How to installing a subpanel

Plan the Load and Feeder Before You Touch a Wire

Start with a load calc per NEC Article 220. Size the subpanel feeder to the calculated load, not the subpanel's bus rating. A 100A subpanel can be fed with a 60A feeder if the load calc supports it, and plenty of detached shops run fine on 50A.

Pick your conductors using NEC Table 310.16 at the 75°C column, and apply any ambient or bundling adjustments from 310.15. Don't forget the 83% rule in 310.12 for dwelling feeders when the feeder carries 100% of the dwelling load. For a detached structure, confirm you only need one supply per 225.30.

Before pulling anything, lock in these numbers:

• Calculated load in amps (and VA for records)
• Feeder OCPD size at the main panel
• Conductor size, insulation type, and raceway or cable method
• EGC size per 250.122, sized to the feeder OCPD
• Distance and voltage drop check (aim under 3% on the feeder)

Four Wires, Always, to a Separate Structure or Detached Subpanel

Since the 2008 cycle, neutrals and grounds must be isolated at any subpanel. That means four conductors on a 240/120V feeder: two hots, one neutral, one EGC. The neutral lands on an insulated bar. The EGC lands on a bonded bar. The bonding screw or strap that ties neutral to the enclosure stays out. Read 250.24(A)(5) and 408.40 together and you'll see why.

For a detached building, 250.32 requires a grounding electrode system at that structure. Drive two rods 6 feet apart unless you can prove 25 ohms with a single rod, or use a Ufer if the slab is accessible. Bond the electrodes to the subpanel's EGC bar, not the neutral.

If you inherit an old three-wire feeder to a detached garage, you cannot extend it or add loads under current code. Pull a new four-wire feeder and retire the bonding jumper at the sub.

Mount, Land, and Torque

Set the panel so the highest breaker handle is no more than 6 feet 7 inches above the floor per 240.24(A). Keep the working space clear: 30 inches wide, 36 inches deep, 6.5 feet high, per 110.26. In a garage or basement, that clearance gets violated fast by shelving and water heaters, so plan the location before you drill.

Land conductors on the correct lugs and torque them to the label value with a calibrated torque screwdriver or wrench. 110.14(D) requires it, and AHJs are writing it up. Re-strip if you nicked a strand. Anti-oxidant on aluminum feeders per the lug listing.

Dress the wires so the deadfront goes on clean:

1. Bring feeders in through a listed connector, no sharp edges
2. Form the neutral and EGC to their separate bars, no crossing the bus
3. Run branch circuits down one side, neutrals to the matching bar position
4. Label every breaker with the circuit it serves, not just "lights"

Branch Circuit Protection at the Sub

Protection requirements follow the branch circuit, not the panel location. GFCI still applies in 210.8 areas: garages, unfinished basements, outdoors, kitchens, laundry, bathrooms, and within 6 feet of a sink. AFCI still applies to the dwelling areas listed in 210.12. If the subpanel feeds a bedroom, those circuits need AFCI whether the breaker sits in the main or the sub.

Watch the neutral on multiwire branch circuits. 210.4(B) requires a simultaneous disconnect, so use a handle tie or a two-pole breaker. On GFCI or AFCI two-pole breakers, the neutral pigtail lands on the breaker, not the neutral bar. Miss that and the breaker trips the moment you energize.

If a GFCI breaker trips on power-up with nothing connected downstream, 90% of the time it's a neutral on the wrong bar or a shared neutral from another circuit.

Grounding, Bonding, and the Isolation Check

Before you energize, verify the neutral-to-ground bond exists in exactly one place: the service disconnect. At the sub, measure between the neutral bar and the EGC bar with a meter. You should read open, or at least high resistance. Any continuity means the bonding screw is still in, or a neutral landed on the ground bar.

For metal raceway feeders, the raceway can serve as the EGC per 250.118, but most inspectors prefer a pulled copper EGC, and it's required for certain flexible methods over 6 feet. If you're running PVC, you must pull an EGC sized to 250.122.

Quick pre-energize checklist:

• Neutral bar isolated, bonding screw removed
• EGC bar bonded to the can
• GES present and connected at detached structures
• All lugs torqued, deadfront secure, knockouts filled
• Feeder breaker at the main is off until final check

Energize, Test, and Document

Energize the feeder breaker first with all sub-breakers off. Verify 240V hot-to-hot, 120V hot-to-neutral on both legs, and near-zero voltage neutral-to-ground. Then turn on branch breakers one at a time and test GFCI and AFCI function with the test button and a plug-in tester.

Fill out the directory with real room names and outlet locations. Take a photo of the open panel for your records before the deadfront goes on, it saves a service call later when something gets added. Note the torque values, feeder size, and load calc on the jobsite file so the inspector and the next electrician both have what they need.