Field guide: installing a subpanel, for master electricians (edition 3)

Plan the load calc before you pick the panel

Before you size conductors, run a real load calc per NEC 220. For a subpanel feeding a shop, garage, or ADU, use Article 220 Part III or the optional method in 220.82 if it qualifies. Don't default to a 100A feeder because it "feels right." A welder, a mini-split, and an EVSE will eat that budget fast.

Document the calc on paper and leave a copy in the panel. Inspectors in most jurisdictions will ask, and it saves a callback when the HO adds a hot tub next spring. If the existing service is already loaded, run 220.87 on 12 months of utility data before you commit to feeder size.

"I staple a laminated load calc inside the dead front. Ten years later, the next electrician knows exactly what's on the bus." ... J.R., master out of Buffalo.

Feeder sizing, conductor, and the 83% rule

For a dwelling unit feeder that carries the entire load, NEC 310.12 lets you use the 83% rule. A 100A feeder can land on 4 AWG copper or 2 AWG aluminum under that allowance, provided it supplies 100% of the dwelling load. For a detached structure subpanel that is not the whole dwelling, you are back to the standard ampacity tables in 310.16.

Size the EGC per 250.122 based on the overcurrent device ahead of the feeder, not the conductor. Oversized feeders for voltage drop still require a proportionally oversized EGC per 250.122(B). This is the write-up inspectors catch most often on long runs to detached garages.

• 100A feeder, dwelling, 83% rule: 4 Cu or 2 Al (75C column)
• 100A feeder, non-dwelling or detached subpanel as separate structure: 3 Cu or 1 Al per 310.16
• EGC for 100A OCPD: 8 Cu or 6 Al per 250.122
• Long runs: upsize EGC proportionally under 250.122(B)

Four-wire feeder, and killing the neutral-ground bond

Since the 2008 NEC, separately derived neutrals to a subpanel are out. Run four wires: two hots, an insulated neutral, and an EGC. The neutral bar in the subpanel must be isolated from the enclosure. Remove the green bonding screw or strap. Ground bar bonds to the can; neutral bar floats.

This applies whether the subpanel is in the same building or detached. The old 250.32(B) exception that allowed a bonded neutral to a detached structure with no metallic path was repealed in 2008. If you are working on a pre-2008 install, you are not required to retrofit, but any replacement panel gets the current rule.

"First thing I do on a service call is check the neutral bar. If it's bonded and there's a separate EGC, I've already found the nuisance-trip problem." ... M.C., Phoenix.

Detached structures and the grounding electrode

A subpanel in a detached structure needs its own grounding electrode system per 250.32(A). Two ground rods 6 feet apart, a Ufer if the slab is new, or a metal water pipe if one qualifies under 250.52(A)(1). Bond the GEC to the ground bar in the subpanel, not the neutral.

Exception: a single branch circuit feeding a detached structure with an EGC does not require a grounding electrode. Two or more circuits, or a feeder, always does. The GEC sizes off 250.66 based on the largest ungrounded feeder conductor, not the OCPD.

• Two driven rods: 6 ft minimum separation, 250.53(A)(2)
• GEC to rods: 6 AWG Cu minimum, 250.66(A)
• GEC to Ufer: 4 AWG Cu minimum, 250.66(B)
• Bond to ground bar only, never the neutral bar

OCPD at the structure, and the six-disconnect rule

If the subpanel is in a detached structure, you need a disconnecting means at that structure per 225.31 and 225.32. The feeder breaker in the main panel does not satisfy this unless it's lockable and readily accessible at the detached building, which almost never works out. Use a main-breaker panel or add a disconnect ahead of a main-lug kit.

The 2020 NEC tightened 230.71 to a single service disconnect at the service, but 225.33 still allows up to six disconnects at a separate building for feeders. Don't confuse the two. If the AHJ is on 2023, double-check emergency disconnect requirements in 230.85 for dwellings, which can affect where you pick up the feeder.

Final inspection prep

Torque every lug to the manufacturer's spec with a calibrated wrench and mark each one with a paint pen. 110.14(D) made this mandatory, and inspectors are looking for the witness marks. Label every breaker with the actual circuit, not "lights." Arc-fault and GFCI requirements in 210.8 and 210.12 follow the load side, so a subpanel in a finished basement triggers dwelling-unit protection rules on every 15 and 20A, 120V circuit.

Before you close the dead front, photograph the panel interior and the torque marks. Saves an argument six months later. Check working clearance per 110.26, minimum 36 inches depth, 30 inches width, and 6.5 ft headroom. A subpanel buried behind shelving will fail every time.

• Torque and mark every termination, 110.14(D)
• AFCI and GFCI per 210.8 and 210.12 on the load side
• Working clearance per 110.26
• Directory complete and specific, 408.4(A)
• Photograph everything before the cover goes on