About the Conductor / Bus Duct Sizing
Conductor and bus duct sizing determines the appropriate cross-section and current rating of busbars (rigid copper or aluminium conductors) or bus ducts (enclosed busbar systems) used to carry large currents within a solar plant's switchgear room, transformer bay, or BESS PCS — where individual cables would be impractical.
How to calculate it
- 1Current density for copper busbars: 1.5–2 A/mm² for enclosed busbars with natural ventilation; 2.5–3 A/mm² for busbars in open air with good airflow.
- 2Required busbar area (mm²) = rated current ÷ current density.
- 3Standard busbar sizes: 25×3, 40×5, 50×6, 63×6, 80×8, 100×10, 125×10 mm (width × thickness).
- 4For three-phase systems: three bars (one per phase) plus a neutral bar (if required).
- 5Short-circuit withstand: busbar must withstand the fault current for 1 second without melting. Check I²t rating: I²t = Isc² × 1 s. Required busbar area (thermal) = Isc ÷ k, where k = 115 for copper (IS 5578).
- 6Bus duct (IPB or non-IPB): used for large currents (> 2,000 A) or in hazardous areas. Rating from manufacturer — specify forced-air cooling if current > 3,500 A.
Worked example
LT busbar for a 1 MVA transformer at 415 V: rated current = 1,000,000 ÷ (1.732 × 415) = 1,390 A. Busbar area at 2 A/mm²: 695 mm². Select 100 × 8 mm copper (800 mm²) per phase — standard size above 695 mm². Short-circuit check: Isc = 27,800 A. Required area = 27,800 ÷ 115 = 242 mm² — well below 800 mm². Thermal check passes.
What you can calculate
- ACSR/AAAC
- IS 398
- Ampacity
- Skin effect
Standards & references
Frequently asked questions
When should I use a bus duct instead of cables?
Bus ducts are preferred when: (1) current exceeds ~1,600 A (too many parallel cable runs become impractical); (2) space is limited and a compact run is needed; (3) the route passes through fire compartments (bus ducts have better fire-stopping properties than cable bundles); (4) flexibility for future tap-off connections is needed. Bus ducts cost 3–5× more than equivalent cabling but are much easier to maintain and expand.
What is the difference between copper and aluminium busbars for solar plants?
Copper busbars carry 60% more current per mm² than aluminium and have better corrosion resistance — preferred for indoor switchgear and BESS connections. Aluminium busbars are lighter, cheaper (aluminium is ~30% the cost of copper per kg), and are standard for large outdoor HT bus in substations. For solar LT switchgear (inverter rooms, ACDB, DCDB), copper is the standard choice. For large distribution busbars in main LT panels above 2,000 A, aluminium bus ducts are increasingly used.
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