Voltage Drop Calculator

About the Voltage Drop Calculator

The Bock Lighting Voltage Drop Calculator helps electricians, lighting designers, landscape architects, and contractors determine voltage drop across wire runs for any lighting circuit. Whether you are designing a low-voltage landscape lighting system or a commercial 277V installation, this tool calculates the exact voltage delivered at each fixture location.

Voltage Drop Formula

The voltage drop formula for single-phase circuits is: VD = (2 × L × I × R) / 1000, where VD is voltage drop in volts, L is the one-way conductor length in feet, I is the current in amperes, and R is the conductor resistance in ohms per 1000 feet. The factor of 2 accounts for both the supply and return conductors in a complete circuit.

AWG Wire Resistance Table (Copper, 75°C)

NEC Voltage Drop Recommendations

The National Electrical Code (NEC) provides voltage drop recommendations in informational notes following NEC 210.19(A) and 215.2(A). While these are recommendations rather than requirements, they represent best practice:

• 3% maximum voltage drop for branch circuits (the wiring from the panel to the outlets/fixtures)
• 5% maximum total voltage drop for the combined feeder and branch circuit (from the service entrance to the final outlet)

For low-voltage landscape lighting systems (12V), keeping voltage drop under 3% is especially critical. A 3% drop on a 12V system is only 0.36V, which can cause noticeable dimming in LED fixtures. Many landscape lighting professionals target less than 2% drop to ensure consistent brightness across all fixtures.

Common Lighting Voltage Systems

• 12V: Low-voltage landscape lighting, under-cabinet lights, LED strip lighting. Most sensitive to voltage drop.
• 24V: Larger landscape systems, commercial LED tape, some track lighting. Less sensitive than 12V but still requires careful wire sizing.
• 120V: Standard residential and commercial branch circuits. Most common in North America for general lighting.
• 277V: Commercial and industrial lighting circuits. Higher voltage means less current for the same wattage, resulting in lower voltage drop.

How to Reduce Voltage Drop

1. Use heavier gauge wire: Going from 14 AWG to 12 AWG reduces resistance by about 37%. Going from 12 AWG to 10 AWG reduces it by another 37%.
2. Shorten wire runs: Voltage drop is directly proportional to wire length. Halving the distance halves the drop.
3. Split into multiple runs: Instead of one long daisy-chain, run separate home runs from the transformer to each group of fixtures.
4. Use hub (tee) method: For landscape lighting, run a heavy trunk line to a central point, then use shorter branch runs to fixtures.
5. Increase system voltage: A 24V system has half the current draw of a 12V system for the same wattage, cutting voltage drop in half.
6. Relocate the transformer: Placing the power source closer to the fixtures reduces wire run lengths.

Landscape Lighting Wire Sizing Guide

For 12V landscape lighting systems, wire sizing depends on total wattage and run length. Common guidelines include: use 16 AWG for runs under 25 feet with under 50W total load; use 14 AWG for runs up to 50 feet with moderate loads; use 12 AWG for runs up to 100 feet (most common choice); use 10 AWG for runs 100-150 feet; and use 8 AWG for runs over 150 feet or high-wattage loads. Always verify with a voltage drop calculation rather than relying solely on rules of thumb.

Low Voltage vs Line Voltage Lighting

Low-voltage lighting systems (12V and 24V) require a transformer to step down from line voltage (120V or 277V). While low-voltage systems are safer and easier to install in outdoor and landscape applications, they are more susceptible to voltage drop due to the higher current required to deliver the same wattage. For example, a 60W load draws 5A at 12V but only 0.5A at 120V — ten times the current means ten times the voltage drop for the same wire gauge and length.