How to Calculate Voltage Drop: The Complete Guide for Electricians
What Is Voltage Drop and Why Does It Matter?
Voltage drop is the reduction in voltage as electrical current travels through a conductor. Every wire has resistance, and that resistance consumes some of the voltage between the panel and the load. If the drop is too large, equipment may underperform, motors may overheat, and lights may dim.
The NEC recommends (not requires — this is an informational note, not a code mandate) that voltage drop on branch circuits not exceed 3%, and that total voltage drop from the service entrance to the farthest outlet not exceed 5%. While not a hard code violation, exceeding these thresholds can cause practical problems and will draw inspector attention.
The Voltage Drop Formula
For single-phase circuits:
VD = (2 × K × I × D) / CM
Where:
- VD = Voltage drop in volts
- K = Resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C)
- I = Current in amperes
- D = One-way distance from panel to load in feet
- CM = Circular mil area of the conductor
For three-phase circuits, replace the 2 with √3 (1.732):
VD = (1.732 × K × I × D) / CM
To express voltage drop as a percentage:
VD% = (VD / Source Voltage) × 100
Circular Mil Values by Wire Gauge
You need the circular mil area for your conductor size. Common values:
14 AWG: 4,110 CM — 12 AWG: 6,530 CM — 10 AWG: 10,380 CM — 8 AWG: 16,510 CM — 6 AWG: 26,240 CM — 4 AWG: 41,740 CM — 3 AWG: 52,620 CM — 2 AWG: 66,360 CM — 1 AWG: 83,690 CM — 1/0: 105,600 CM — 2/0: 133,100 CM — 3/0: 167,800 CM — 4/0: 211,600 CM
Worked Example 1: Residential Branch Circuit
Problem: A 120V, 20A circuit runs 100 feet from the panel to a workshop outlet using 10 AWG copper wire. What’s the voltage drop?
Calculation: VD = (2 × 12.9 × 20 × 100) / 10,380 VD = 51,600 / 10,380 VD = 4.97 volts
Percentage: 4.97 / 120 × 100 = 4.14%
Assessment: This exceeds the NEC 3% recommendation for branch circuits. The circuit will work, but you’re leaving less margin for the feeder voltage drop. Consider upsizing to 8 AWG (VD = 3.12%, which is marginal) or shortening the run if possible.
Worked Example 2: Long Feeder Run
Problem: A 240V, 50A feeder runs 150 feet using 6 AWG copper. What’s the voltage drop?
Calculation: VD = (2 × 12.9 × 50 × 150) / 26,240 VD = 193,500 / 26,240 VD = 7.37 volts
Percentage: 7.37 / 240 × 100 = 3.07%
Assessment: Just over the 3% branch circuit recommendation, but this is a feeder. The NEC recommends 5% total (feeder + branch). If the branch circuits after this feeder have minimal drop, the total system stays within 5%.
Worked Example 3: Short Residential Run
Problem: A 120V, 15A circuit runs 50 feet using 12 AWG copper. What’s the voltage drop?
Calculation: VD = (2 × 12.9 × 15 × 50) / 6,530 VD = 19,350 / 6,530 VD = 2.96 volts
Percentage: 2.96 / 120 × 100 = 2.47%
Assessment: Under 3%. This is a typical residential circuit that passes voltage drop requirements comfortably.
Common Mistakes
Forgetting to use one-way distance. The formula already accounts for the round trip with the “2” multiplier. D is the distance from the panel to the load — not the total wire length.
Using the wrong K value. K = 12.9 for copper at 75°C. K = 21.2 for aluminum at 75°C. If you’re running aluminum feeders (common for larger services), using the copper K value will give you an optimistic and incorrect result.
Ignoring temperature. The K values above assume 75°C conductor temperature. At higher ambient temperatures or with bundled conductors, effective resistance increases and voltage drop worsens. The standard formula gives you a baseline — extreme conditions require adjustment.
Confusing code requirement with recommendation. The NEC 3%/5% voltage drop guidance is in informational notes, not prescriptive code language. It’s best practice and inspectors expect it, but it’s not technically a code violation to exceed it.
When to Upsize the Conductor
If your voltage drop calculation exceeds 3%, you have three options: upsize the conductor (more copper = less resistance = less drop), shorten the run (move the panel closer or route the wire more directly), or reduce the load (split the circuit or reduce the connected amperage).
In practice, upsizing the conductor is the most common solution. Going from 10 AWG to 8 AWG nearly halves the voltage drop for the same circuit.
Try It Yourself
Use our free voltage drop calculator to run your own scenarios. Enter the wire gauge, amperage, distance, and voltage — get the voltage drop and percentage instantly. For advanced scenarios including three-phase circuits, aluminum conductors, and temperature correction, the FieldLab Electrician NEC Calculator handles the full range.