Solar Voltage Drop Input Form
Example Data Table
| Scenario | System | Voltage (V) | Current (A) | Length (m) | Size (mm²) | Material | Temp (°C) | Drop Limit (%) |
|---|---|---|---|---|---|---|---|---|
| String to combiner | DC | 150 | 18 | 35 | 6 | Copper | 55 | 3 |
| Combiner to inverter | DC | 600 | 32 | 48 | 16 | Copper | 50 | 2 |
| Inverter to AC panel | Single-Phase AC | 240 | 40 | 22 | 16 | Aluminum | 45 | 3 |
| Central inverter feeder | Three-Phase AC | 415 | 85 | 60 | 50 | Copper | 40 | 2.5 |
Formula Used
Temperature-adjusted resistivity:
ρT = ρ20 × [1 + α × (T − 20)]
This adjusts conductor resistance for hotter operating conditions, which usually increase cable losses.
Resistance per meter:
Rm = ρT ÷ (A × parallel runs)
A larger cross-sectional area or additional parallel runs reduce resistance.
Voltage drop:
DC and single-phase basis: Vdrop = factor × L × I × (R cosφ + X sinφ)
Three-phase factor = √3, DC and single-phase factor = 2
For DC, cosφ = 1 and sinφ = 0, so reactance has no effect.
Total drop including connections:
Vtotal = cable drop + [I × total contact resistance]
Drop percentage: Vtotal ÷ system voltage × 100
How to Use This Calculator
- Enter a useful circuit segment name, such as string homerun or inverter feeder.
- Choose the system type and conductor material.
- Provide nominal voltage, design current, one-way route length, and cable size.
- Add operating temperature, power factor, reactance, and any design current margin.
- Enter the expected number of electrical connections and contact resistance per connection.
- Set the maximum acceptable voltage drop percentage for your design target.
- Press the calculation button to show the result above the form.
- Use the suggested standard size and status badge to refine conductor selection.
Frequently Asked Questions
1. Why is voltage drop important in solar systems?
Excessive drop reduces delivered voltage, wastes energy as heat, and can lower inverter or controller performance. Managing it helps improve system efficiency and equipment reliability.
2. Why does the calculator ask for one-way length?
The one-way route is easier to measure in the field. The calculator applies the proper path factor automatically for DC, single-phase, and three-phase circuits.
3. What does design margin do?
Design margin increases the entered current before calculations. It helps account for future loading, conservative engineering practice, and mild estimation uncertainty.
4. Why include contact resistance?
Terminals, breakers, lugs, and connectors add small resistances. In high-current or low-voltage solar circuits, these extra losses can meaningfully affect total voltage drop.
5. Does temperature really change cable losses?
Yes. Conductors usually become more resistive as temperature rises. Hot rooftop runs and conduit installations can produce noticeably higher voltage drop than cool conditions.
6. When should I care about reactance?
Reactance matters mainly on AC feeders, longer runs, and larger conductors. For short DC strings, it usually contributes little compared with pure resistance.
7. Is the suggested cable size always code-compliant?
No. It is a voltage-drop recommendation only. Final conductor selection must also satisfy ampacity, insulation rating, installation method, fault duty, and local code rules.
8. What drop percentage is usually acceptable?
Many designers target around 1% to 3% for important solar segments, but the best limit depends on equipment sensitivity, energy yield goals, and project standards.