(\displaystyle \text{One-way resistance} = \frac{\text{One-way length} \times \text{Conductor resistance}}{1000}\)

It then applies the phase-model multiplier:

text{Phase multiplier} =
begin{cases}
2 & text{single-phase two-conductor} \
sqrt{3} & text{balanced three-phase}
end{cases}

Voltage drop is calculated as:

\(\displaystyle \text{Voltage drop} = \text{Phase multiplier} \times \text{Circuit current} \times \text{One-way resistance}\)

The displayed percentage is:

\(\displaystyle \text{Voltage-drop percent} = \frac{\text{Voltage drop}}{\text{System voltage}} \times 100\)

The calculator also estimates voltage available at the load:

\(\displaystyle \text{Estimated end voltage} = \text{System voltage} - \text{Voltage drop}\)

Approximate conductor loss is based on the calculated voltage drop and circuit current:

\(\displaystyle \text{Approximate circuit loss} = \text{Voltage drop} \times \text{Circuit current}\)

This loss represents resistive heating in the circuit conductors under the entered load condition.

Calculation Example

A 120V lighting branch circuit supplies 12A of lighting load at a one-way distance of 150 ft. The selected conductor resistance is 2 ohm/1000 ft, and the circuit is modeled as Single-phase two-conductor.

Input	Entered value
Circuit current	12 A
One-way length	150 ft
Conductor resistance	2 ohm/1000 ft
System voltage	120 V
Phase model	Single-phase two-conductor

First, calculate one-way conductor resistance:

\(\displaystyle \frac{150 \text{ ft} \times 2 \text{ ohm/1000 ft}}{1000} = 0.3 \text{ ohm}\)

For a single-phase two-conductor circuit, the phase multiplier is 2:

\(\displaystyle 2 \times 12 \text{ A} \times 0.3 \text{ ohm} = 7.2 \text{ V}\)

The calculator result is:

Result	Value
One-way resistance	0.3 ohm
Phase multiplier	2 x
Voltage drop	7.2 V
Voltage drop	6%
Estimated end voltage	112.8 V
Approximate circuit loss	86.4 W

At the entered circuit current, the lighting load is estimated to receive 112.8V rather than the 120V source voltage. The conductors also dissipate approximately 86.4W as resistance loss.

A designer or installer may use that result to compare alternate conductor sizes. If a larger AWG or kcmil conductor has a lower resistance value, entering that new resistance will show the expected reduction in voltage drop and circuit loss. The ampacity selection remains a separate step: conductor size must still satisfy the applicable load calculation, terminal rating, insulation temperature rating, adjustment factor, correction factor, and installation conditions.

Lighting Circuit Application

Voltage-drop review is especially useful where lighting circuits include long homeruns, exterior poles, parking-lot luminaires, remote lighting panels, warehouse lighting rows, or large groups of LED drivers. Voltage at the end of a long run can affect driver operating range, starting behavior, lighting output, control-device operation, and nuisance performance issues.

For a lighting branch circuit, use the calculated voltage drop alongside the actual circuit design:

• Confirm the Circuit current (A) represents the anticipated operating load, not merely the breaker handle rating.
• Measure or estimate the actual One-way length (ft) along the conductor route, including vertical risers, offsets, and the real raceway path where applicable.
• Use conductor resistance corresponding to the installed copper or aluminum conductor, AWG or kcmil size, and intended temperature basis.
• Select the correct Phase model for the circuit arrangement. A single-phase two-conductor line-to-neutral or line-to-line circuit uses 2; a balanced three-phase circuit uses sqrt{3}.
• Review whether downstream lighting loads are distributed along the run rather than concentrated at the far end. The calculator treats the entered current and length as the basis for its voltage-drop arithmetic.

A circuit with loads distributed throughout a building wing or site route may not behave like a full load located at the final luminaire. Conversely, a load added later at the end of a circuit can increase both voltage drop and conductor loss beyond the original design condition.

Field Verification and Limits

This worksheet performs lighting circuit voltage-drop arithmetic only. It does not select a conductor, determine ampacity, evaluate overcurrent protection, calculate breaker sizing, account for conductor reactance, analyze power factor, evaluate unbalanced three-phase loading, or determine compliance with NEC requirements or AHJ approval.

Confirm the final installation separately for:

• Conductor ampacity, including applicable adjustment factors, correction factors, current-carrying conductors, insulation temperature rating, and terminal rating.
• Branch-circuit or feeder load calculation, continuous-load treatment where applicable, and overcurrent protective device selection.
• Raceway fill, box fill, conduit routing, pull difficulty, conductor bending space, equipment grounding conductor requirements, and termination compatibility.
• Actual source voltage, feeder voltage drop upstream of the lighting circuit, conductor temperature, connection quality, and voltage measured under load.
• Equipment manufacturer requirements for LED drivers, emergency lighting equipment, controls, contactors, transformers, ballasts, or other connected equipment.
• Applicable project specifications, local amendments, and AHJ requirements.

The result is most useful as a transparent electrical check: it shows how current, conductor resistance, route length, system voltage, and circuit configuration combine to reduce voltage available to the lighting load.