Field guide: installing a subpanel, high-altitude considerations (edition 4)

Why altitude matters for subpanel work

Thinner air means less convective cooling and lower dielectric strength. Breakers, transformers, and enclosures rated at sea level run hotter and arc easier above 3,300 ft. NEC 110.20(B) and the 2023 110.21(A)(2) marking rules require equipment to be evaluated for the altitude where installed, and most listed gear is rated to 6,600 ft (2,000 m) without derating.

Above that, you start applying manufacturer correction factors. Square D, Eaton, Siemens, and GE all publish altitude derate tables. Dielectric correction usually kicks in around 3,300 ft; current correction starts at 6,600 ft for most molded case breakers. Get the cut sheet before you order the panel, not after rough-in.

If the AHJ is in Denver, Flagstaff, Reno, Albuquerque, or anywhere on the Front Range or in the Sierras, expect questions about derate calcs on any subpanel above 100A. Bring documentation.

Sizing the feeder and the panel

Start with the load calc per NEC Article 220. For dwelling units, the standard method (220.40 through 220.61) or the optional method (220.82) both work, but optional usually wins for all-electric homes with heat pumps. Add the subpanel's calculated load to the service total before you size the feeder.

Feeder ampacity comes from NEC 215.2 and Table 310.16. At altitude, ambient temps swing harder, so check the 30-day high for your install location and apply 310.15(B)(1) ambient correction. A garage subpanel in Leadville sees different ambients than one in a conditioned basement.

• Feeder OCPD per 215.3, sized to protect the conductor
• Neutral sized per 220.61, accounting for nonlinear loads
• EGC per 250.122, based on the upstream OCPD
• Grounding electrode conductor not required at the subpanel if a separate building, see 250.32

For a typical 100A subpanel feeder, #4 copper THHN at 75C terminations is common, but verify against your actual load and any altitude-related ambient correction.

Grounding and bonding at the subpanel

This is where most failed inspections happen. At a subpanel in the same structure, the neutral and ground must be isolated. Remove the main bonding jumper. The neutral bar floats; the ground bar bonds to the enclosure. NEC 250.24(A)(5) prohibits a neutral-to-ground connection on the load side of the service disconnect.

If the subpanel feeds a separate structure, NEC 250.32 applies. You need a grounding electrode system at the second building, and as of the 2008 NEC you must run a 4-wire feeder with isolated neutral and ground. The old 3-wire feeder rule for separate buildings is dead.

Field tip: pull the bonding screw out of the can and tape it to the inside of the door before you energize. Inspector sees it, you don't get a callback, and the next guy working the panel knows it was checked.

Enclosure selection and mounting at altitude

UV exposure is brutal above 7,000 ft. Standard gray powder coat fades and chalks within a few years. Spec a NEMA 3R with UV-stabilized finish for outdoor mounts, and verify the gasket material is silicone or EPDM, not standard nitrile. Nitrile cracks fast at high-altitude UV and freeze-thaw cycles.

Working clearance per NEC 110.26 still applies: 36 inches deep, 30 inches wide, 6.5 ft headroom. At altitude, also think about snow load and drift. A subpanel mounted on a north wall in Telluride needs to clear average snow depth plus drift, or you'll be digging it out every February.

For mountain cabins and outbuildings, mount the subpanel inside a heated space if possible. Breakers nuisance-trip in extreme cold, and manufacturer trip curves are calibrated at 40C ambient. A panel sitting at -10F behaves differently than the curve says.

Conductor terminations and torque

Cold weather and altitude don't change torque specs, but they do affect how techs work. NEC 110.14(D) requires terminations be torqued to manufacturer specs using a calibrated tool. This has been enforced since the 2017 cycle and inspectors are looking for it.

1. Calibrated torque screwdriver or wrench, not a click-and-pray
2. Verify the lug spec on the breaker, not just the panel label
3. Re-torque aluminum feeders after 24 to 48 hours of load
4. Mark each terminal with a paint pen after torquing

Aluminum SER feeders are common on subpanel runs. Use a listed antioxidant compound on aluminum, follow 110.14(B) for splices, and double-check that the breaker lugs are AL-CU rated. Most modern panels are, but resi load centers from the 90s and earlier sometimes aren't.

Inspection prep and documentation

Bring the load calc, the altitude derate documentation from the manufacturer, and the torque values you used. If you ran AFCI or GFCI breakers (210.12 and 210.8 as applicable), have the test buttons exercised before the inspector shows up.

Field tip: take a photo of the panel with the dead front off, breakers labeled, neutrals and grounds on separate bars, and the bonding screw bagged. Email it to yourself with the address. Saves you on the callback when somebody else's work fails next door and the inspector wants to verify yours.

Label the subpanel directory legibly per NEC 408.4(A). Inspectors flag handwritten chicken scratch. Print the directory or use a label maker. The name of the upstream panel and breaker goes on the inside of the door per 408.4(B), required since the 2011 cycle.