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 October 9, 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
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Watch the step-by-step video

Watch it explained

A step-by-step video using your numbers: what to enter, what the result means, and a “what if” example. On-screen instructions follow each step as it happens.

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What the animation shows

The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.

Finished frame of the voltage drop calculator animation for the default example
The last frame of the animation for the default example.

What you enter

  • Conductor material: Copper.
  • Conductor size: 12 AWG.
  • One-way circuit length: 100 ft. Distance from the panel to the load, not the round trip.
  • Load current: 20 A.
  • Voltage: 120 V.
  • System: Single-phase (or DC).

What happens, step by step

  1. Load on: 20 A flows out on one wire and back on the other.
  2. The 12 AWG copper resists the current, so voltage falls along all 100 ft.
  3. 112.1 V reaches the load: 7.9 V lost, a 6.59% drop.
  4. Over the 3% guideline: use a larger wire or a shorter run.

The answer: voltage drop is 6.59%.

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.