Solar Farm AC Combiner Feed to Substation

Case Study 2: Solar Farm AC Combiner Feed to Substation

Scenario

A 2.4 MW utility-scale solar PV plant in the arid San Luis Valley, Colorado (elevation 2,200 m, summer ambient up to 42°C), requires an AC interconnection feeder from the central inverter combiner cabinet to the onsite 34.5 kV/400 V step-down substation. The 120 m route traverses rocky, dry soil with minimal shading — installed via direct burial in desert-grade HDPE ductbank. Constraints include IEEE 1547-compliant voltage regulation (±5% at point of interconnection), thermal stability under diurnal cycling, and avoidance of aluminum due to high soil resistivity (1,200 Ω·m) increasing corrosion risk. Local code mandates minimum 120% continuous rating for renewable generation circuits.

Given Data

• Voltage: 400 V (LV side of transformer)
• Power: 2,400,000 W (2.4 MW aggregate inverter output)
• Power Factor: 0.98 (grid-support inverters with reactive power capability)
• Length: 120 m
• Material: Copper
• Installation Method: Direct Buried

Calculation

Using the Cable Size Calculator:

1. Design current (3-phase, 120% continuous rating):
   ( I = \frac{1.2 \times P}{\sqrt{3} \times V \times \text{PF}} = \frac{1.2 \times 2{,}400{,}000}{1.732 \times 400 \times 0.98} \approx 4,272 , \text{A} )
2. Since single conductors cannot carry >4,000 A practically, parallel runs are required. The tool evaluates per-conductor sizing assuming 3 parallel sets (common industry practice):
   • Per-set current: ( \frac{4{,}272}{3} \approx 1{,}424 , \text{A} )
   • Direct burial in dry soil (42°C) applies derating factor ≈ 0.82 (NEC Table 310.15(B)(3)(c) + IEEE 835 adjustment).
   • Required ampacity per conductor: ( \frac{1{,}424}{0.82} \approx 1{,}737 , \text{A} )
3. Voltage drop limit: 5% of 400 V = 20 V max.
   • Using ( \Delta V = \frac{2 \times K \times L \times I}{S} ) with ( K = 25 , \text{mΩ·mm²/m} ), ( L = 120 , \text{m} ), ( I = 1{,}424 , \text{A} ):
     • For 500 mm²: ( \Delta V = \frac{2 \times 25 \times 120 \times 1{,}424}{500} \approx 17.09 , \text{V} ) ✅
     • Ampacity of 500 mm² Cu (direct buried, 42°C): ~1,850 A (per ICEA P-54-44, 3-conductor, 90°C XLPE) → derated = 1,517 A ❌ (insufficient)
     • 630 mm²: Ampacity ~2,100 A → derated = 1,722 A ⚠️ (tight margin)
     • 800 mm²: Ampacity ~2,450 A → derated = 2,009 A ✅; ( \Delta V = \frac{2 \times 25 \times 120 \times 1{,}424}{800} \approx 10.68 , \text{V} ) ✅

Result and Decision

The calculator recommends 800 mm² copper cable per parallel run (3 runs total), rated 90°C XLPE, direct buried. This met all thermal, voltage drop (<2.7%), and fault-current withstand requirements. Final specification: 3× (3C × 800 mm² Cu, XLPE, LSZH, armored) in separate ducts to mitigate mutual heating.

Lesson

For high-power, long-duration DC/AC feeders in renewables, parallel conductor configurations shift the dominant constraint from voltage drop to thermal derating — especially in high-ambient, low-dissipation environments. Always cross-check the calculator’s ampacity output against actual manufacturer thermal data for the specific insulation and burial condition, not generic tables.