Practical tools for measured work
DC Wire Gauge & Voltage Drop Calculator
Find the thinnest standard copper wire gauge that keeps voltage drop at or below your target for a 12V, 24V or 48V DC circuit.
Recommended size is the thinnest of the standard sizes checked (16 AWG to 4/0) that keeps drop at or below your target.
How it works
How to use it: enter your system voltage (12, 24 or 48V), the load's continuous current draw, the one-way distance from source to load, and the maximum voltage drop you can accept. The tool checks standard copper wire sizes from 16 AWG to 4/0 and recommends the thinnest one that keeps drop at or below your target.
Formula. Round-trip circuit length is twice the one-way distance, since current has to return to the source: length = 2 × one-way distance. Wire resistance comes from copper's resistivity and the wire's cross-sectional area from its AWG size: R = resistivity × length ÷ area. Voltage drop is V = I × R, and drop percentage is drop % = V ÷ system voltage × 100. Power lost as heat in the wire is P = I² × R.
Worked example. A 20A load, 15ft one-way (30ft round trip) on a 12V system with a 3% target: 12 AWG copper has about 0.0052 ohm/m, so 30ft (9.14m) of it is about 0.0477 ohm total. Drop = 20A × 0.0477 = 0.95V, or 7.9% of 12V - well over a 3% target, so the tool steps up to a thicker gauge until the drop percentage clears the target.
Reference: drop and loss by wire size for a fixed 20A, 25ft one-way, 12V scenario:
| Wire size | Drop at 20A, 25ft one-way, 12V | Power lost |
|---|---|---|
| 14 AWG | 2.53 V (21.0%) | 50.5 W |
| 10 AWG | 1.00 V (8.3%) | 20.0 W |
| 8 AWG | 0.63 V (5.2%) | 12.6 W |
| 6 AWG | 0.40 V (3.3%) | 7.9 W |
| 4 AWG | 0.25 V (2.1%) | 5.0 W |
| 2 AWG | 0.16 V (1.3%) | 3.1 W |
| 1/0 AWG | 0.10 V (0.8%) | 2.0 W |
| 3/0 AWG | 0.06 V (0.5%) | 1.2 W |
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- System voltage
- 12 V
- Load current
- 20 A
- One-way cable length
- 15 ft
- Max acceptable drop
- 3 %
Primary result: Recommended wire size: 6 AWG.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
Resistance is calculated at 20°C for annealed copper and does not include a temperature-rise correction, connector or crimp resistance, or aluminum conductors. Real installed runs are usually slightly worse than this ideal-conductor estimate, so treat the result as a minimum wire size, not a guaranteed maximum drop.
The suggested fuse or breaker rating is continuous current × 1.25, rounded up to the next standard size - a common starting rule of thumb, not a substitute for your equipment manufacturer's rating, your wire's actual ampacity, or your applicable electrical code and inspection requirements.
3% is a common design target for sensitive DC loads; less sensitive loads often tolerate more.
Frequently asked questions
Why does the calculation use round-trip length instead of just the distance to the load?
A DC circuit needs a complete loop - current flows out to the load on one conductor and back to the source on another. Both conductors add resistance, so the electrical length is twice the physical one-way distance, even when the two wires are bundled together.
What voltage drop target should I use?
3% is a widely used target for lighting and sensitive electronics. Motors, pumps and less sensitive loads often tolerate 5-10%. Longer or higher-current runs get expensive to hold at 3%, so treat the target as a design choice, not a fixed rule.
Does this tell me if my fuse or breaker is code-compliant?
No. It suggests a rating relative to your load's continuous current only. It does not check your wire's ampacity, ambient temperature derating, bundling, or any electrical code or marine/RV standard - confirm those separately before wiring a system.
Can I use this for AC household wiring?
This tool is built for low-voltage DC circuits (solar, battery, RV, marine and automotive accessory wiring). AC circuits, three-phase systems and code-compliance calculations for household or commercial wiring need a qualified electrician and jurisdiction-specific code references.