Solar Cable Size Calculator

Formula used

1) Current

• DC: I = P / (V x eta)
• AC 1phi: I = P / (V x PF x eta)
• AC 3phi: I = P / (sqrt(3) x V x PF x eta)

2) Voltage drop (resistive planning model)

• DC and AC 1phi: Vdrop = 2 x L x I x (rhoT / A)
• AC 3phi: Vdrop = sqrt(3) x L x I x (rhoT / A)

3) Resistivity temperature correction

• rhoT = rho20 x (1 + alpha x (T - 20))
• Planning constants: copper rho20 ~= 0.0175 ohm*mm2/m, aluminum rho20 ~= 0.0282 ohm*mm2/m.

4) Ampacity check (planning)

• Design current: I_design = I_load x safety_factor
• Derated ampacity: I_allow = I_base x F_temp x F_ambient x F_group x F_install
• The recommended cable is the smallest standard size meeting both checks.

This is an engineering sizing helper. Always verify with local codes, manufacturer data, and protection settings.

How to use this calculator

1. Select DC or AC and choose how you will enter the load.
2. Enter voltage, one-way length, material, and site temperature details.
3. Set your voltage drop limit, then choose an appropriate safety factor.
4. Press Submit to see the recommended size above the form.
5. Download CSV or PDF to attach the sizing record to your project.

Load current and design margin

Cable sizing begins with operating current. DC uses I = P/(V×η); AC adds power factor and phase. Example: 7.2 kW at 600 V DC and 98% efficiency draws 12.24 A. With a 1.25 continuous margin, design current becomes 15.30 A, improving thermal headroom during sustained generation.

Voltage drop limits and performance

Voltage drop is modeled as resistive loss along the run. DC and single phase use 2× one‑way length; three phase uses √3× length. A 2% limit at 600 V allows 12 V. Keeping drop near 1–2% helps inverter tracking, reduces nuisance trips, and lowers I²R heating.

Resistivity, temperature, and conductor choice

Material changes resistivity: copper is about 0.0175 Ω·mm²/m at 20°C, aluminum about 0.0282. Resistivity rises with temperature using ρT = ρ20(1+α(T−20)). At 90°C, copper increases roughly 27% versus 20°C, so hot routes often require a larger standard size.

Derating for ambient, grouping, and routing

Allowable current is reduced by site factors. Ambient correction drops capacity as air warms, grouping accounts for mutual heating of nearby circuits, and installation reflects heat dissipation. For example, two grouped circuits in conduit at 40°C can derate capacity by roughly 30% compared with a single open run near 30°C.

How the recommendation is selected

The calculator finds two minimum sizes: one meeting the drop limit at load current, and one meeting derated ampacity at design current. The final recommendation is the larger standard size. If voltage drop dominates, shorten routing or raise voltage. If ampacity dominates, reduce grouping, improve ventilation, or select higher temperature rated cable.

Records that support commissioning

CSV export stores inputs, targets, and the chosen size for project files. The PDF snapshot supports inspection and handover. Pair the output with cable datasheets, verify actual route length, confirm terminations and protection settings, and check that installed rating and conduit fill match construction details. For troubleshooting, compare measured voltage under load with predicted drop, then inspect terminations, connectors, and splices for heat, looseness, or corrosion early.

FAQs

1. What length should I enter for the run?

Enter the one-way route length from source to load. The calculator applies the correct return-path factor for DC, single phase, or three phase internally.

2. Why does the tool use a safety factor?

Continuous solar output can run for hours. A margin, often 1.25, converts operating current to a design current so the selected size stays cooler and lasts longer.

3. Is the voltage drop model accurate for AC cables?

It uses a resistive planning model. For long AC feeders with higher reactance, confirm with detailed impedance data from the cable manufacturer and local design practice.

4. How do ambient temperature and grouping affect sizing?

Higher ambient reduces heat shedding, and grouped circuits heat each other. Both reduce allowable current, so a larger cable may be needed even if voltage drop looks acceptable.

5. When should I prefer aluminum over copper?

Aluminum can lower cost and weight, but needs larger cross-section for similar drop and ampacity. Use approved lugs, proper torque, and anti-oxidation practices where required.

6. What do CSV and PDF exports include?

They capture key inputs, computed currents, drop targets, required sizes, and the final recommendation. Attach them to design notes, commissioning checklists, and inspection packages.