Field guide: installing a subpanel, hot weather considerations (edition 3)

Sizing the feeder before you cut anything

Subpanel feeder sizing starts with the calculated load per NEC 220, not the panel's bus rating. A 100A subpanel does not require a 100A feeder if the calculated load is 62A. Run the math, then pick the conductor.

For dwelling units, apply NEC 220.42 lighting demand factors and NEC 220.82 optional method where it fits. Detached structures fall under NEC 225, and a single feeder to a separate building means you are also dealing with NEC 225.30 (one feeder rule) and NEC 225.31 through 225.33 for disconnect location.

Conductor ampacity comes from NEC 310.16 at the 75C column for terminations on most modern breakers and lugs. Verify the lug rating stamped inside the panel. If you find 60C lugs on a small subpanel, you derate to that column, which catches people on 100A aluminum SER feeds in summer heat.

Grounding and bonding, the part everyone gets wrong

Same structure subpanel: four wires. Two hots, one neutral, one equipment grounding conductor. The neutral bar floats. The bonding screw or strap comes out. Ground bar bonds to the enclosure. This is NEC 250.24(A)(5) and 408.40.

Separate structure subpanel fed after 2008: still four wires, neutral isolated, EGC bonded to the enclosure, and a grounding electrode system at the second structure per NEC 250.32(A) and (B). The old three-wire feeder to a detached garage is no longer permitted for new installs.

• Neutral bar isolated from the can on every subpanel, no exceptions for same-structure work.
• EGC sized per NEC 250.122 based on the feeder OCPD, not the conductor.
• Grounding electrodes at separate structures bonded with a GEC sized per NEC 250.66.
• Metallic raceway between panels can serve as the EGC if it qualifies under NEC 250.118, but pull a wire EGC anyway. It costs nothing and saves callbacks.

Hot weather ampacity, what the table does not tell you

NEC 310.15(B) ambient temperature correction is the line item that bites you in July. The table assumes 30C ambient. An attic at 1pm in Phoenix runs 55C to 60C. A 100A feeder in 90C THHN with four current-carrying conductors and a 55C ambient is no longer a 100A feeder.

Run the correction: ampacity at 90C column, multiply by the 310.15(B)(1) ambient factor, then multiply by the 310.15(C)(1) adjustment factor if you have more than three CCCs in the raceway. Then check the result against the 75C terminal rating, and use the lower number. The terminal rating almost always wins.

If the feeder runs through an unconditioned attic in the Southwest, size the conductor for the attic, not the panel room. A 1/0 CU feeder that calcs fine at 30C can fall below 100A once you correct for a 50C attic.

Rooftop conduit on a hot roof is its own problem. NEC 310.15(B)(2) was rewritten in 2017, but the rooftop adder still matters: conduit on a roof in direct sun runs hotter than the air around it. If you can route through the attic instead of across the roof, do it.

Working a panel in the heat

Panel work in summer is a safety problem before it is a code problem. Service equipment and feeders carry real fault energy, and you are sweating into your gloves. Plan the de-energization, do not freelance it.

1. Lock out and tag the upstream OCPD. Test dead with a meter you tested live first.
2. Stage the work outside the can when you can: pre-cut, pre-strip, pre-label.
3. Hydrate before the truck rolls. Heat exhaustion turns into mistakes around live bus.
4. If the homeowner needs the AC back on, set a temporary feed off a generator or a different circuit. Do not rush a termination to chase the customer's comfort.

NEC 110.26 working clearance still applies when the attic is 130F. Three feet of depth, 30 inches of width, 6.5 feet of headroom. Hot weather is not an exception, and an inspector will absolutely measure it.

Conduit fill, expansion, and the summer reality

PVC expansion is the silent killer on long outdoor runs. NEC 352.44 gives the expansion table: a 100 foot run of PVC sees roughly 4 inches of movement across a 100F temperature swing. No expansion fitting, the conduit bows, the LB cracks, or the connection at the panel pulls loose.

Conduit fill per NEC Chapter 9, Table 1, is 40 percent for three or more conductors. In hot ambient, derating from CCC count compounds with ambient correction. Pulling four 4/0 SER through a tight 2 inch run in August can drop your usable ampacity below what the breaker calls for.

Pull SER through conduit underground or under slab? You are now subject to NEC 338.10(B)(4)(b), which sends you back to the 60C column. That kills a lot of feeders that looked fine on paper.

Final inspection checklist

Before the inspector shows up, walk the install with the same eye they will. The common failures on subpanels are not the splashy ones, they are the small things that should have been caught at rough.

• Neutral isolated, EGC bonded, bonding screw removed and bagged inside the can.
• Feeder OCPD at the supply end, not the load end, sized to the conductor or below.
• Handle ties or two-pole breakers on all multiwire branch circuits per NEC 210.4(B).
• Open knockouts closed. Unused breaker spaces filled. Directory legible and circuit-specific, not "lights."
• GFCI and AFCI protection per NEC 210.8 and 210.12 for the circuits in scope.
• Working clearance verified, not estimated.

The hot weather work is the same code as the cool weather work. The difference is the ambient correction, the expansion, and the time pressure to finish before the heat wins. Slow down on the math, speed up on the prep, and the install holds up.