Voltage Drop on a Long Barn Feed
A farmer wants a 40 A single-phase submain run 95 m from the origin to a barn distribution board. The load is a 9 kW grain drier plus lighting and sockets, diversity applied gives a design current of 34 A. The electrician proposes 16 mm² copper PVC-insulated singles in buried conduit. Measured loop impedance at the barn board on the existing supply is 0.62 Ω with a 63 A BS 88 fuse at origin. The farmer is pressing to reuse a drum of 10 mm² cable left over from another job to save money, arguing the drier only runs at harvest. The electrician is unsure whether the 10 mm² is acceptable and whether the fuse will disconnect on a fault at the far end.
1. Assess the voltage drop for both cable sizes and state which is compliant. Show your method.
Read the worked answer
Voltage drop is calculated as mV/A/m × design current × length ÷ 1000. Using typical tabulated values for two-core copper PVC singles (verify against the actual IS 10101 tables before committing):
16 mm² ≈ 2.9 mV/A/m VD = 2.9 × 34 × 95 ÷ 1000 ≈ 9.4 V → 4.1% of 230 V
10 mm² ≈ 4.4 mV/A/m VD = 4.4 × 34 × 95 ÷ 1000 ≈ 14.2 V → 6.2% of 230 V
The conventional lighting limit is 3% and the general power limit 6% from origin. The 16 mm² sits comfortably within 6% for power and leaves headroom; the 10 mm² is marginal to non-compliant once the drier starting current and any future load are considered, and would certainly breach the 3% lighting limit on the same board.
Conclusion: 16 mm² is the defensible choice. The 'it only runs at harvest' argument is irrelevant - voltage drop is a fixed property of the cable, load current and length, not of duty cycle.
2. The electrician must decide whether the 63 A BS 88 fuse will disconnect a fault at the barn board. Explain the check and the outcome.
Read the worked answer
This is an earth fault loop impedance check against the disconnection time requirement.
Measured Zs at the barn board = 0.62 Ω. For a 63 A BS 88 fuse on a TN system, the maximum permitted Zs for the relevant disconnection time is typically around 0.7–0.9 Ω depending on the time/current characteristic used (confirm from the fuse table in IS 10101). On the face of it, 0.62 Ω is within the limit - but with little margin.
Cautions the electrician must apply:
- The measured value must be corrected for conductor operating temperature (apply the rule-of-thumb factor of 0.8 to the measured value when comparing, or use the tabulated maximum directly against the measured cold value as the standard requires - be consistent).
- A 0.62 Ω reading at the board does not guarantee adequate Zs at the furthest point on a final circuit fed from it; each final circuit needs its own check.
- If the corrected value exceeds the maximum, the fuse is not acceptable and an RCD or a different protective device is required.
Given the thin margin, the electrician should not sign off on the fuse alone without confirming the corrected value and the furthest-point readings.