Field guide: installing a subpanel, hot weather considerations (edition 6)

Sizing the feeder before you cut a hole

Subpanel jobs go sideways at the load calc, not the install. Run NEC 220 Part III on the subpanel's expected load before you pick conductor size, and remember the feeder neutral can be sized per 220.61 if the load is mostly 240V. Don't copy the main panel's amperage out of habit. A 100A subpanel fed from a 200A main is common, and oversizing the feeder past what 408.36 and 215.2 require just costs you money and bending effort.

Voltage drop is the silent killer on detached structures. NEC 210.19 Informational Note recommends 3% on branch circuits and 5% total. On a 150 foot run to a garage subpanel at 60A, #4 copper is often where the math lands, not #6. Run the numbers, don't eyeball it.

Confirm the feeder OCPD at the main matches the subpanel's bus rating per 408.36. Feeding a 100A bus from a 125A breaker because that's what was in the truck is how panels cook.

Grounding and bonding, the part everyone gets wrong

In a separate structure, the subpanel must have neutrals and grounds isolated. Pull the bonding screw or strap. NEC 250.32(B) requires an equipment grounding conductor run with the feeder, and the neutral stays floating in the subpanel. This has been the rule since the 2008 cycle. If you find a bonded neutral on a remodel, fix it before you energize anything new off that panel.

Detached structures still need a grounding electrode system per 250.32(A), typically two ground rods 6 feet apart unless you can prove 25 ohms or less with one. The GEC bonds to the ground bar in the subpanel, not the neutral bar.

If the inspector finds the bonding screw still in a subpanel, you're failing on the spot. Keep a labeled bag in the truck with screws pulled from subpanels so you stop reinstalling them out of muscle memory.

Conductor and conduit choices for the run

For outdoor and underground feeders, NEC 300.5 sets your burial depths. PVC at 18 inches under a driveway, 24 inches under landscape if no concrete encasement. Mixing wet and dry locations means THWN-2 is the safer call across the whole run, since 310.4 lets you use it in both. Don't splice in a pull box you'll bury and forget about.

Aluminum SER and aluminum URD are common for cost, and they're fine if you torque correctly with the listed antioxidant per 110.14. The failures aren't the metal, they're the technique. Use a calibrated torque screwdriver, not a feel screwdriver.

• 4/0-4/0-2/0 aluminum SER for 200A feeders, per 310.12 dwelling rule.
• 2/0-2/0-1 aluminum for 150A dwelling feeders.
• #2 copper or 1/0 aluminum for 100A dwelling feeders.
• Always verify against the panel manufacturer's lug range before you cut.

Hot weather derating, the math you can't skip

Summer attic and rooftop runs get punished by 310.15(B) ambient temperature correction and 310.15(C) bundling adjustments. Rooftop conduit in direct sun adds an ambient temperature adder per 310.15(B)(2), which the 2017 cycle revised to a fixed 60F adder for conduit above the roof in sunlight up to 7/8 inch from the surface. That changes derating fast.

Example: 75F design ambient on the roof plus 60F adder gets you 135F. Look at the 90C column on Table 310.16, apply the correction factor, then your final ampacity often drops a wire size below what looks correct on paper. If you size off the 75C column out of habit because the terminations are 75C rated, you're double penalized.

Bundle four current carrying conductors in a conduit and 310.15(C)(1) hits you with an 80% adjustment. Stack ambient correction on top and a #6 THWN-2 you thought was good for 65A is suddenly closer to 50A.

Working safely in heat

Heat illness on a service change is a real failure mode. OSHA doesn't have a federal heat standard yet, but several states do, and the General Duty Clause covers it. Hydrate before the job, not at lunch. Schedule the panel swap for early morning if the meter pull leaves the house without AC.

PPE doesn't get to come off because it's hot. Arc flash gear stays on for the energized work per NFPA 70E 130.7. Cotton long sleeves under FR is more comfortable than the bare arms move, and it actually helps with sweat evaporation.

If you're working in an attic over 110F, set a 20 minute timer on your phone. Come down, drink water, check your tools. Mistakes in heat look exactly like mistakes from being tired, and your hands get clumsy before your head notices.

Final checks before you energize

Walk the install with a checklist before you close the cover. The boring inspection items are the ones that fail.

1. Bonding screw removed in the subpanel.
2. EGC landed on ground bar, neutrals on neutral bar, no crossover.
3. Lugs torqued to spec, marked with torque seal.
4. Feeder OCPD at or below subpanel bus rating per 408.36.
5. Working clearances per 110.26, 36 inches deep, 30 inches wide.
6. Panel directory filled in legibly, not pencil scribbles.

Energize one circuit at a time if you can. A clamp meter on the feeder while you flip breakers tells you in 30 seconds whether something is miswired or shorted to ground. Cheaper than a callback.