Voltage Drop on a Long Barn Feeder
A farm owner wants a 100 A subpanel in a barn 350 ft from the main service. The load is a 5 HP 240 V single-phase motor (nameplate 28 A, 240 V) plus lighting and receptacles totaling 30 A at 120 V. The owner proposes 1/0 copper THHN in PVC conduit buried 24 in. deep. The utility voltage at the main is 240 V. The owner complains that a similar installation at a neighbor's farm causes lights to dim when the motor starts. The electrician must decide conductor size, considering both ampacity and voltage drop, and must also address the equipment grounding conductor and burial depth.
1. Calculate the voltage drop for 1/0 copper at 100 A over 350 ft and determine whether it is acceptable.
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Voltage drop for single-phase: VD = 2 × K × I × L / CM, where K = 12.9 for copper, I = 100 A, L = 350 ft, CM for 1/0 copper = 105,600.
VD = 2 × 12.9 × 100 × 350 / 105,600 = 903,000 / 105,600 = 8.55 V.
Percent drop = 8.55 / 240 = 3.56%. NEC 210.19(A) Informational Note recommends no more than 3% on a branch circuit and 5% total on feeder plus branch circuit. At 3.56% on the feeder alone, the total will exceed 5% once the branch circuit is added, and motor starting will cause a much larger momentary dip. The 1/0 is acceptable for ampacity (1/0 copper THHN at 75°C = 150 A, well above 100 A), but marginal for voltage drop. The electrician should recommend upsizing to 2/0 or 3/0 copper, or reducing the run, to keep the feeder drop at or below 3%. The judgement call is that ampacity compliance alone does not satisfy the design requirement for motor starting and lighting flicker.
2. What equipment grounding conductor is required, and what burial depth applies to the PVC conduit?
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For a 100 A feeder, the equipment grounding conductor is sized per NEC Table 250.122. A 100 A overcurrent device requires an 8 AWG copper equipment grounding conductor. If the conductors are upsized for voltage drop, 250.122(B) requires the EGC to be proportionally upsized. For example, if the ungrounded conductors go from 1/0 (105,600 CM) to 3/0 (167,800 CM), the EGC must be increased by the same ratio: 8 AWG (16,510 CM) × (167,800/105,600) = 26,230 CM, which rounds up to 6 AWG copper (26,240 CM). This is a commonly missed requirement.
Burial depth: PVC conduit requires 18 in. of cover for a 100 A feeder under NEC Table 300.5, not 24 in. The owner's proposed 24 in. exceeds the minimum and is acceptable, but the electrician should not assume 24 in. is required. If the conduit is under a driveway, depth increases to 24 in. The judgement call is applying 250.122(B) when conductors are upsized for voltage drop - a step many installers skip.
3. The motor is a 5 HP, 240 V single-phase unit with a nameplate 28 A. What branch-circuit conductors and overload protection are required, and how does the motor's starting current affect the feeder design?
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Motor branch-circuit conductors are sized at 125% of the motor full-load current from NEC Table 430.248, not the nameplate. For a 5 HP 240 V single-phase motor, Table 430.248 gives 28 A FLC. Conductors = 28 × 1.25 = 35 A, so 8 AWG copper at 75°C (50 A) is adequate. The branch-circuit short-circuit and ground-fault protective device is sized per Table 430.52: for a single-phase motor, 250% of FLC = 70 A, so a 70 A breaker or time-delay fuse is permitted; if it trips on starting, 430.52 allows the next higher size.
Overload protection is separate: it is sized to the motor's service factor and nameplate, typically 115–125% of nameplate current, and is provided by the motor's overload relay or a listed overload device. The starting current (locked-rotor, roughly 6× FLC) is what causes the neighbor's lights to dim; the feeder must be sized so starting voltage dip is tolerable. That is why voltage drop, not just ampacity, governs the barn feeder. The judgement call is separating overload protection (motor) from short-circuit protection (branch breaker) and recognizing that the feeder sees starting current.