Fill % = (Total Wire Area / Conduit Area) × 100 Maximum fill: 40% (NEC) for 3+ wires
25mm conduit (314mm²), 4×2.5mm² wires: Wire area ≈ 15.9mm² each | Total = 63.6mm² Fill = 63.6/314 = 20.3% ✅
Frequently Asked Questions
NEC (US) and most standards allow 40% fill for 3+ conductors, 31% for 2 conductors and 53% for 1 conductor. Keeping fill below 40% allows heat dissipation and easier cable pulling.
Overfilled conduits are harder to pull wires through, cause cable damage during installation, and reduce heat dissipation leading to cable overheating and reduced current capacity.