Input Parameters
Example data table
The table below shows typical outcomes for a 48 V DC system with various loads and lengths. Values are indicative and based on copper conductors.
| System voltage (V) | Load current (A) | Length (m) | Max drop (%) | Material | Ambient (°C) | Insulation (°C) | Conductors | Recommended size (mm²) | Estimated drop (V) |
|---|---|---|---|---|---|---|---|---|---|
| 48 | 10 | 20 | 3 | Copper | 30 | 70 | 2 | 4 | 1.3 |
| 48 | 25 | 30 | 3 | Copper | 35 | 70 | 4 | 16 | 3.9 |
| 48 | 40 | 40 | 3 | Copper | 40 | 90 | 6 | 35 | 4.6 |
| 24 | 20 | 15 | 5 | Aluminium | 30 | 70 | 3 | 16 | 1.8 |
Formula used
1. Voltage drop along a two-core DC cable
R_total = 2 × ρ × L / A
V_drop = I_design × R_total
ρ= conductor resistivity (Ω·mm²/m)L= one-way length (m)A= cross-sectional area (mm²)I_design= design current after safety and derating factors (A)
2. Area from voltage drop limit
A_vd = 2 × ρ × L × I_design / V_drop_allow
where V_drop_allow = V_system × (%) / 100.
3. Area from current carrying capacity
A_current = I_design / (J_base × k_overall)
J_base≈ 6 A/mm² for copper, 4 A/mm² for aluminiumk_overall= product of base, temperature and grouping factors
4. Final selection
The calculator takes the larger of A_vd and A_current as the minimum area and then rounds up to the next standard cable size.
How to use this calculator
- Enter the system voltage and continuous load current for the circuit you are designing.
- Specify the one-way cable length, from the DC source to the load.
- Choose an acceptable voltage drop percentage according to your design standards or equipment limits.
- Select the conductor material used for the cable, usually copper for compact installations.
- Set a safety factor on current to allow for overloads and possible future load increases.
- Enter a base derating factor if you need extra reduction for special situations.
- Provide ambient temperature, insulation rating and number of loaded conductors to refine automatic derating.
- Click “Calculate cable size” to obtain the recommended cross-sectional area and detailed intermediate results.
- Use the CSV or PDF download buttons to capture the results and attach them to your design documentation or project reports.
Where DC cable sizing matters most
Voltage drop and ampacity are critical in low-voltage DC systems such as off-grid solar, battery banks, telecom power, and RV or boat wiring. Long runs at modest currents can still lose significant voltage without careful cable section selection.
Coordinating DC and AC cable selection
Many projects combine DC battery wiring with AC distribution. Use this tool for battery and inverter DC runs, and the AC cable size calculator for mains side conductors to keep both sides of the system consistent.
Impact of voltage drop on LED lighting
LED strips and low-voltage luminaires are sensitive to voltage drop. Undersized DC cables cause dimmer light at the far end of runs. Combine this calculator with the basement lighting calculator when planning power and illuminance together.
Temperature, bundling and derating effects
Elevated ambient temperature, higher insulation ratings and multiple loaded conductors in a raceway all influence current capacity. This tool combines a base derating factor with automatic temperature and grouping factors to approximate these influences in a single overall derating coefficient.
Planning for system growth and overloads
The safety factor on current lets you oversize cables slightly today to accommodate future load increases or occasional overloads. A modest margin helps maintain acceptable voltage and conductor temperature without excessively inflating cable material cost.
Documenting cable calculations for projects
Use the CSV and PDF exports to capture assumptions, intermediate values and recommended cable sizes. Attaching these files to design records, maintenance manuals or regulatory submissions makes future troubleshooting, upgrades and audits faster and more transparent.
Frequently asked questions
1. What voltage drop limit should I choose?
For most low-voltage DC circuits, designers often keep total voltage drop within three to five percent. Sensitive electronics or long cable runs may need tighter limits, while non-critical loads can tolerate slightly higher percentages.
2. Can I use this calculator for solar PV battery cabling?
Yes. Enter the DC bus voltage, maximum charging or discharging current, and realistic cable length between batteries, charge controller and inverter. Confirm final choices against manufacturer recommendations and local standards for photovoltaic and battery installations.
3. Is this tool suitable for automotive or marine wiring?
The formulas apply to any DC circuit, including vehicles and boats, provided you use correct voltages, currents and lengths. Always cross-check against automotive or marine wiring guidelines, fuse ratings and environmental protection requirements for harsh locations.
4. Why is the recommended cable size larger than handbook values?
Handbook tables might assume cooler temperatures, fewer bundled conductors, smaller safety margins or different installation methods. This calculator includes safety and derating factors that often increase the required cross-sectional area compared with simple nameplate current ratings.
5. How should I handle very long DC cable runs?
For long distances, voltage drop usually dominates over ampacity. Reduce current by raising system voltage where practical, keep routes as short as possible, and consider splitting loads or relocating equipment closer to the power source.
6. Do I need a separate tool for AC cable sizing?
Yes. AC circuits involve different ratings and installation assumptions. Use this page for DC runs and the AC cable size calculator for mains or distribution feeders on the alternating current side.
7. Does this calculator replace wiring codes or manufacturer data?
No. Results are engineering-style estimates for early design and checking. Final conductor selection must comply with national wiring rules, equipment datasheets, protective device coordination requirements and any project-specific engineering standards.