Voltage Drop on a Long Barn Feeder
A farm owner wants to feed a 240 V single-phase subpanel in a detached barn from the service panel at the house. The run is 95 m of 3-conductor copper cable in a buried conduit, and the calculated load at the barn is 60 A continuous. The owner has already pulled 6 mm² copper conductors (about 10 AWG equivalent, resistance roughly 1.8 ohms per 1000 m per conductor). He argues that the breaker is 60 A and the cable is rated for it, so nothing else matters. You are asked to review the design before a permit is issued. Ambient temperature is 30 C, and the terminations are rated 75 C.
1. Calculate the approximate voltage drop at full load and determine whether the design is acceptable for an inspector's approval.
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Round-trip conductor length is 2 × 95 m = 190 m. Resistance of 6 mm² copper is about 1.8 ohms per 1000 m, so R = 1.8 × 0.190 = 0.342 ohms. Voltage drop = I × R = 60 × 0.342 ≈ 20.5 V. As a percentage of 240 V that is 20.5 / 240 ≈ 8.5%. That is well beyond the commonly applied 3% branch-circuit / 5% total guideline used for sensible design, and it means the barn panel will sit near 219 V at full load. The inspector should not simply pass the installation because the ampacity matches the breaker. Voltage drop is a design issue, not an overload issue, but a 60 A continuous load at 8.5% drop will cause poor motor starting, dimming, and possible nuisance tripping of equipment. The correct judgement is to require a larger conductor, such as 16 mm², which roughly cuts the drop to about 3%. Document the calculation and the basis for the correction in the plan-review notes.
2. The owner says voltage drop is only a recommendation, not a code rule. How should the inspector respond, and what is the defensible position?
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The inspector should acknowledge the technical point: many codes treat voltage drop as a fine-print note or advisory rather than an absolute mandatory limit for all feeders. That does not make the installation automatically acceptable. The inspector's authority rests on the approved plans and on the requirement that equipment operate safely and as intended. A feeder that delivers 219 V to a 240 V panel under normal load can cause motors to overheat and controls to malfunction, which is a genuine hazard. The defensible position is to state the calculated drop, cite the applicable advisory provision or the plan-review condition, and require correction before energization, or require the owner to submit an engineered justification. Keep the reasoning factual and put the numbers in writing. If the owner appeals, the record should show that the decision was based on measured calculation and safety, not personal preference.