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Wire size calculator / 12V DC wire size

12V DC wire size, and what this does not say about solar

Low voltage makes the same volt drop a much larger percentage. These rows are generic DC examples, not a solar design.

At 12 V, a drop that would be a small share of a 120 V branch can be a large share of the source. The resistance model is the same Chapter 9 Table 8 stranded value at 75°C. DC uses the factor of two, out and back, on the one-way length. Reactance is not part of a DC estimate. This page is not a solar, battery, or charge-controller design.

PV source circuits, battery fault current, controller terminals, and temperature-corrected array current are outside this calculator. The presets labeled as 12 V, 24 V, and 48 V DC examples were renamed for that reason: they are hypothetical loads. Use them to see percentage drop. Do not use them as a wiring diagram.

Copper drop at the 12 V preset

Each row is 20 A DC at 12 V and 20 ft one way, using that gauge’s stranded copper resistance. The percent column is the drop divided by 12 V. A row can show a huge percentage and still be the wrong conductor for ampacity. Read both.

Calculated Table 8 voltage drop for the 12 V DC preset current and length. Not a solar-array wiring table and not an ampacity waiver.
SizeCu Ω/kftDropPercent of 12 V
14 AWG 3.14 2.51 V 20.93%
12 AWG 1.98 1.58 V 13.20%
10 AWG 1.24 0.99 V 8.27%
8 AWG 0.778 0.62 V 5.19%
6 AWG 0.491 0.39 V 3.27%

Same conductor, three source voltages

10 AWG copper at 20 A and 20 ft. Only the source voltage changes. Ampacity cells do not.

Voltage changes the percentage, not the Table 8 volt drop, when current, length, and resistance stay fixed.
SourceDropPercent
12 V 0.99 V 8.27%
24 V 0.99 V 4.13%
48 V 0.99 V 2.07%

Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.

Sources for these cells: existing Table 310.16 numerical cells in NFPA’s public development record, pp. 142–143 (proposed changes are not treated as adopted code) and NFPA 70 (2017), Chapter 9 Table 8, printed p. 689, in a third-party-hosted copy. The same publication supplies 110.14 (terminals and aluminum), 210.19/215.2 (continuous loads), 240.4(D) (small conductors), 250.24/250.32 (grounding), and 625.41/625.42 (EV loads). Schneider Electric’s voltage-drop formulas support the arithmetic; Table 8 AC results here omit reactance and assume unity power factor. Resistance and geometry are construction- and temperature-specific. Adopted editions and local amendments can differ.

Related guides

Open the wire size calculator · Ampacity chart · Voltage drop calculator · Printable chart

Run the 12 V DC example

The form starts from this page’s example. The address stays on this guide until you edit a field. After an edit, a shared link includes those settings in the URL, and opening that link sends the URL to the host.

Circuit

Headline metal

Both metals are compared. Verify equipment compatibility; some equipment permits copper only.

System

General NEC notes suggest 3% on a branch or feeder and 5% combined. Special equipment and local codes may require other limits; upstream drop is not included.

Estimated size

Enter the load current and the one-way distance. The size appears here, with the limit that set it.

Temperature and bundling

These conditions affect the table ampacity used for the comparison.

Conductor insulation

Use the rating marked for wet or dry conditions. THHN is 90°C dry; XHHW is 90°C dry or 75°C wet. For NM-B use 90°C insulation and a 60°C cap below.

Terminal / cable ampacity cap

Use the lowest applicable rating at either end, including cable limits. The general unmarked default is 60°C for equipment rated ≤100 A or marked for 14–1 AWG. Confirm equipment markings before selecting 75°C; NM-B remains capped at 60°C.

Equipment grounding conductors are excluded. Verify neutrals, harmonic loads, and all other current-carrying conductors in the raceway or cable; the automatic count does not check these.

AC impedance (Chapter 9 Table 9)

At power factor 1, the reactance term is zero, but Table 9 AC resistance still applies. Unlisted sizes have no Table 9 result.

Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.

Questions

Does this page size solar panel wire?

No. The rows are generic DC circuits at the current, length, and voltage written in the table. PV source circuits, charge controllers, batteries, and inverter inputs have additional current, voltage, temperature, and protection rules. This tool does not apply them. The home-page 12 V, 24 V, and 48 V presets are the same kind of hypothetical load.

Why does 12 V need a larger conductor than 48 V?

The Table 8 volt drop does not know the source voltage. The percentage does, because it divides by that voltage. Hold 10 AWG, 20 A, and 20 ft fixed and the volt drop stays put while the percentage shrinks as the source rises from 12 V to 24 V to 48 V. That comparison is not a PV string calculation.

Is DC voltage drop calculated one way or round trip?

This model uses two equal conductors for DC and for two-wire single-phase: 2 × I × R × L / 1,000, where L is the one-way length and R is Table 8 uncoated stranded resistance at 75°C. It is not a one-way resistance and it does not include fault current, controller limits, or grounding.