Long 240 V Feeder to a Detached Workshop: Ampacity vs. Voltage Drop, and the EGC Consequence
A 240 V single-phase feeder is being installed from a 200 A residential service to a detached workshop on a rural lot. The calculated load is 80 A, and the owner wants a 100 A feeder breaker so the shop can be expanded later. The one-way run is 400 ft of copper THWN-2 in Schedule 40 PVC, buried at 24 in. Terminations are rated 75°C and the ambient is 30°C. The owner's stated goal is to keep voltage drop at or below 3% at the calculated load, and the inspector has already said the equipment grounding conductor will be checked carefully against Article 250. The installer proposes 3 AWG copper ungrounded conductors, reasoning that 3 AWG carries 100 A at 75°C and therefore matches the breaker.
1. Determine the minimum ungrounded conductor size that satisfies both ampacity and the owner's 3% voltage-drop limit at 80 A. Show the calculation and state the conductor you would install.
Read the worked answer
Ampacity check (Table 310.16, 75°C column): 3 AWG Cu = 100 A, which equals the 100 A breaker, so 3 AWG is the smallest conductor that satisfies the load and the overcurrent device.
Voltage-drop check: the design limit is 3% of 240 V = 7.2 V. For a single-phase circuit, VD = 2 x L x I x R / 1000, so the maximum permitted resistance is:
R = (7.2 V x 1000) / (2 x 400 ft x 80 A) = 0.1125 ohm/kft
Chapter 9, Table 8 (uncoated copper): 1/0 = 0.122 ohm/kft (VD = 7.81 V, 3.25% - fails), 2/0 = 0.0967 ohm/kft (VD = 6.19 V, 2.58% - passes).
Answer: 2/0 Cu. Ampacity alone would have allowed 3 AWG, but the owner's 3% criterion governs. Two practical follow-ups: 2/0 Cu may not fit the lugs of a 100 A breaker, so verify termination capacity (a larger enclosure or a listed reducing splice may be needed), and note that the 3% figure is a design/contract requirement, not a mandatory Code rule (210.19(A) Informational Note suggests 3% feeder plus 2% branch for a 5% total).
2. What size equipment grounding conductor is required, and how does upsizing the ungrounded conductors for voltage drop affect it?
Read the worked answer
Start with Table 250.122: for a 100 A overcurrent device, the minimum copper EGC is 8 AWG (16,510 cmil).
Because the ungrounded conductors were increased in size above the ampacity minimum, 250.122(B) requires the EGC to be increased proportionally, using circular mil area:
- 3 AWG Cu (ampacity minimum) = 52,620 cmil
- 2/0 Cu (installed) = 133,100 cmil
- Ratio = 133,100 / 52,620 = 2.53
- Required EGC = 16,510 cmil x 2.53 = 41,764 cmil
4 AWG Cu is 41,740 cmil - about 24 cmil short of the computed value - so the defensible answer is 3 AWG Cu (52,620 cmil). The teaching point is that the EGC cannot stay at 8 AWG just because the breaker did not change; the proportion rule follows the conductor, not the breaker. The EGC must be run in the same PVC raceway with the circuit conductors, be a continuous conductor, and be sized on the circuit overcurrent device for the Table 250.122 starting point.
3. The owner later asks whether the 3% limit applies to the whole circuit from the service to the shop panel and then out to a 240 V welder receptacle. How should the electrician answer?
Read the worked answer
The 3% figure was written for the feeder - service panel to shop panel. Everything downstream of that panel is a branch circuit, and the conventional design guidance (210.19(A) Informational Note, mirrored for feeders by 215.2(A) Informational Note) is 3% on the feeder plus 2% on the branch circuit, for a total of 5% from the service point to the outlet.
So yes, the concern extends to the whole path, but the budget is split. If the welder branch circuit is long, its own drop should be checked separately at its own load. Because the welder is a 240 V load on a short branch from the shop panel, the feeder has already consumed most of the allowance, so keeping the receptacle close to the panel matters more than it would appear.
Also confirm that none of this is a pass/fail Code item: the inspector enforces ampacity, overcurrent protection, and EGC sizing. The 3% number is enforceable only because the owner put it in the scope - so document the design basis and the calculation in the job file, because a future load addition will reopen the question.