How to Calculate Voltage Drop in Commercial Lighting Circuits

Voltage drop is an important consideration when designing commercial lighting circuits. Voltage drop occurs when voltage is lost as power travels through wires, resulting in the lights receiving lower voltage than what is supplied at the panelboard. Excessive voltage drop can cause lights to flicker or operate incorrectly. Follow these steps to calculate voltage drop for commercial lighting circuits.

Select the Wire Size

The first step is to select the wire size for the lighting circuit based on the ampacity needed. Refer to the National Electrical Code for details on selecting the minimum wire size based on the amp load. Common wire sizes for 15 amp and 20 amp 120 volt lighting circuits are:

• 14 AWG for 15 amp circuits
• 12 AWG for 20 amp circuits

Larger wire sizes like 10 AWG may be needed for longer circuit runs. The wire size affects voltage drop, with larger wires reducing voltage drop.

Determine Wire Length

Next, measure the length of the wire run from the panel to the last light on the circuit in feet. Include the total length of the feeder wires, any branch circuit wiring, and the return wires. For multi-wire home runs, use the length of the farthest wire.

Accurately measuring wire lengths is crucial for calculating voltage drop. For existing wiring, lengths can be approximated based on building plans.

Calculate Resistance

** Determine the resistance (R)** of the wire based on the gauge, length, and copper conductivity.

Use this formula:

R = (L * 2) / Kcmil

Where:

• R is resistance in ohms
• L is one-way wire length in feet
• Kcmil is conductivity of copper wire based on size

For example, 150 feet of #12 AWG copper wire:

R = (150 * 2) / 16.8 = 17.86 ohms

Refer to reference tables for the Kcmil value for different wire sizes.

Determine Current Load

Calculate the current in amps for the circuit based on total wattage divided by voltage:

Amps = Watts / Volts

For example, a 20 amp 120 volt lighting circuit with 2,400 watts of load:

Amps = 2400 / 120 = 20 amps

Be sure to derate ampacity for continuous loads. Actual load current may be less for intermittent lighting use.

Use Voltage Drop Formula

With the resistance and current known, calculate voltage drop using this formula:

Voltage Drop = Amps x Resistance

For the examples above, a 20 amp load on #12 wire with 17.86 ohms resistance:

Voltage Drop = 20 amps x 17.86 ohms = 3.57 volts

Evaluate Acceptable Voltage Drop

Compare the calculated voltage drop to acceptable levels, which is around 3% for feeders and 5% for branch circuits.

For a 120 volt circuit, this would be:

• 3.6 volts maximum for feeders
• 6 volts maximum for branch circuits

If voltage drop exceeds these levels, reduce length or increase wire size.

Alternative Voltage Drop Calculation

An alternative is to use voltage drop tables that list maximum lengths for different wire sizes and currents. This simplifies calculations.

Voltage drop must be verified for lighting circuits during design. Proper wire sizing and runs within allowable lengths are key to preventing problems. Let me know if you need any clarification on these voltage drop calculation steps!