Field guide: installing a subpanel, before you start (edition 4)

Scope the job before you pull a permit

A subpanel install lives or dies on the first site walk. You need to know the feeder length, the ampacity of the existing service, available bus space in the main, and whether the structure is detached or attached. All four change the parts list.

Measure the feeder run twice. Voltage drop on a 100A feeder at 150 feet in #2 Cu copper is already pushing the 3% recommendation in NEC 210.19(A) Informational Note 4. If the customer wants the subpanel in the detached shop 220 feet out, you are moving to 1/0 or aluminum 2/0, and that changes your conduit size and your lug terminations.

Confirm the main panel can actually feed it. A 200A service already loaded with a heat pump, a range, and an EV charger will not carry a 100A subpanel without a load calc under NEC 220. Do the math on paper before you quote.

Feeder sizing and the neutral question

Size the feeder conductors per NEC 310.16 and the overcurrent device per 408.36. A 100A subpanel typically runs #3 Cu or #1 Al at 75C terminations. Check the breaker and lug rating stamped inside the panel door, not what you assume.

The neutral is where guys get sloppy. For a detached structure fed after the 2008 cycle, you must run four wires: two hots, a neutral, and a separate equipment grounding conductor, per NEC 250.32(B)(1). No more bonding the neutral at the remote structure. No more three-wire feeders with a local ground rod doing double duty.

If you pop the cover on an older detached garage and see a three-wire feeder with the neutral bonded, that is a grandfathered install under the old 250.32(B) Exception. The moment you replace that panel, you are pulling a fourth wire. Price it in or walk away.

Grounding and bonding at the subpanel

This is the single most failed item on rough inspection. At any subpanel, the neutral bar floats and the ground bar bonds to the enclosure. Remove the green bonding screw or bonding strap from the neutral bus. The neutral and ground are only tied together once, at the service disconnect, per NEC 250.24(A)(5).

For a detached structure, drive two ground rods 6 feet apart or prove 25 ohms with a single rod, per NEC 250.53(A)(2). Bond to the grounding electrode conductor landing on the ground bar, not the neutral. If there is a metal water line or a concrete encased electrode (Ufer) present, those are required electrodes too under 250.50.

• Neutral bar: isolated, floating, no bonding screw
• Ground bar: bonded to enclosure, lands EGC and GEC
• Detached: two driven rods or tested single rod, 6 ft apart minimum
• Metal raceway entering the panel: bonded per 250.92 if feeding from the service

Overcurrent, location, and working space

If the subpanel is in a detached structure, you need a disconnecting means at the structure per NEC 225.31 and 225.32. A main breaker panel solves this in one box. A main lug panel does not, unless you add a fused disconnect ahead of it.

Working space is non-negotiable. 30 inches wide, 36 inches deep, 6.5 feet high, per NEC 110.26(A). Do not mount the subpanel behind the water heater, above a laundry sink, or in a closet with shelving. Inspectors fail this constantly and they are right to.

Check the panel's short circuit current rating against the available fault current at the point of installation, per NEC 110.10. On a short feeder from a 200A service with a tight utility transformer, you can exceed 10 kAIC on a budget panel. Use a series-rated combination or bump the panel.

Conduit, conductors, and derating traps

Run the feeder in the right raceway. PVC underground needs 18 inches of cover under a driveway, 24 under other locations per NEC Table 300.5. Use a sweep, not a pulled 90, on anything 1 inch and up. Rigid at the riser, PVC below, bushings on the metal.

Watch your fill. A 100A feeder of four #3 THHN plus a ground fits 1 inch EMT but barely. On a long run with pulling tension, step up to 1.25 inch and save yourself the fight. Ambient derating under NEC 310.15(B) kicks in on rooftop and attic runs, which burns a lot of residential subpanel jobs in the south.

On one 150 ft underground feeder, we pulled 1/0 aluminum SER inside schedule 40 and the inspector red tagged it. SER is not listed for wet locations. Use USE-2 or XHHW-2 in conduit underground. Know your conductor listings.

Final walkthrough before you energize

Torque every lug with a calibrated driver to the value stamped in the panel, per NEC 110.14(D). Hand tight is not a torque spec. A loose neutral on a 100A feeder will cook in a year.

Label the subpanel with the source and the overcurrent device per NEC 408.4(B). Update the main panel directory too. AFCI and GFCI requirements still apply to the branch circuits you are landing, per 210.8 and 210.12, and that includes the new kitchen, laundry, or garage circuits you just made live.

1. Verify neutral isolation and ground bond
2. Torque feeder and branch lugs to spec
3. Confirm working space and clearances
4. Megger or insulation test long feeders before energizing
5. Label source, amperage, and circuit directory

Do these six things in order on every subpanel and you will pass rough and final on the first trip. Skip any one and you will be back in a truck with a ladder and a bad attitude.