ElectricalToolbox

12V Voltage Drop Calculator

System inputs

Uses NEC Chapter 9 Table 8 DC resistance at 75°C (conservative). A conductor running cooler will drop less. This is a physics calculation, not a code-compliance result.

This calculator finds DC voltage drop on 12V circuits — automotive, RV, marine, solar, battery banks, and LED runs — from conductor size, load current, and one-way run length. Conductor resistance comes from NEC Chapter 9, Table 8; the result is a physics calculation, not a code-compliance determination.

How to use this calculator

  1. Confirm system voltage (12V by default; use the presets for 24V or 48V banks, or type any value).
  2. Enter the load current in amps.
  3. Select conductor size and material (copper or aluminum).
  4. Enter the one-way distance from source to load in feet.
  5. Read voltage drop, percent drop, and voltage at the load. Resistance and circular mils are under "Show calculation details."

NEC reference

Resistance values are from the National Electrical Code (NEC) Chapter 9, Table 8 — DC resistance, uncoated copper and aluminum, at 75°C (NEC 2020; these values are unchanged across the 2017, 2020, and 2023 editions). Most 12V DC systems are outside NEC scope, but conductor resistance is governed by physics, not by which edition applies — so the same table values produce a valid voltage-drop result.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

Formula and basis

DC voltage drop on a two-conductor circuit is VD = 2 × R × I × L ÷ 1000, where R is conductor DC resistance in ohms per 1000 ft from NEC Chapter 9, Table 8; I is load current in amps; and L is the one-way run length in feet. The factor of 2 covers the round trip — current flows to the load on one conductor and back on the other. Percent drop is VD divided by system voltage; on 12V, small absolute losses become large percentages, so conductor sizing and run length matter far more than on 120V or 240V wiring.

Worked example

A 12V circuit feeds a 10A load 20 ft from the battery using 10 AWG copper. From Table 8, 10 AWG uncoated copper is 1.24 Ω/1000 ft, so VD = 2 × 1.24 × 10 × 20 ÷ 1000 = 0.496 V, or 4.13% of 12V. That exceeds a 3% target for sensitive loads. Stepping up to 8 AWG (0.778 Ω/1000 ft) drops it to about 2.59%, and 6 AWG (0.491 Ω/1000 ft) to about 1.64% — the usual fix for a high 12V run is larger conductor, a shorter run, or both.

Common mistakes

  • Entering round-trip length. Use the one-way distance; the formula already doubles it.
  • Treating 3% as a hard limit. On 12V DC it is a design target, not code. Sensitive electronics may need tighter; non-critical loads tolerate more.
  • Sizing only for ampacity. A conductor can be thermally adequate yet still drop too much voltage over a long 12V run. Ampacity and voltage drop are separate checks.
  • Ignoring material. Aluminum carries roughly 1.6× the resistance of copper for the same size, so an aluminum run drops proportionally more.

Frequently asked questions

What is an acceptable voltage drop for a 12V system?

No single code limit applies, since most 12V DC systems sit outside NEC scope. A common design target is 3% or less for sensitive electronics and lighting; up to 10% is sometimes tolerated on non-critical loads. The NEC notes in 210.19 and 215.2 reference 3% and 5%, but those address premises wiring.

Why is my 12V drop higher than on a 120V circuit?

Percent drop is volts lost divided by system voltage. The same conductor and current lose the same volts, but on 12V those volts are a much larger share — a 0.5V loss is 0.4% at 120V and about 4.2% at 12V.

One-way or round-trip distance?

Enter one-way distance. The calculator applies the round-trip factor of 2 for you.

Does it account for conductor temperature?

It uses Table 8 resistance at 75°C — the published, conservative figure. A conductor running cooler (a typical 20–30°C) carries roughly 15–20% less resistance, so real drop is usually lower than shown.

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