Voltage Drop on a Long Barn Feeder
A farm owner wants a 100 A subpanel in a barn 180 m from the main service. The load is mostly motors: a 5 kW irrigation pump, a 2 kW feed auger, and lighting. The electrician proposes 35 mm² copper conductors in PVC conduit buried at 900 mm. The calculated load is 70 A. The owner reports that the existing smaller feeder to a shed flickers lights when the pump starts. The soil is rocky and the trench crosses a driveway. The electrician wants to use the same 35 mm² conductor and rely on the 100 A breaker for protection. Voltage at the service is 240 V single-phase.
1. Assess whether the proposed conductor size is adequate, considering voltage drop and motor starting.
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The 35 mm² copper conductor may satisfy ampacity for 70 A, but voltage drop over 180 m is the controlling issue. For a single-phase 240 V circuit, voltage drop is proportional to current, length, and conductor resistance. At 70 A over 180 m, even with 35 mm² copper, the drop can be several percent, and motor starting current can be five to six times running current, producing a momentary drop large enough to make lights flicker and contactors chatter.
The owner's report of flickering at the shed is consistent with excessive voltage drop during motor starting. I would calculate the running voltage drop and the starting voltage drop, and compare against the recommended limits, typically 3 percent for a feeder and 5 percent total. If the running drop exceeds the limit, the conductor must be upsized.
Relying on the 100 A breaker does not address voltage drop, because the breaker protects the conductor from overcurrent, not from poor performance. I would require a voltage drop calculation and likely a larger conductor, and I would confirm motor starting methods such as reduced-voltage starting if upsizing is impractical.
2. What installation concerns arise from the trench route and soil, and how do you handle them?
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The rocky soil and driveway crossing raise mechanical protection and burial depth concerns. PVC conduit at 900 mm may be acceptable for the general run, but the driveway crossing may require greater depth or concrete encasement depending on local rules, and rocky backfill can damage PVC. I would require inspection of the trench before backfill, verification of depth at the crossing, and protection such as a concrete cap or rigid conduit sleeve under the driveway.
I would also require the conduit to be bedded and backfilled without sharp rocks against it, and I would verify that expansion and movement at the barn are addressed. The grounding at the barn subpanel must be separate from the neutral, with a grounding electrode at the barn, because this is a separate structure.
My decision is to hold approval until the trench is inspected, the crossing protection is confirmed, and the voltage drop calculation supports the conductor size. I would document depths and photographs before backfill.