Voltage Drop Calculator

The NEC informational notes suggest limiting voltage drop to about 3% on a branch circuit and 5% total, for efficiency and equipment performance. These are recommendations, not mandatory limits in most cases.

Important: This calculation provides a preliminary estimate. Final conductor and overcurrent protection sizing must account for the NEC as adopted locally, conductor material, insulation temperature rating, ambient temperature, installation method, bundling, terminal ratings, equipment specifications and local amendments. Electrical work should be done or checked by a qualified electrician.

Tapeline — Voltage Drop Calculator

Printed September 29, 2026

Conductor materialCopper
Conductor size12 AWG
One-way circuit length100 ft
Load current20 A
Voltage120 V
SystemSingle-phase (or DC)

Results update as you type.

Distance from the panel to the load, not the round trip.

A

How it’s calculated

  • Single-phase: VD = 2 × K × I × L ÷ cmil
  • Three-phase: VD = √3 × K × I × L ÷ cmil
  • K = 12.9 (copper) or 21.2 (aluminum) Ω·cmil/ft at about 75 °C
  • cmil from the AWG definition: d = 0.005 × 92^((36 − n) ÷ 39) in

Worked example

12 AWG copper, 100 ft one way, 20 A at 120 V single-phase.

Inputs

Conductor material
Copper
Conductor size
12 AWG
One-way circuit length
100 ft
Load current
20 A
Voltage
120 V
System
Single-phase (or DC)

Results

Voltage drop
6.59%
Voltage drop
7.9 V
Voltage at the load
112.1 V
Conductor area
6,530 cmil

Load this example into the calculator

Assumptions and limits

  • DC resistance method; reactance is ignored (acceptable for small conductors at 60 Hz).
  • Conductor temperature near 75 °C.

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

What is an acceptable voltage drop?

The NEC suggests (in informational notes) 3% for a branch circuit and 5% for feeder plus branch combined.

Does voltage drop depend on the breaker?

No — it depends on the actual current, conductor size, material and length.