12V Wire Size Calculator — DC Gauge & Voltage Drop
Enter the load current, one-way wire length, and the maximum voltage drop you can accept. The calculator finds the thinnest AWG copper wire that keeps the drop within your limit and shows the actual voltage drop and power lost in the wire.
A
ft
Length unit
%
- 1
Allowable voltage drop
3 % × 12 V ÷ 100 = 0.36 V - 2
Required circular mils
2 × 10.75 × 10 × 10 ÷ 0.36 = 5,972The smallest AWG gauge whose CM ≥ this value is the minimum safe wire size.
How does this calculator work?
Use CM = 21.5 × I × L / V_drop to find the required circular mils, then pick the smallest AWG copper gauge that meets or exceeds that value. For 12 V systems, keep voltage drop at 3 % (0.36 V) or less; always verify the gauge also handles the load current within its ampacity rating.
Formula
How this is calculated
In a 12 V DC circuit the current must travel to the load and back, making the effective resistive path twice the one-way wire length. Copper's resistivity is expressed as 10.75 Ω·CM/ft, where CM is the wire's cross-sectional area in circular mils. Re-arranging V = I × R gives the required circular mils: CM = 2 × 10.75 × I × L / V_drop. The calculator finds the thinnest standard AWG gauge whose circular-mil area meets or exceeds that requirement.
Voltage drop is a key concern in low-voltage DC systems because the same absolute drop represents a larger fraction of the supply: a 0.5 V drop on a 12 V system is over 4 %, yet on a 120 V system it would be only 0.4 %. The NEC recommends keeping voltage drop to 3 % or less on branch circuits. RV, marine, and solar installations typically use 3 % (0.36 V) as the limit.
The ampacity column shows the wire's maximum safe continuous current rating (NEC 60 °C, open air). Always confirm the chosen gauge can also handle the load current without overheating — if the current exceeds the ampacity, you must move to a thicker gauge regardless of voltage-drop math. Wire routing, bundling, and temperature derating can reduce ampacity further; consult the NEC for site-specific adjustments.
Frequently asked questions
For the same power, a 12 V load draws ten times more current than a 120 V load (P = V × I). Higher current increases resistive voltage drop proportionally, so a much larger conductor is needed to keep the drop within acceptable limits.
Enter the one-way (single-run) length. The calculator automatically doubles it to account for the return path, matching the industry-standard formula.
If the required circular mils exceed AWG 4/0 (211 600 CM), you need multiple conductors in parallel or a higher-voltage system. Splitting the run into two parallel AWG 4/0 cables halves the resistance and doubles the effective CM.
TG we-Calculate Editorial Team. (2026). 12V Wire Size Calculator — DC Gauge & Voltage Drop [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/12v-wire-size-calculator
TG we-Calculate Editorial Team. "12V Wire Size Calculator — DC Gauge & Voltage Drop." TG we-Calculate. 2026. https://we-calculate.com/calculator/12v-wire-size-calculator.
TG we-Calculate Editorial Team, "12V Wire Size Calculator — DC Gauge & Voltage Drop," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/12v-wire-size-calculator
@misc{wecalculate_12v_wire_size_calculator, title = {12V Wire Size Calculator — DC Gauge & Voltage Drop}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/12v-wire-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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