Voltage Drop on a Long Farm Supply
A farm building in St. Lucy is fed from a meter position at the road by a 90 m run of 25 mm² two-core armoured copper cable, buried, to a distribution board supplying a 7.5 kW irrigation pump (continuous), lighting, and a workshop socket circuit. The supply is single-phase 230 V. The measured voltage at the meter is 228 V; at the farm board with the pump running it falls to 205 V. The pump motor overheats and trips on its thermal overload after about twenty minutes. The electrician proposes increasing the pump motor's overload setting.
1. Calculate the expected voltage drop on the 90 m run and compare it with the measured drop.
Read the worked answer
The pump current at 230 V is approximately 7500 / 230 ≈ 32.6 A, assuming unity power factor and ignoring motor efficiency; with a realistic power factor and efficiency the running current may be somewhat higher, but 32.6 A is a reasonable starting figure.
For a two-core cable, the voltage drop is approximately mV/A/m × current × length / 1000. For 25 mm² copper, the tabulated mV/A/m value is around 1.75 for two-core cable at operating temperature. So the drop is about 1.75 × 32.6 × 90 / 1000 ≈ 5.1 V - only about 2.2% of 230 V.
The measured drop is 228 − 205 = 23 V, roughly 10%. That is far larger than the cable calculation predicts. The discrepancy means the problem is not primarily the 90 m run: it lies elsewhere - a high-resistance joint or termination, a corroded or undersized connection at the meter position, a poor neutral, or a supply-side issue. The measured value must be trusted over the calculation, and the installation must be investigated with the pump off and on, measuring at each accessible point along the run to localise the drop.
2. Explain why raising the overload setting is dangerous and what the correct remedy is.
Read the worked answer
The motor is overheating because it is being operated at reduced voltage. A motor running at 205 V instead of 230 V draws more current for the same mechanical output, so the thermal overload is doing exactly its job by tripping. Raising the setting would allow the motor to run hotter, accelerating insulation degradation and risking a winding failure or fire. It would also leave the motor unprotected against genuine overload once the underlying fault is corrected.
The correct remedy is to find and eliminate the excessive voltage drop. Steps:
- Measure voltage at the meter, at the farm board, and at the pump starter with the pump running, to localise the drop.
- Inspect and remake any joint or termination showing heat damage, corrosion, or high resistance; measure resistance across joints where accessible.
- If the drop is genuinely in the supply cable, recalculate for the actual load and replace with a larger conductor or provide a locally transformed supply.
- Confirm the voltage at the pump terminals is within the motor manufacturer's tolerance, typically ±5% to ±10% of rated voltage.
- Only after the voltage is corrected should the overload setting be verified against the motor nameplate full-load current.
Finally, check that the motor's overload device is correctly rated for the motor and that the circuit's protective device still provides fault protection.