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Contents
- 1. System Parameters for a Representative 10kW Installation
- 2. Voltage Drop Calculation Methodology
- 3. Worked Calculation for a 10kW System
- 4. Cable Size Recommendation Table for 10kW Systems
- 5. Thermal Ampacity Verification
- 6. The Cost of Undersizing: A 25-Year Perspective
- 7. SORIVO Cable Recommendations for a 10kW System
- Q&A: Cable Sizing Calculations
- Q&A: Economic Considerations
- Q&A: Combined Strings vs. Separate Strings
A 10kW photovoltaic system—whether installed on a residential rooftop, a commercial building, or as a ground-mount array—represents the most common capacity class in the distributed generation market. It operates at DC string voltages typically between 300V and 600V, with string currents in the range of 10 to 20 amperes depending on module specifications and string configuration.
The cable that connects the PV array to the inverter carries this DC power for a distance that can range from 10 metres in a compact residential installation to 50 metres or more on a large rooftop or ground-mount structure. The cross-sectional area of that cable is not an arbitrary choice. It must be calculated against voltage drop criteria, thermal ampacity limits, and the economics of lifetime energy loss. Undersizing the DC cable by even one cross-section step—for example, specifying 4 mm² where 6 mm² is required—can result in avoidable generation losses that compound over 25 years into a significant financial penalty.
This article provides a complete, practical cable sizing methodology for a nominally 10kW PV system, with worked voltage drop calculations and SORIVO solar cable recommendations.
1. System Parameters for a Representative 10kW Installation
To make the analysis concrete, we define a representative 10kW system using 18 × 550Wp modules (9,900Wp total DC capacity—nominally 10kW) with the following parameters:
| Parameter | Value | Notes |
|---|---|---|
| System capacity (DC) | 9,900 Wp (nominally 10 kW) | STC rating, 18 × 550 Wp modules |
| Module power | 550 Wp | Typical high-efficiency mono PERC module |
| Number of modules | 18 (2 strings × 9 modules) | 9,900 W total |
| Module Vmp | 41.5 V | At STC |
| Module Imp | 13.25 A | At STC |
| String Vmp | 373.5 V | 9 × 41.5 V |
| String Imp | 13.25 A | Modules in series |
| Number of strings | 2 | Parallel at combiner box or inverter inputs |
| Total array current (if combined) | 26.5 A | After parallel connection |
| Inverter type | Single-phase or three-phase, dual MPPT | String inverter with 2 MPP trackers |
| Cable run distance (array to inverter) | 20 m (short), 40 m (medium), 60 m (long) | One-way distance |
System topology: Each string of 9 modules feeds one MPPT input of the inverter. No DC combiner box is used at the array level for the separate strings. The DC cable from each string runs directly to the inverter. This is the most common configuration for a 10kW system.
2. Voltage Drop Calculation Methodology
DC voltage drop in a cable is governed by Ohm's Law and the resistivity of copper:
Vdrop = (2 × L × I × ρ) / A
Where:
- L = one-way cable length in metres
- I = string current in amperes (Imp at STC)
- ρ = resistivity of copper at operating temperature (Ω·mm²/m)
- A = conductor cross-sectional area in mm²
- Factor 2 accounts for the return path (positive + negative conductors)
Resistivity of Copper at Operating Temperature
The resistivity of copper increases with temperature. The relationship is:
ρ(T) = ρ20 × [1 + α × (T − 20)]
Where:
- ρ20 = 0.01724 Ω·mm²/m (resistivity at 20°C)
- α = 0.00393 /°C (temperature coefficient for copper)
Calculated values:
| Temperature | Resistivity (Ω·mm²/m) |
|---|---|
| 20°C | 0.0172 |
| 60°C | 0.0200 |
| 90°C | 0.0220 |
For a conservative design in a hot climate (Middle East rooftop, for example), we use the 90°C value of 0.022 Ω·mm²/m. This ensures the calculated voltage drop is not worse than expected under worst-case summer conditions.
Voltage Drop Expressed as a Percentage
% Vdrop = (Vdrop / String Vmp) × 100
Industry Voltage Drop Targets
| Standard / Practice | Recommended Maximum |
|---|---|
| IEC 60364-7-712 | 3% DC voltage drop |
| Leading EPC specifications | 1–2% for high-performance systems |
| Best practice | ≤1.5% for long cable runs |
A 3% voltage drop represents approximately 3% of potential generation lost to cable heating. Over 25 years, this loss compounds into significant financial impact.
Q: Why does voltage drop matter for a PV system?
A: Voltage drop directly reduces the power delivered to the inverter. If your array produces 10kW but 3% is lost in the DC cables, only 9.7kW reaches the inverter. Over 25 years, this 3% loss compounds into thousands of dollars of foregone revenue. Unlike module degradation (which is unavoidable), cable voltage drop is entirely preventable through proper sizing.
Q: Should I calculate voltage drop at STC current (Imp) or actual operating current?
A: For cable sizing, use Imp at STC (Standard Test Conditions: 1000 W/m², 25°C). This provides a conservative design that covers peak production conditions. Actual operating current will typically be 80–95% of Imp depending on irradiance and temperature, but sizing for Imp ensures the cable performs well under all conditions.
Q: Why use 90°C resistivity when the cable is rated for 120°C?
A: The 90°C value represents a realistic operating temperature for a cable exposed to rooftop conditions in hot climates, where:
- Ambient temperature can reach 45–50°C
- Cable surface temperature rises 20–30°C above ambient due to solar irradiance and current heating
Using 90°C resistivity provides a safety margin for worst-case conditions. Using the 120°C resistivity would be overly conservative and result in oversized cables.
Q: What if my cable runs through a shaded area or conduit?
A: If the cable is shaded or in conduit (not exposed to direct sunlight), the operating temperature will be lower. You can use a lower resistivity value (e.g., 60°C = 0.020 Ω·mm²/m) for the calculation. However, for simplicity and conservatism, many designers use the 90°C value regardless of installation conditions.
3. Worked Calculation for a 10kW System
We calculate the minimum cable cross-section required to keep voltage drop below 3% for three representative cable run distances: 20 metres, 40 metres, and 60 metres.
String parameters:
- String Vmp = 373.5 V
- String Imp = 13.25 A
- Allowable voltage drop = 3% × 373.5 V = 11.2 V
20-Metre Cable Run
Vdrop = (2 × 20 × 13.25 × 0.022) / A = 11.66 / A
A = 11.66 / 11.2 = 1.04 mm²
A = 11.66 / 11.2 = 1.04 mm²
A 1.5 mm² cable would technically meet the 3% criterion. However, thermal ampacity and mechanical robustness considerations push the practical minimum higher. Most 10kW installations use 4 mm² as the minimum for mechanical strength and to account for module bifacial gain, higher irradiance, and future expansion.
SORIVO recommendation: 4 mm² minimum for 20 m
40-Metre Cable Run
Vdrop = (2 × 40 × 13.25 × 0.022) / A = 23.32 / A
A = 23.32 / 11.2 = 2.08 mm²
A = 23.32 / 11.2 = 2.08 mm²
Next standard size: 2.5 mm² or 4 mm². To maintain drop well below 3% and accommodate real-world temperature variations:
| Size | Voltage Drop | % Drop |
|---|---|---|
| 4 mm² | 5.83 V | 1.56% |
| 6 mm² | 3.89 V | 1.04% |
SORIVO recommendation: 6 mm² for optimal balance
60-Metre Cable Run
Vdrop = (2 × 60 × 13.25 × 0.022) / A = 34.98 / A
A = 34.98 / 11.2 = 3.12 mm²
A = 34.98 / 11.2 = 3.12 mm²
| Size | Voltage Drop | % Drop | Assessment |
|---|---|---|---|
| 4 mm² | 8.75 V | 2.34% | Under 3% but approaching limit |
| 6 mm² | 5.83 V | 1.56% | Comfortable margin |
SORIVO recommendation: 6 mm² minimum; 10 mm² for high-temperature environments or bifacial modules
4. Cable Size Recommendation Table for 10kW Systems
The table below summarises the recommended SORIVO H1Z2Z2-K cable cross-section for a 10kW system with string configuration of 2 strings × 9 modules (373.5 V, 13.25 A per string). Values are rounded up to the nearest standard size and include a safety margin for high-temperature conditions.
| One-Way Distance | Minimum Cross-Section | SORIVO Recommended | Vdrop at Recommended | Comments |
|---|---|---|---|---|
| Up to 25 m | 2.5 mm² | 4 mm² | ≤ 1.0% | 4 mm² provides mechanical robustness and future margin |
| 25–40 m | 4 mm² | 6 mm² | 1.0–1.6% | Optimal balance of cost and performance |
| 40–60 m | 6 mm² | 6 or 10 mm² | 1.6–2.4% | 10 mm² recommended for high-ambient or bifacial |
| > 60 m | 10 mm² | 10 mm² | ≥ 2.4% at 10 mm² | Run separate strings to inverter; avoid combining |
Important Note for Combined Strings: If the two strings are combined in a rooftop DC combiner box before a single cable run to the inverter, the current doubles to 26.5 A. The cable cross-section must be recalculated accordingly, resulting in one to two size steps larger. SORIVO recommends using the dual MPPT configuration (one cable per string) for 10kW systems to minimise cable cost and improve MPPT performance under partial shading.
5. Thermal Ampacity Verification
Voltage drop determines the minimum cable size for energy efficiency. Thermal ampacity confirms the cable can carry the current safely without exceeding insulation temperature limits.
For SORIVO H1Z2Z2-K cables installed in free air, the ampacity values (at 60°C ambient, derated per IEC 60364-5-52) are:
| Cross-Section (mm²) | Ampacity in Free Air (A) | Sufficient for 10kW String (13.25 A)? |
|---|---|---|
| 2.5 | 30 A | ✅ Yes, with margin |
| 4 | 40 A | ✅ Yes |
| 6 | 55 A | ✅ Yes |
| 10 | 74 A | ✅ Yes |
In all recommended sizes, the thermal ampacity is not the limiting factor for a single string of 13.25 A. Voltage drop drives the sizing decision.
Ampacity Derating Factors
When cables are installed in conditions that limit heat dissipation, ampacity must be derated:
| Installation Condition | Derating Factor | Example: 4 mm² Ampacity |
|---|---|---|
| Free air | 1.00 | 40 A |
| In conduit on wall | 0.87 | 35 A |
| In conduit in insulated wall | 0.70 | 28 A |
| Buried in ground | 0.80 | 32 A |
| Multiple cables touching | 0.75–0.85 | 30–34 A |
Even with derating, all recommended cable sizes have adequate ampacity for a 13.25 A string.
Q: Why is ampacity not the limiting factor for PV string cables?
A: PV string currents are relatively low (typically 8–15 A for residential/commercial modules). Even a 2.5 mm² cable can carry 30 A in free air—far more than a typical string current. The voltage drop criterion (energy efficiency) is almost always more restrictive than thermal ampacity for PV DC cables.
Q: When would ampacity become the limiting factor?
A: Ampacity becomes critical when: (1) combined strings double or triple the current, (2) utility-scale systems with multiple strings combined before the inverter, (3) high ambient temperature exceeding 50°C on rooftops, (4) cables in conduit with limited ventilation, (5) multiple cables bundled together causing mutual heating.
Q: How do I apply derating factors?
A: Multiply the base ampacity by all applicable derating factors: Effective ampacity = Base ampacity × Factor₁ × Factor₂ × ...
Example: 4 mm² cable in conduit on a rooftop (60°C ambient)
Base ampacity (free air, 30°C): 55 A × Temperature derating (60°C): 0.58 × Conduit derating: 0.87 = 27.8 A — still sufficient for 13.25 A.
Example: 4 mm² cable in conduit on a rooftop (60°C ambient)
Base ampacity (free air, 30°C): 55 A × Temperature derating (60°C): 0.58 × Conduit derating: 0.87 = 27.8 A — still sufficient for 13.25 A.
6. The Cost of Undersizing: A 25-Year Perspective
Consider a 10kW system in a high-irradiance location producing 16,000 kWh/year. A 1% reduction in yield due to DC cable voltage drop represents 160 kWh/year of lost generation.
Financial Impact Calculation
| Parameter | Value |
|---|---|
| Annual production | 16,000 kWh |
| Voltage drop loss | 1% (160 kWh/year) |
| Electricity tariff | $0.08/kWh (commercial) |
| Annual loss value | $12.80/year |
| 25-year NPV (5% discount) | ~$170 |
Upsizing Cost vs. Benefit
| Upsizing Option | Additional Cable Cost | 25-Year Savings | Net Benefit |
|---|---|---|---|
| 4 mm² → 6 mm² (40 m run) | ~$25 | ~$85 | +$60 |
| 6 mm² → 10 mm² (60 m run) | ~$40 | ~$55 | +$15 |
The additional cost of upsizing is recovered within 2–5 years. After that, the larger cable generates a net positive return for the remaining 20+ years.
The Real Cost of Undersizing: Undersizing the cable to save $20–$30 on the bill of materials is one of the most economically irrational decisions in a PV project. The lost generation compounds over 25 years into hundreds of dollars of foregone revenue—far exceeding the initial savings.
Q: How do I calculate the financial impact of voltage drop for my project?
A: Use this formula:
Annual energy loss (kWh) = System capacity (kW) × Peak sun hours × Voltage drop % × Performance ratio
Annual cost = Annual energy loss × Electricity tariff ($/kWh)
25-year NPV = Annual cost × Annuity factor (typically 14–16 at 5% discount rate)
Example: 10kW system, 1600 peak sun hours, 2% voltage drop, 80% PR, $0.10/kWh
Annual loss = 10 × 1600 × 0.02 × 0.80 = 256 kWh
Annual cost = 256 × $0.10 = $25.60
25-year NPV ≈ $25.60 × 14 = $358
Annual energy loss (kWh) = System capacity (kW) × Peak sun hours × Voltage drop % × Performance ratio
Annual cost = Annual energy loss × Electricity tariff ($/kWh)
25-year NPV = Annual cost × Annuity factor (typically 14–16 at 5% discount rate)
Example: 10kW system, 1600 peak sun hours, 2% voltage drop, 80% PR, $0.10/kWh
Annual loss = 10 × 1600 × 0.02 × 0.80 = 256 kWh
Annual cost = 256 × $0.10 = $25.60
25-year NPV ≈ $25.60 × 14 = $358
Q: Is it worth upsizing cable if electricity prices are low?
A: Even at $0.05/kWh, the economics favour proper cable sizing. A 1% loss on 16,000 kWh = 160 kWh = $8/year, 25-year NPV ≈ $100+. Upsizing cost: $20–$30. The payback is still 3–4 years, with 20+ years of net benefit.
Q: What about residential systems with net metering?
A: With net metering, the value of lost generation equals the retail electricity rate you would otherwise offset. If your retail rate is $0.15/kWh: 160 kWh/year × $0.15 = $24/year, 25-year NPV ≈ $340. The economics are even more favourable for residential systems with high retail rates.
Q: Should I oversize cables for future expansion?
A: Consider future expansion if roof space is available for additional modules, the inverter has spare capacity, and local regulations allow system expansion. Upsizing from 4 mm² to 6 mm² costs ~$0.10/metre extra but provides flexibility for 30–50% current increase. This is cheap insurance for future-proofing.
7. SORIVO Cable Recommendations for a 10kW System
SORIVO's PV DC cables are manufactured to EN 50618 and IEC 62930, TÜV certified, with tinned copper conductors and XLPE insulation with an LSZH (XLPO) outer sheath—rated for 1500 V DC and a 25-year design life. For systems operating at 1000 V DC or below, the earlier TÜV 2PfG 1169 / PV1-F standard is also applicable; SORIVO supplies both standards.
| System Component | SORIVO Cable Specification | Notes |
|---|---|---|
| PV string to inverter (DC) | H1Z2Z2-K 1×4 mm² or 1×6 mm² | Tinned copper, 1500 V DC rated, EN 50618 / IEC 62930, XLPE insulation, LSZH sheath |
| Inverter to AC distribution board (AC) | SORIVO LV XLPE/SWA/PVC 3-core or 5-core | Typical 10kW three-phase inverter: ~15 A per phase → 4 mm² or 6 mm² AC cable |
| Earthing conductor | SORIVO H1Z2Z2-K or PVC earth cable, min. 6 mm² | Sized per local earthing regulations |
Quick Reference: Decision Tree
- Determine string current (Imp)
- Measure one-way cable distance
- Calculate minimum cross-section for 3% voltage drop
- Round up to next standard size
- Add one step for: High ambient temperature (>45°C) Bifacial modules (>10% gain) Future expansion potential Conservative design preference
- Verify ampacity (usually not limiting)
- Document calculation for project records
Standard Cable Sizes for 10kW Systems
| Distance | Standard Module (13 A) | Bifacial Module (15 A) |
|---|---|---|
| ≤ 25 m | 4 mm² | 4 mm² |
| 25–40 m | 4–6 mm² | 6 mm² |
| 40–60 m | 6 mm² | 6–10 mm² |
| > 60 m | 10 mm² | 10 mm² |
Combined Strings vs. Separate Strings
Q: When should I combine strings vs. run separate cables to the inverter?
A: The choice depends on your system architecture:
| Configuration | Advantages | Disadvantages |
|---|---|---|
| Separate cables (dual MPPT) | Lower current per cable; smaller cable size; better shading performance; no combiner box cost | More cable runs; more inverter connections |
| Combined strings (single cable) | Fewer cable runs; simpler inverter connection | Larger cable required; combiner box cost; single MPPT loses shading tolerance |
Recommendation: For 10kW systems, separate cables to dual MPPT inverter inputs is almost always the better choice. The savings from avoiding a combiner box and using smaller cables typically outweigh the cost of an extra cable run.
Q: How do I size the cable if I combine two 13.25 A strings?
A: Double the current and recalculate:
Combined current = 2 × 13.25 A = 26.5 A
For 40 m run:
Vdrop = (2 × 40 × 26.5 × 0.022) / A = 46.64 / A
For 3% drop (11.2 V): A = 46.64 / 11.2 = 4.16 mm² → Use 6 mm²
For 1.5% drop (5.6 V): A = 46.64 / 5.6 = 8.33 mm² → Use 10 mm²
Combined strings require significantly larger cables than separate string cables.
Combined current = 2 × 13.25 A = 26.5 A
For 40 m run:
Vdrop = (2 × 40 × 26.5 × 0.022) / A = 46.64 / A
For 3% drop (11.2 V): A = 46.64 / 11.2 = 4.16 mm² → Use 6 mm²
For 1.5% drop (5.6 V): A = 46.64 / 5.6 = 8.33 mm² → Use 10 mm²
Combined strings require significantly larger cables than separate string cables.
Q: What if my inverter only has one MPPT input?
A: You have two options:
Option 1 — Combine strings at the array: Use a DC combiner box and size the combined cable for 26.5 A.
Option 2 — Use a Y-connector at the inverter: Run two smaller cables (one per string) and combine at the inverter input.
Option 2 is often more economical because you avoid the combiner box cost and can use smaller cables.
Option 1 — Combine strings at the array: Use a DC combiner box and size the combined cable for 26.5 A.
Option 2 — Use a Y-connector at the inverter: Run two smaller cables (one per string) and combine at the inverter input.
Option 2 is often more economical because you avoid the combiner box cost and can use smaller cables.
Q: Does cable colour (red vs. black) affect sizing or performance?
A: No. The colour of the cable jacket is for polarity identification only (red for positive, black for negative in DC systems). Both colours use identical conductor, insulation, and sheath materials. Never use cable colour as a substitute for proper polarity labelling — always use permanent markers or heat-shrink labels at both ends.
Q: How does cable sizing differ for a single-phase vs. three-phase inverter?
A: The DC side sizing is identical — voltage drop depends only on string current, cable length, and cross-section. For the AC side: single-phase uses 2-core cable (L + N) with Vdrop = (2 × L × I × ρ) / A; three-phase uses 4-core or 5-core cable with Vdrop = (√3 × L × I × ρ × cosφ) / A. For the same power, three-phase AC current is approximately half of single-phase, allowing smaller AC cable sizes.
Size It Right. Specify SORIVO.
SORIVO H1Z2Z2-K solar cables are available in cross-sections from 1.5 mm² to 35 mm², manufactured to EN 50618 and IEC 62930, TÜV certified, and supplied with full test certification.
Explore our renewable energy solutions | View solar cable range | Request a free quote
For a project-specific voltage drop calculation and cable schedule for your PV system, contact our engineering team at sale@sorivocable.com or call +86 192 8290 5529. 7×24×365 support.
SORIVO H1Z2Z2-K solar cables are available in cross-sections from 1.5 mm² to 35 mm², manufactured to EN 50618 and IEC 62930, TÜV certified, and supplied with full test certification.
Explore our renewable energy solutions | View solar cable range | Request a free quote
For a project-specific voltage drop calculation and cable schedule for your PV system, contact our engineering team at sale@sorivocable.com or call +86 192 8290 5529. 7×24×365 support.