Field guide: installing a subpanel, code-compliant approach (edition 3)

Plan the load and feeder before you touch a knockout

Start with the load calc per NEC Article 220. Subpanels fail inspection when the feeder is sized off a guess instead of a calculation. Pull the nameplate data on everything the sub will serve, run a standard or optional calculation, and size the feeder conductors from NEC 215.2 using the 60/75/90 C column that matches your terminations (NEC 110.14(C)).

Pick your feeder ampacity with headroom. A 100A sub fed with 3 AWG copper at 75 C works on paper, but if the run is long, voltage drop eats you alive. NEC 210.19 and 215.2 informational notes recommend 3% max on feeders, 5% total. For anything over 75 feet at 100A, bump to 2 AWG or 1 AWG and stop fighting physics.

Decide the feeder wiring method up front. SER cable is legal in many jurisdictions for interior dwelling feeders (NEC 334.10), but check local amendments. For detached structures, you are in NEC Article 225 and 250.32 territory, which changes the grounding rules.

Four wires, always, on a separately structure or not

Since the 2008 NEC, subpanels require a 4-wire feeder: two hots, a neutral, and a separate equipment grounding conductor. The neutral and ground must be isolated at the subpanel. This is not optional. NEC 250.24(A)(5) and 408.40 are the hammer.

Remove the main bonding jumper from the sub. Pull the green bonding screw, cut the bond strap, whatever the manufacturer uses. The neutral bar floats. The ground bar bonds to the enclosure. If you leave them bonded, you create parallel neutral paths on the EGC, and every inspector worth their ticket will catch it.

Field tip: before you energize, ohm from the neutral bar to the enclosure. You should read open. If you read continuity, you missed a bond somewhere, usually a factory jumper hiding behind the neutral bar.

• Two ungrounded (hot) conductors sized per load calc
• One grounded (neutral) conductor, isolated at the sub
• One equipment grounding conductor sized per NEC 250.122
• No main bonding jumper installed at the subpanel

Detached structures: the 250.32 wrinkle

Feeding a subpanel in a detached garage, shop, or barn? NEC 250.32(B)(1) requires the 4-wire feeder and a grounding electrode system at the separate structure. That means a ground rod, or two rods 6 feet apart if a single rod does not hit 25 ohms (NEC 250.53(A)(2)).

The EGC from the main building bonds to the ground bar at the sub. The grounding electrode conductor from the rods also lands on the ground bar or the enclosure per NEC 250.32(B). Do not bond neutral to ground at the detached sub, same rule as interior subs since 2008.

If the detached structure only has one branch circuit and no metallic path back, 250.32(A) exception lets you skip the electrode, but it is rarely worth the argument with the inspector. Drive the rods.

Overcurrent protection and panel ratings

The feeder OCPD lives at the main panel, sized to protect the feeder conductors per NEC 240.4. The subpanel itself does not need a main breaker if it qualifies as a main-lug-only panel, but most electricians install a main breaker sub for a clean disconnect and easier future work.

Match the panel bus rating to the feeder breaker. A 100A feeder breaker on a 125A bus panel is fine. A 150A breaker on a 100A bus is a code violation and a fire waiting to happen. NEC 408.36 is clear: OCPD shall not exceed the panelboard rating.

1. Confirm feeder OCPD size from your load calc
2. Verify panel bus ampacity on the label inside the dead front
3. Check breaker compatibility, classified breakers are not always accepted, see 110.3(B)
4. Torque every lug to the manufacturer spec, not by feel

Working space, AFCI, GFCI, and the details that fail inspections

NEC 110.26 working space: 30 inches wide, 36 inches deep, 6.5 feet high, clear. Inspectors measure. Do not mount a sub behind a water heater or inside a clothes closet (NEC 240.24(D) and (E)).

Branch circuits off the sub follow the same rules as any other panel. Bedroom, kitchen, laundry, and most other dwelling circuits need AFCI per NEC 210.12. GFCI where 210.8 says so. If the sub feeds a detached garage or outbuilding, the outlets in that structure follow 210.8(A) and (B) accordingly.

Field tip: label the feeder breaker at the main with the sub location, and label the sub dead front with where the feeder originates. Ten years from now, someone, maybe you, will thank you.

Last pass before you close it up: torque check, dead front on, directory filled out in pen, neutral-to-ground continuity verified open, every unused opening closed with a proper knockout seal (NEC 110.12(A)). Then energize and test under load.