60 A Feeder to a Detached Workshop
A homeowner wants a 60 A feeder from the 200 A main panel in the house to a new detached workshop 180 ft away. The proposed route is Schedule 80 PVC buried 18 in. deep, with 2 AWG aluminum XHHW-2 phase conductors and a 6 AWG aluminum equipment grounding conductor. Shop loads are a 5 HP 240 V compressor, a 30 A welder receptacle, lighting, and general-use receptacles. The shop slab contains 20 ft of bare 4 AWG copper tied to the rebar in the footing, a 1/2 in. copper water line is stubbed up for a future sink, and the building has a bolted steel frame. The owner wants to omit the equipment grounding conductor 'because the neutral goes back to the house anyway.'
1. Verify the voltage drop for the proposed conductors at the 60 A feeder rating, and determine whether 4 AWG aluminum could be substituted.
Read the worked answer
Ampacity check first: 2 AWG Al XHHW-2 at 75 °C is 90 A, well above the 60 A feeder rating. Ampacity is not the limiting factor here; voltage drop is.
Voltage drop, single phase: VD = 2 × K × I × L ÷ CM, with K ≈ 21.2 for aluminum.
2 AWG Al (CM = 66,360): VD = (2 × 21.2 × 60 × 180) ÷ 66,360 = 457,920 ÷ 66,360 = 6.9 V, or 6.9 ÷ 240 = 2.9%
4 AWG Al (CM = 41,740): VD = 457,920 ÷ 41,740 = 11.0 V, or 4.6%
Conclusion: 2 AWG aluminum lands just under the 3% feeder target; 4 AWG aluminum is well over it and should be rejected. Note that the 3% figure comes from an informational note in 210.19(A) and is not an enforceable rule - it is a design and contract issue. On a 180 ft run feeding a compressor, the practical concern is starting: a 6.9 V drop at 60 A becomes far larger during the 5 HP motor's inrush, causing long starts, overload trips, and contactor chatter. If the shop load may grow, upsize to 1/0 aluminum now rather than trenching twice.
Also confirm the 18 in. burial depth for Schedule 80 PVC against Table 300.5, and provide protection where the conduit emerges from grade per 300.5(D)(1).
2. Evaluate the owner's proposal to omit the equipment grounding conductor and use the neutral as the ground-fault return path.
Read the worked answer
The proposal is a serious code violation and a shock hazard. Reject it.
- 250.32(B)(1): A feeder supplying a separate building or structure must include an equipment grounding conductor run with the feeder conductors. Using the grounded conductor for grounding is not permitted.
- 250.32(A): The shop must have its own grounding electrode system, connected to the equipment grounding conductor of the feeder.
- 250.24 / 250.32: The neutral must be isolated from the enclosure and from the grounding electrode at the shop - no bonding jumper, no neutral landed on the can, and the neutral bar must not be connected to the enclosure.
- Why it matters: If the neutral is also the ground-fault path, every fault return current flows on it, and a loose or open neutral energizes the panel enclosure and the steel frame at line potential. It also creates a parallel return path through the earth and the water line.
- EGC sizing: A 60 A feeder requires only a 10 AWG copper / 8 AWG aluminum EGC per Table 250.122. The proposed 6 AWG aluminum is larger than required, which is fine - oversizing an EGC is permitted.
Bonding at the shop: Bond the interior metal water piping (250.104(A), jumper sized per Table 250.102(C)(1)) and the structural steel frame (250.104(C)) to the equipment grounding conductor and grounding electrode system.
3. The shop footing contains 20 ft of bare 4 AWG copper tied to the rebar. Must it be used as part of the grounding electrode system, and how does it interact with the other electrodes present?
Read the worked answer
Yes - it must be used.
- 250.52(A)(3): A concrete-encased electrode is at least 20 ft of 1/2 in. or larger reinforcing steel, or at least 20 ft of bare 4 AWG copper conductor, encased in at least 2 in. of concrete near the bottom of a footing. Twenty feet of bare 4 AWG copper in the footing qualifies.
- 250.50: All grounding electrodes described in 250.52(A)(1) through (A)(7) that are present at each building or structure must be bonded together to form the grounding electrode system. It is not optional, and you cannot choose a rod instead because it is easier.
- Interaction with other electrodes: The metal water line, if it is in direct contact with the earth for 10 ft or more, is itself a grounding electrode under 250.52(A)(1) and must be bonded in. The building steel frame is bonded under 250.104(C) whether or not it qualifies as a grounding electrode. All of them are tied together with bonding jumpers sized per 250.66 (electrode bonding jumpers) or 250.102(C)(1) as applicable.
- Practical point: If the slab was poured without a CEE and only the rebar is present without a copper conductor, the rebar itself may qualify. Confirm with the AHJ before assuming the footing is unusable - and never cut the copper stub out of the footing to 'simplify' the job.