How DC voltage drop and wire-size screening work
Use Check Existing Wire to evaluate a chosen AWG size, or Find Wire Size to compare available gauges against your voltage-drop goal. The finder reports the smallest wire that meets the drop target in its 11-size dataset, plus a separately labeled option that does not exceed available preliminary ampacity and protection thresholds. Neither means that a wire has been approved for installation.
Low-voltage DC circuits (12V, 24V, and 48V) are extremely sensitive to conductor resistance. Because current (I) is inversely proportional to voltage for a given power level (P = V × I), a 12V system requires four times more current than a 48V system to deliver the same power, resulting in 16 times more resistive power dissipation (P = I2R).
1. Loop Conductor Resistance
In direct current circuits, current flows from the positive terminal to the load and returns through the negative conductor. The total conductor length is therefore twice the physical one-way route distance:
Total Path (ft) = 2 × Physical One-Way Distance (ft)
Total Resistance (Ω) = Total Path × (Conductor Ohms per 1,000 ft ÷ 1,000)
Worked example: 12 V, 20 A and 25 ft one-way
This example uses the same 75°C stranded copper reference as the calculator. The 25-foot one-way distance is a 50-foot complete positive-and-negative conductor path. For 10 AWG (1.24 Ω per 1,000 feet): resistance is 0.0620000 Ω, cable drop is 1.240 V, or 10.33% of source voltage. Estimated load voltage is 10.760 V and cable loss is 24.80 W.
With the same source, current and run, the larger 4 AWG conductor (0.308 Ω per 1,000 feet) has 0.308 V drop, approximately 2.57%, and 6.16 W cable loss. It clears a 3% voltage performance goal, but ampacity, terminals and protection still require installation-specific review. The finder searches the supported conductor set and deliberately distinguishes this voltage-only minimum from a reference-screened candidate.
How 12 V, 24 V and 48 V compare at the same 240 W source-DC input
This comparison holds nominal source DC input power at 240 W, 10 AWG uncoated stranded copper and the 25-foot one-way route constant. Because planning current is source watts divided by source voltage, raising nominal system voltage reduces current and resistive cable loss in this fixed-current estimate.
240 W source-DC estimate, 10 AWG copper, 25 ft one way | Source V DC | Planning current | Drop V | Drop % | Cable loss W |
|---|
| 12 V | 20.0 A | 1.240 V | 10.33% | 24.80 W |
| 24 V | 10.0 A | 0.620 V | 2.58% | 6.20 W |
| 48 V | 5.0 A | 0.310 V | 0.65% | 1.55 W |
These are not interchangeable voltage settings for one piece of equipment. Battery, appliances, inverter and all connected components must actually be designed for the chosen system voltage. Entering AC output watts from an inverter as source-DC watts underestimates demand without a verified efficiency allowance; use known DC input current where possible.
Maximum modeled one-way cable distance for a selected 3% drop
These static reference tables invert the same fixed-current voltage-drop equation used above. Each cell is a maximum modeled one-way route length in feet for a specified source voltage, steady current and equal-gauge copper positive and negative conductors. They use the existing verified 75°C stranded, uncoated copper resistance references, a 3% voltage-performance planning target, and two conductor legs (round trip). Figures are rounded down to 0.1 ft, never up past the calculated limit.
12 V DC source: maximum modeled one-way copper cable length (feet) at 3% drop | Conductor AWG | 10 A | 20 A | 30 A |
|---|
| 10 AWG | 14.5 ft | 7.2 ft | 4.8 ft |
|---|
| 8 AWG | 23.1 ft | 11.5 ft | 7.7 ft |
|---|
| 4 AWG | 58.4 ft | 29.2 ft | 19.4 ft |
|---|
24 V DC source: maximum modeled one-way copper cable length (feet) at 3% drop | Conductor AWG | 10 A | 20 A | 30 A |
|---|
| 10 AWG | 29.0 ft | 14.5 ft | 9.6 ft |
|---|
| 8 AWG | 46.2 ft | 23.1 ft | 15.4 ft |
|---|
| 4 AWG | 116.8 ft | 58.4 ft | 38.9 ft |
|---|
48 V DC source: maximum modeled one-way copper cable length (feet) at 3% drop | Conductor AWG | 10 A | 20 A | 30 A |
|---|
| 10 AWG | 58.0 ft | 29.0 ft | 19.3 ft |
|---|
| 8 AWG | 92.5 ft | 46.2 ft | 30.8 ft |
|---|
| 4 AWG | 233.7 ft | 116.8 ft | 77.9 ft |
|---|
Voltage drop is not wire safety approval. Values are for steady DC current, not constant-power loads; different supply voltages do not imply interchangeable devices. This model excludes connector/fuse resistance, conductor temperature variation, battery sag and load transients. Ampacity, overcurrent protection, insulation, terminals, installation method, local rules and manufacturer limits must be checked separately; even where the selected current is within published preliminary thresholds, the table does not verify any installed circuit. Gauge labels are AWG metal areas, not promises about equivalent metric cable sizes. Consult the separate reference and screening tables below.
2. Voltage-Drop Design Recommendations
- ≈ 3% branch-circuit design target: A commonly cited NEC informational recommendation for reasonable voltage performance. It is a design guideline, not a universal mandatory maximum.
- ≈ 5% feeder + branch design target: A commonly cited combined design recommendation. Equipment voltage tolerance, manufacturer instructions, the locally adopted code edition, and installation-specific requirements still control the actual design.
Stranded Conductor Resistance and Limited Reference Ampacity
Reference dataset: stranded-75C-NFPA70-2023-excerpt-v2. Cross-checked against University of Maryland Extension EB-2023-0676 (2023 NEC excerpt, Appendix A1/A2). Source temperature is a conductor-resistance reference; base 75°C ampacity assumes 30°C ambient, not 75°C ambient. Installed ampacity must be verified.
Resistance values use verified stranded (7- or 19-strand) conductor entries at 75°C, uncoated copper and corresponding aluminum, cross-checked between Ω/km and Ω/1,000 ft. The base 75°C ampacity checks use material-specific Table 310.16 reference values before correction or adjustment. They are planning references only: insulation rating, terminal temperature limits, ambient conditions, conductor count, installation method, continuous-load rules, overcurrent protection, equipment instructions, and local amendments can change the allowable design.
| Wire Gauge | Metric mm² | Copper Ω/1,000ft | Aluminum Ω/1,000ft | Copper 75°C Ampacity | Aluminum 75°C Ampacity |
| 14 AWG | 2.08 | 3.14 Ω | 5.17 Ω | 20 A | —* |
| 12 AWG | 3.31 | 1.98 Ω | 3.25 Ω | 25 A | 20 A |
| 10 AWG | 5.261 | 1.24 Ω | 2.04 Ω | 35 A | 30 A |
| 8 AWG | 8.367 | 0.778 Ω | 1.28 Ω | 50 A | 40 A |
| 6 AWG | 13.3 | 0.491 Ω | 0.808 Ω | 65 A | 50 A |
| 4 AWG | 21.15 | 0.308 Ω | 0.508 Ω | 85 A | 65 A |
| 2 AWG | 33.62 | 0.194 Ω | 0.319 Ω | 115 A | 90 A |
| 1/0 AWG | 53.49 | 0.122 Ω | 0.201 Ω | 150 A | 120 A |
| 2/0 AWG | 67.43 | 0.0967 Ω | 0.159 Ω | 175 A | 135 A |
| 3/0 AWG | 85.01 | 0.0766 Ω | 0.126 Ω | 200 A | 155 A |
| 4/0 AWG | 107.2 | 0.0608 Ω | 0.1 Ω | 230 A | 180 A |
* This calculator does not provide a general-power 75°C ampacity reference for 14 AWG plain aluminum. Copper-clad aluminum is a distinct conductor type with code-edition-specific provisions and is not modeled as ordinary aluminum here.
Voltage drop versus ampacity: why the two wire sizes can differ
Voltage-drop minimum means the smallest verified available AWG metal area that meets your percentage target with a positive modeled load voltage. Reference-screened candidate means the smallest such conductor that also does not exceed the limited, available base 75°C ampacity and applicable small-conductor protection references. For example, at 12 V, 20 A and just 1 foot one-way, 14 AWG copper can pass the voltage calculation, but the 20 A planning current exceeds that reference's 15 A small-conductor protection threshold. The screened answer becomes 12 AWG. That comparison does not size an OCPD or prove an installed wire is appropriate.
Common DC wire-sizing mistakes
- Counting length twice: the tool doubles a one-way route, but not a length already entered as the combined positive and negative conductor path.
- Equating AWG area with a nominal metric cable: the mm² labels describe the metal in the referenced AWG conductor; use actual cable specifications when buying.
- Entering AC inverter output watts as battery-side DC demand: enter source-DC input current or an independently estimated DC input power. No assumed inverter efficiency is added.
- Forgetting connectors, fuses and battery sag: these real losses are excluded and can increase the practical drop.
- Assuming 75°C resistance means 75°C ambient: it does not. The ampacity column's ambient basis is 30°C, with prescribed reference installation conditions.
What this calculator does not establish
- Code compliance, conductor suitability, or final overcurrent-device sizing for a specific installation.
- Temperature correction, bundled-conductor adjustment, terminal ratings, or installation-method derating.
- Connector/contact resistance, battery-terminal losses, fuse/breaker voltage drop, or equipment-specific low-voltage thresholds.
Frequently asked questions
Is 3% voltage drop a mandatory maximum?
Not universally. Three percent is a common branch-circuit design guideline and five percent a common total feeder/branch guideline. Actual circuit performance, equipment requirements and applicable local rules govern the design. You can choose a different planning target within 0.5–10%.
Should I enter the distance one way or both directions?
Select the matching field mode. If the source is 25 feet from your load and the same-size negative conductor runs back, choose 25 ft one-way, not 50 ft one-way. A full positive-plus-negative path of 50 ft produces the same result.
Can I calculate cable for 12 V, 24 V and 48 V systems?
Yes, for modeled DC circuits at 1–250 V using the fixed-current assumptions. A higher voltage is not permission to connect incompatible loads; actual equipment must match the system architecture. AC circuits and solar PV string design are outside this tool.
Does the wire finder choose a fuse or certify ampacity?
No. The limited reference screen compares the planned load current with published base 75°C conductor ampacity and applicable small-conductor thresholds. This cannot select a fuse, breaker or approved cable, and does not apply installation adjustments.
Why is my estimated load voltage unavailable?
If modeled cable drop reaches or exceeds the source voltage, a fixed current cannot be sustained at a positive load voltage using this model. The tool shows an infeasible diagnostic rather than a misleading working-circuit result. Equipment may stop functioning at a much smaller drop.
Why are some copper and aluminum wire references different?
The two materials have different electrical resistances and base ampacity references. One aluminum row also lacks an applicable general-purpose ampacity reference in this dataset, so it cannot pass the reference screen even when it satisfies the mathematical voltage target. Material substitutions require hardware, terminals and code checks.
Related Practical Guides
Prepared by the Clean Power Guide Editorial Team under the site's editorial policy. Calculator and reference data updated October 15, 2026. Hand-worked expected values and source records are documented in the project methodology. No independent licensed electrical-installation approval is claimed. Found an error? Use the corrections process or contact us.