Voltage Drop Calculator
Your estimate
- Voltage drop (%)
- 6.43%
- Voltage at the load
- 112.28 V
- Result
- Over 5 % — use a larger wire
Single or three phase, AWG or mm² — see if a run stays under the 3 % recommendation.
Updated 2026-10-20 · Based on: NEC Chapter 9 Table 8 and 310.16; DIN VDE 0100-520 for metric.
How to use the Voltage Drop Calculator
- Choose copper or aluminum and the wire size.
- Enter the one-way length of the run.
- Enter the load current and the supply voltage.
- Keep the drop under 3 % for branch circuits (5 % total).
Formula
Single phase: Vd = 2 × L × I × R. Three phase: Vd = √3 × L × I × R, with R the resistance per foot (NEC Chapter 9, Table 8, at 75 °C).
Metric: Vd = 2 × L × I ÷ (κ × A), with κ = 56 for copper and 35 for aluminum.
Worked example
With the values preset in the calculator above:
- Material
- Copper
- One-way length
- 100 ft
- Current
- 20 A
- Voltage
- 120 V
- Phase
- Single phase
- Wire size
- 12
- Voltage drop
- 7.72 V
- Voltage drop (%)
- 6.43%
- Voltage at the load
- 112.28 V
- Result
- Over 5 % — use a larger wire
Related guides
Frequently asked questions
What is the voltage drop on 12 AWG at 100 ft and 20 A?
7.72 V on 120 V — 6.43%, too much. 10 AWG gives 4.03%; 8 AWG gets under 3 % (2.55%).
What is an acceptable voltage drop?
The NEC recommends no more than 3 % on a branch circuit and 5 % in total (feeder + branch).
Does voltage drop depend on the breaker?
No — it depends on wire material, size, length and the actual current flowing.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.