260 ft Feeder to a Detached Shop: Ampacity Versus Voltage Drop

Ampacity check (310.16)

• 2 AWG aluminum at 75 C = 90 A
• A 100 A feeder requires conductors with at least 100 A ampacity. 240.4(B) does not rescue this: 90 A is itself a standard rating in 240.6, so the next-size-up rule does not apply. The 100 A breaker would be protecting a 90 A conductor.
• Minimum acceptable aluminum for a 100 A feeder at 75 C is 1 AWG (100 A).

Voltage drop check

Using 2-wire drop: VD = 2 x L x I x R / 1000, with R for uncoated aluminum in ohms per 1000 ft.

• 2 AWG Al, R = 0.319 ohm/kft: VD = 2 x 260 x 78 x 0.319 / 1000 = 12.9 V, or 5.4% of 240 V
• 1/0 Al, R = 0.200 ohm/kft: VD = 2 x 260 x 78 x 0.200 / 1000 = 8.1 V, or 3.4%
• 2/0 Al, R = 0.159 ohm/kft: VD = 2 x 260 x 78 x 0.159 / 1000 = 6.4 V, or 2.7%

The 2 AWG fails on ampacity outright. It also fails the 3% feeder recommendation by nearly a factor of two, which is what the saw motor and any future air compressor will feel as hard starting and brownout on a long run. The practical answer is 2/0 aluminum for the ungrounded conductors, with a 100 A breaker protecting it.

Equipment grounding conductor

• 250.122 sets the EGC for a 100 A overcurrent device at 8 AWG copper or 6 AWG aluminum. The 4 AWG aluminum in the purchased cable is larger than required and is fine.
• The EGC must be run with the feeder in the same trench/cable assembly as the circuit conductors.

Grounding at the separate structure - 250.32

• A grounding electrode system is required at the shop. With no metal water pipe and no building steel, that means a ground rod per 250.52(A)(4) (typically two rods unless a single rod proves 25 ohms or less, per 250.53(A)(2)).
• The feeder must be a 4-wire feeder: two ungrounded conductors, one grounded (neutral) conductor, and one equipment grounding conductor.
• At the shop panel, the neutral terminates on an isolated neutral bar and the EGC terminates on the grounding bar; the neutral is not bonded to the panel enclosure or to the ground rod. The main bonding jumper exists only at the service.

Why separation matters

If the neutral is bonded at the shop, the EGC path between buildings becomes a parallel return path for neutral current. That current will flow on the shop panel enclosure, the grounding electrodes, and any metal connected to them, creating a shock hazard and a potential difference between the two buildings' grounded surfaces. Because there are no other metallic paths here, the NEC requires the EGC regardless - the grounded-conductor exception in 250.32(B) does not apply to new installations.

What the Code actually enforces

Voltage drop limits appear as informational notes (210.19(A), 215.2(A)(1)), not as mandatory rules. An inspector cannot reject a feeder for exceeding 3% if the conductors are adequate for ampacity. But here the 2 AWG fails the mandatory rule: 90 A ampacity does not permit a 100 A breaker. So the rejection happens on 240.4/310.16, not on voltage drop.

Options to present

1. Keep the 100 A panel but install a 90 A feeder breaker. This is legal: 2 AWG Al at 90 A, 4 AWG Al EGC is still more than the 6 AWG required, and the shop's 78 A calculated load fits. The tradeoff is real: 5.4% drop at full load, and worse during motor starting, which is why the saw will run hot and any future welder will be a problem.
2. Pull the 2 AWG out and install 2/0 Al. Highest first cost, lowest lifetime cost, and the only option that leaves room for the shop to grow.
3. Reduce the actual load - a smaller heater, or a 240 V heat pump instead of resistance heat - which drops current and therefore drop, and may let the 2 AWG work at 90 A comfortably.
4. Do not backfill today. The trench is the cheap part of the job. Backfilling over an undersized feeder converts a wire problem into an excavation problem, and the customer will blame the electrician who backfilled it.

Document the measured drop and the recommendation in writing, and let the owner choose among 1, 2 and 3 - but not among 1 and doing nothing.