Field guide: installing a subpanel, hot weather considerations (edition 5)

Sizing the feeder before you cut anything

Subpanel feeder sizing starts with the calculated load per NEC 220, not with whatever breaker the homeowner heard was "plenty." Run the numbers, then pick the conductor. A 100A subpanel feeding a detached garage with a mini split, EV charger, and lighting can hit 80A continuous fast, and 75C terminations on most residential gear cap your ampacity at the 75C column of NEC 310.16.

For dwelling feeders, 310.12 still lets you use the reduced conductor table when the feeder carries the entire load of the dwelling, but a subpanel almost never qualifies. Size to 310.16 unless you can prove the exception applies. Voltage drop on long runs to detached structures is the other killer. Keep it under 3% on the feeder, 5% total, per the informational note in 210.19.

Pulled #2 aluminum 180 feet to a shop last spring. Math said fine at 90A load. Customer ran the welder and saw 108V at the receptacle. Bumped it to 1/0 and the problem walked away.

Grounding and bonding at the subpanel

This is where most failed inspections happen. At a subpanel, the neutral and ground must be separated. Remove the bonding screw or strap, land neutrals on the insulated neutral bar, and grounds on a separate equipment grounding bar bonded to the enclosure. NEC 250.24(A)(5) prohibits a neutral-to-case bond on the load side of the service disconnect.

For a detached structure, you need a grounding electrode system at the subpanel per NEC 250.32. That means a ground rod (or two if you can't prove 25 ohms with one, per 250.53(A)(2)), or a Ufer if available. The equipment grounding conductor still runs with the feeder. The 2008 code killed the old "three-wire feeder to detached building" allowance, so don't repeat what your foreman did in 1995.

• Four-wire feeder: two hots, neutral, equipment ground
• Neutral isolated from enclosure at subpanel
• Grounding electrode at the structure, bonded to the EGC and neutral bar's ground bar
• Bonding screw removed and bagged inside the panel for the inspector

Hot weather derating, the part everyone skips

Summer attic and exterior wall installs get punished by NEC 310.15(B). Ambient correction starts mattering around 86F (30C), and an attic in Phoenix or a south-facing wall in Dallas hits 130F+ on a July afternoon. At 51-55C ambient, your 75C THWN-2 ampacity drops to 0.41 of the table value. That #6 copper rated 65A at 75C becomes 26A.

Add conduit-on-rooftop adders from 310.15(B)(2) if the run is exposed to sunlight on or above a roof, though the 2017 cycle removed the temperature adder table and now relies on the ambient correction directly. Combine ambient correction with conductor bundling per 310.15(C)(1) when you have more than three current-carrying conductors in a raceway, and the numbers compound.

Ran a 60A subpanel feed across a hot attic in August, used #6 because that's what the spec said. Breaker tripped under load by week two. Re-pulled in #4 and it has run clean for three years.

Working the panel in the heat

Hot weather is a safety issue, not a comfort issue. Sweat on your hands, fogged safety glasses, and a 140F attic make stupid mistakes likely. Plan the cuts, drilling, and conduit assembly for early morning. Save the panel terminations for shaded or conditioned space whenever you can stage it that way.

Hydrate before you climb, not after you feel it. Heat exhaustion sneaks up on guys in their 40s and 50s who think they can still push through it like they did at 25. If you're working a live panel, OSHA 1910.269 and NFPA 70E still apply regardless of how miserable it is, so the PPE stays on.

1. Start exterior and attic work before 9 AM
2. Pre-bend and pre-cut conduit on the ground in the shade
3. Torque terminations to spec, NEC 110.14(D), even when you're tired
4. Re-check torque after the panel cools if you terminated hot

Breaker selection and panel labeling

Match the breaker to the panel listing. Classified breakers (UL listed for use in a competitor's panel) are legal but the AHJ may push back, and the panel manufacturer's warranty usually walks. NEC 110.3(B) requires you follow the listing instructions, which means same-brand breakers in most cases unless the panel label explicitly lists the classified option.

Label every circuit per NEC 408.4(A) with a description specific enough that someone else can find it in an emergency. "Bedroom" is not enough if there are three bedrooms. Include the panel's source on the cover per 408.4(B), so the next electrician knows where the feeder originates.

Final checks before you close it up

Megger the feeder before energizing if the run is over 100 feet or pulled through wet conditions. Phase rotation does not matter on single-phase residential, but on a 208/120 three-phase subpanel feeding motors, verify rotation with a phase tester before you fire up equipment.

Walk the panel one more time. Bonding screw out, neutrals and grounds separated, torque marks on every lug, dead front secured, directory filled in, working clearance per NEC 110.26 maintained. The inspector is looking for the same five things every time, and so should you.