Detached Garage Feeder: Ampacity, Burial Depth, and Protection
A 100 A feeder leaves a 200 A dwelling panel and runs to a detached garage. Route: 130 ft of 1-1/2 in PVC Schedule 40 buried 12 in deep across the lawn, 40 ft of EMT through an attic that reaches 130 F in summer, then a riser down the inside of the garage wall with no expansion fitting. Conductors are three 1/0 Al XHHW-2 (two ungrounded plus neutral) and one 6 AWG Al XHHW-2 equipment grounding conductor; all terminations are rated 75 C. Garage loads are 120/240 V: a 50 A welder receptacle, a 30 A compressor, three 125 V general-purpose receptacles, and lighting. The electrician plans a 2-pole 100 A breaker at the house and a main-lug panel in the garage, and intends to GFCI-protect only the receptacle beside the overhead door, saying the 240 V outlets never need it.
1. How many current-carrying conductors are in this raceway, and is the 1/0 Al adequate for the attic portion and the terminations?
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Conductor count and ampacity check
- Current-carrying conductors = 2. The neutral of a 120/240 V single-phase, 3-wire feeder carries only unbalanced current and is not counted under 310.15(E)(1). No bundled-conductor adjustment applies (two conductors, no derating table entry).
- Ambient correction for the attic. 130 F is about 54 C. Using the next-higher row of the ambient correction table (55 C) is conservative: factor 0.76.
- Adjusted ampacity. 1/0 Al XHHW-2 is a 90 C conductor, so correction starts from the 90 C column: 135 A x 0.76 = 102.6 A. The 75 C termination limit for 1/0 Al is 120 A, and 102.6 A does not exceed it, so the final feeder ampacity is 102.6 A.
- Result: 102.6 A is at least the 100 A breaker rating, so the installation is code-compliant - but with about 2% margin. There is no permission to upsize only the attic section: ampacity is a property of the conductor, so the same conductor is limited over its full length.
Recommended judgment: because the attic is only 40 ft, an accessible splice box at the attic boundary with the attic portion upsized, or simply running 2/0 Al throughout (90 C ampacity 150 A x 0.76 = 114 A), gives room for the 3% voltage-drop recommendation. Voltage drop on the 180 ft run is 2 x 21.2 x 100 x 180 / 105,600 = about 7.2 V, or 3.0% at 240 V - acceptable but at the advisory limit.
2. Address the 12 in burial depth and the missing expansion fitting. What protection is required where the conduit emerges from grade?
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Burial depth
- Table 300.5 requires 18 in of cover for PVC conduit in earth that is not under a building, street, driveway, or roadway (24 in where it is). Depth is measured from the top of the raceway to the finished grade, so 12 in is a rejection.
- This is a re-dig, not a cover-with-dirt-and-hope item. If the run crosses any driveway, plan on 24 in for that segment.
Physical protection at grade
- 300.5(D)(1): the raceway emerging from the ground must be protected from physical damage up to at least 18 in above grade - typically a Schedule 80 PVC or RTRC sleeve over the riser.
- Schedule 40 PVC is not permitted where it is subject to physical damage, so the sleeve is not optional at a garage wall where ladders, mowers, and doors are present.
Expansion fitting
- 352.44 requires an expansion fitting where a PVC raceway is exposed to ambient temperature change that would cause length change of 1/4 in or more per 100 ft. An outdoor riser on a garage wall routinely exceeds that between winter and summer; install the expansion fitting at the transition from direct-buried to exposed, and set it at the mid-range installation temperature with the piston positioned per the manufacturer's instructions (110.3(B)).
- Also confirm the expansion fitting is listed and that the conduit is supported per 352.30 at the riser; an unsupported, unanchored riser is the usual companion finding.
3. Which garage branch circuits require GFCI protection, and how should the welder branch circuit be approached?
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GFCI scope
- 210.8(A)(2) covers all 125 V, 15 and 20 A receptacles in a garage - so all three general-purpose receptacles need GFCI protection, not just the one by the overhead door. The device or breaker must be readily accessible.
- The 240 V welder and compressor receptacles are outside 210.8(A), which is limited to 125 V, 15/20 A receptacles. GFCI is not required for them (and a GFCI on a welder is a known nuisance-trip source).
- AFCI is not required for garage branch circuits; 210.12 lists the rooms and occupancies that require it, and a garage is not one of them. Do not add AFCI "for consistency" without a local amendment.
Welder branch circuit - do not just match the receptacle
A 50 A receptacle does not mean a 50 A breaker and 50 A conductors. Article 630 bases the branch-circuit conductor ampacity and overcurrent protection on the welder's nameplate primary current and duty cycle (multiplication factors in 630.11 for arc welders, and the OCPD sizing rules in 630.12). Get the welder nameplate first; a 50 A circuit may be oversized for a light-duty unit and undersized for a production machine.
Also verify: the garage panel is a main-lug type fed by a 100 A breaker at the house - that is acceptable, but the feeder's 6 AWG Al equipment grounding conductor (good for a 100 A device per Table 250.122) and the separate-building grounding and neutral-isolation rules must be confirmed before energizing.