Field guide: installing a subpanel, hot weather considerations (edition 4)

Sizing the feeder before you touch a knockout

Subpanel feeder sizing starts with the calculated load, not the breaker you happen to have in the truck. Run Article 220 Part III for the dwelling, or Part IV for optional method, then size conductors per 310.16 with the 75°C column unless terminations say otherwise (110.14(C)). Don't forget the neutral can be reduced per 220.61 once you account for the non-linear loads that stay full size.

For a typical 100A subpanel feeder off a 200A service, #3 Cu or #1 Al at 75°C lands you at 100A. EGC sizes off 250.122 from the overcurrent device ahead of the feeder, not the feeder ampacity. A 100A feeder gets a #8 Cu EGC, not a #6, unless you upsized the ungrounded conductors for voltage drop, in which case 250.122(B) kicks in and the EGC scales proportionally.

Voltage drop is not code (it's an FPN in 210.19 and 215.2), but it is a callback. For runs over 100 feet, bump a size. Your customer's well pump dropping out at startup is your problem after you leave.

Four-wire feeder, separated neutrals and grounds

Since 2008, separately derived buildings and subpanels in the same structure require a 4-wire feeder with isolated neutral and ground bars at the subpanel (250.32(B), 408.40). The bonding screw or strap comes out. The grounded (neutral) bar floats. The equipment ground bar bonds to the enclosure.

This is the single most common failure point on inspection for residential subpanels. If the panel shipped with the bonding screw installed, back it out and tape it to the inside of the deadfront so the next person knows it was intentional. Do not rely on the homeowner or the next electrician to read your mind.

If you find a 3-wire feeder to a detached building installed before 2008, you can leave it under existing-installation rules, but the moment you replace the panel or pull a new feeder, you are on the current code. Price the trench accordingly.

Grounding electrodes at a separate structure

A subpanel inside the same building as the service does not get its own grounding electrode system. A subpanel in a detached structure does, per 250.32(A), unless it's a single branch circuit feeding the building (250.32(A) Exception).

Drive two ground rods 6 feet apart, or one rod with a documented 25-ohm reading (250.53(A)(2)). Ufer if the slab is fresh. Bond all electrodes present (250.50). The grounding electrode conductor sizes off 250.66 from the feeder, not 250.122, and that conductor lands on the ground bar, not the neutral, in the subpanel.

• Two rods, 5/8" copper-clad, 8 feet, driven full depth
• #6 Cu GEC for feeders up to 1/0 Cu (250.66)
• Acorn clamps listed for direct burial (250.70)
• Single continuous run from rod to rod, irreversible splice if you must

Hot weather derating that actually bites

Attics and south-facing exterior walls in summer punish anyone who skipped 310.15(B). Ambient correction starts at 30°C. An attic that hits 60°C ambient drops THHN's 90°C ampacity by a factor of 0.71. A #3 Cu rated 110A at 90°C carries 78A after correction, which won't protect a 100A breaker.

Layer that with 310.15(C) conduit fill adjustment when you have more than 3 current-carrying conductors in a raceway, and the math gets ugly fast. A four-wire feeder counts the neutral as current-carrying if it's serving non-linear loads (310.15(E)).

For rooftop conduit, 310.15(B)(2) adds a temperature adder based on height above the roof. Less than 1/2" off the roof adds 33°C to ambient. This is why you see PV installers strapping conduit on standoffs.

Working in the heat without making mistakes

Torque values drift when terminations are hot. A lug torqued at 110°F in an attic is going to loosen as the panel cools and the conductor contracts. Hit your torque values per 110.14(D) and the manufacturer's label, then check them again on a cooler day if you can get back.

Personal pace matters. Heat illness shows up as sloppy work first, dizziness second. If you're rereading the same page of the codebook, you're done for the day on that panel. Schedule subpanel work for early morning when the attic is still under 100°F.

Pre-make your feeder terminations on the truck tailgate in the shade. Strip, land, torque, label. Then carry the assembled panel up. You'll spend half the time in the hot space.

Final walk before you energize

Before you flip the feeder breaker, walk the install cold. Verify the bonding screw is out, neutrals and grounds are on separate bars, the EGC lands on the ground bar, and every breaker is off. Megger the feeder if the run is long or got wet during the pull.

1. Confirm feeder OCPD matches conductor ampacity after all derating
2. Verify GEC connection at the subpanel and at each electrode
3. Check torque on every lug, including the main lugs
4. Label the subpanel with feeder source and AIC rating (408.4, 110.24)
5. Update the directory on the service panel pointing to the new sub

Energize the feeder with branch breakers off, verify voltage at the bus, then bring circuits up one at a time. If something trips, you know exactly which circuit owns it.