Voltage Drop on a Long Barn Feeder
A rural client wants a 100 A subpanel in a detached barn 260 ft from the house service. The load is a 5 hp, 230 V single-phase irrigation pump motor plus lighting and a welder outlet. The existing house service is 200 A, 120/240 V single-phase. The installer proposes 2 AWG aluminum direct-burial feeder in PVC conduit for the entire run, reasoning that 2 AWG aluminum is listed for 100 A at 75 degrees C and the run is short enough. The pump starts under load and the client reports dimming lights in the house when it kicks on. The soil is rocky and the trench crosses a driveway.
1. Evaluate the proposed feeder against ampacity, voltage drop, and starting-current concerns, and recommend a design.
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Ampacity alone is not the design basis. A 5 hp, 230 V motor draws roughly 28 A at full load, and starting current can be five to six times that, so the feeder must carry a large transient while keeping voltage drop acceptable. On a 260 ft run, 2 AWG aluminum at 100 A will show significant drop; the motor start will compound it. The dimming in the house confirms the drop is already visible. The better design is to increase the feeder size, for example to 1/0 or larger aluminum, and to compute drop at the motor starting current rather than only at the steady load. A separate disconnecting means and motor branch protection at the barn are also required. The installer should calculate drop with the actual one-way length, conductor resistance, and load current, and show the result to the client before trenching, because upsizing later means re-excavating the rocky trench and the driveway crossing.
2. The installer says the dimming is caused by the utility, not the feeder. What measurements would distinguish the two causes, and how should the driveway crossing affect the installation method?
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Measure voltage at the house service and at the barn panel simultaneously while the pump starts. If the barn voltage sags far more than the service voltage, the feeder is the cause; if both sag equally, the utility supply or the service itself is implicated. A recording meter left for several start cycles gives better data than a single reading. Also measure running current to confirm the motor is not drawing above nameplate. For the driveway crossing, direct burial at insufficient cover is a hazard; the crossing should use rigid metal or Schedule 80 PVC sleeving, or be bored deeper, and the conductor should be protected from rock damage with sand or a listed bedding. The crossing method is part of the design, not an afterthought.