Voltage Drop Formula Explained
Voltage drop is caused by the resistance of the conductor. The longer the run or the higher the current, the more voltage is lost before it reaches the load. The standard formulas are:
- Single-phase: VD = (2 × K × I × L) / CM
- Three-phase: VD = (1.732 × K × I × L) / CM
Where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way length in feet, and CM is the conductor cross-sectional area in circular mils.
The factor of 2 in the single-phase formula accounts for the round-trip conductor length (line and neutral). The 1.732 factor for three-phase is √3, which accounts for the phase relationship between conductors in a balanced three-phase system.
Worked example: 75 ft run to a workshop
A 120 V single-phase circuit feeds a 15 A load in a detached workshop. The one-way run from the panel is 75 ft of 12 AWG copper (6,530 circular mils).
- VD = 2 × 12.9 × 15 × 75 ÷ 6,530 = 4.44 V
- Percentage: 4.44 ÷ 120 = 3.7%, flagged Marginal (3 to 5%)
- Receiving voltage: 115.6 V on the 120 V source
Because the drop is above 3%, the calculator suggests the next size up. In 10 AWG copper the same run drops 2.80 V, or 2.3%, which is inside the NEC recommendation for branch circuits. If the load or the distance grows, run the check again, since the drop rises in direct proportion to both.
NEC Voltage Drop Recommendations
The NEC does not mandate a specific maximum voltage drop — it provides recommendations in the Fine Print Notes (now Informational Notes) to NEC 210.19 and 215.2:
- Branch circuits: No more than 3% voltage drop at the farthest outlet.
- Feeders: No more than 2% voltage drop.
- Combined total: No more than 5% from the service point to the farthest outlet.
While these are recommendations rather than hard code requirements, many AHJs treat them as mandatory and energy codes increasingly require compliance. For sensitive equipment (medical, data, motor loads), keep total drop under 3%.
The calculator colour-codes results: green for <3%, yellow for 3–5%, and red for >5%. It also suggests the next AWG size up if your current wire exceeds the 3% threshold.
Reducing Voltage Drop on Long Runs
If the calculator shows excessive voltage drop, you have several options:
- Increase conductor size: Going up one AWG size reduces resistance by about 20%. The calculator shows the recommended upgrade size.
- Use copper instead of aluminum: Copper has lower resistivity (K = 12.9 vs 21.2), so the same AWG carries more current with less drop.
- Reduce one-way length: Move the panel closer to the load, or add a sub-panel. Halving the distance halves the drop.
- Increase voltage: A 240 V circuit carries the same power at half the current of a 120 V circuit, reducing drop by 75%.
- Use three-phase power: For large motor loads, three-phase distribution reduces the current per conductor.
For runs over 100 feet on 120 V circuits or over 200 feet on 240 V circuits, always check voltage drop before finalising the design.
Frequently asked questions
What is the maximum allowed voltage drop per NEC?
The NEC recommends (but does not mandate) a maximum of 3% on branch circuits and 2% on feeders, for a combined maximum of 5%. Many jurisdictions and energy codes treat these as requirements. For sensitive electronics and motor loads, aim for 3% total or less.
Do I use one-way or total distance in the formula?
Enter the one-way distance (from panel to load). The formula automatically doubles it (for single-phase) or uses the √3 factor (for three-phase) to account for both the outgoing and return conductors.
Why is aluminum wire more prone to voltage drop?
Aluminum has higher electrical resistivity than copper. The K constant for aluminum is 21.2 vs 12.9 for copper — about 64% higher. An aluminum conductor of the same AWG will have roughly 64% more voltage drop than the equivalent copper conductor at the same current and distance.
How does three-phase voltage drop compare to single-phase?
Three-phase voltage drop uses the multiplier 1.732 (√3) instead of 2. This means a balanced three-phase circuit has about 13% less voltage drop than a single-phase circuit of the same wire, current and distance because the three conductors share the return path.
Can excessive voltage drop damage equipment?
Yes. Motors running on low voltage draw higher current, overheat and fail prematurely. Electronic equipment may malfunction, restart unexpectedly or have its power supply work harder and shorten its lifespan. LED drivers and variable-speed drives are particularly sensitive. Keep drop under 3% for any equipment with a manufacturer-specified voltage tolerance.