Beginner's guide to installing a subpanel

Plan the load and location first

Before pulling a single wire, size the subpanel to the actual load it will serve. Run a load calculation per NEC Article 220, Part III. Add up continuous loads at 125%, then non-continuous at 100%. Do not just match the main panel rating because the homeowner asked for "100 amps in the garage."

Pick the location with working clearance in mind. NEC 110.26 requires 36 inches of depth, 30 inches of width, and 6.5 feet of headroom in front of the panel. No shelving, no water heaters, no stored paint cans crowding the dead front. If the subpanel feeds a detached structure, also verify NEC 225.32 placement rules for the disconnect.

Confirm the feeder path before you commit. Measure the run, count the bends, and check for fire-rated assemblies you will need to firestop. A 90 foot run through three penetrations changes your conduit fill and your labor estimate.

Size the feeder and overcurrent protection

The feeder OCPD at the main panel sets the ampacity floor for the whole subpanel install. Size conductors per NEC 310.16 using the 75 degree C column for terminations rated at that temperature, which covers nearly all modern breakers and lugs. A 100 amp feeder typically lands on 3 AWG copper or 1 AWG aluminum, but always verify against the actual termination ratings.

The equipment grounding conductor sizes off NEC Table 250.122 based on the upstream OCPD, not the feeder ampacity. For a 100 amp feeder, that is 8 AWG copper. Do not undersize this. It is the path that clears a fault and keeps the enclosure from going hot.

• 60 amp feeder: 6 AWG Cu conductors, 10 AWG Cu EGC
• 100 amp feeder: 3 AWG Cu conductors, 8 AWG Cu EGC
• 125 amp feeder: 1 AWG Cu conductors, 6 AWG Cu EGC
• 200 amp feeder: 2/0 AWG Cu conductors, 6 AWG Cu EGC

If the run is long, check voltage drop. NEC 210.19(A) Informational Note recommends keeping feeder drop under 3%, and combined feeder plus branch under 5%. On a 100 foot, 100 amp run at 240V, you may need to bump up a size.

Run four wires, not three

Modern subpanel feeders require four conductors: two ungrounded, one grounded (neutral), and one equipment grounding conductor. The three-wire feeder with a bonded neutral at the subpanel is no longer permitted for new installs per NEC 250.32(B). The only narrow exception applies to existing installations meeting specific conditions, and even then most inspectors will push back.

Field tip: the moment you open a subpanel and see the neutral bar bonded to the can with a green screw, stop. Pull that bond screw, install a separate ground bar kit, and move every EGC over. Mixed neutrals and grounds downstream of the main bonding jumper create parallel paths on the EGC, and that is how panels start humming and hot tubs start tingling.

Order a panel that ships with an isolated neutral bar and a separate ground bar, or buy the ground bar kit listed for that enclosure. Do not improvise with a sheet metal screw and a chunk of bus.

Bond, ground, and terminate cleanly

At the subpanel, the neutral floats. The EGC lands on the ground bar, which is bonded to the enclosure. If the subpanel is in a separate building, NEC 250.32(A) requires a grounding electrode at that structure, typically two 8 foot ground rods spaced 6 feet apart, bonded with 6 AWG copper. The grounding electrode conductor connects to the ground bar, never the neutral.

Torque every termination to the manufacturer specification listed inside the panel door. NEC 110.14(D) made calibrated torque tools mandatory for listed termination requirements. A loose 100 amp lug will glow red before it trips a breaker.

1. De-energize the source and verify dead with a meter you just tested on a known live source
2. Land the two hots on the main lugs or backfed breaker, observing phase rotation
3. Land the neutral on the isolated neutral bar
4. Land the EGC on the ground bar
5. Torque every lug to spec, mark each with a paint pen

Get the inspection right the first time

Label the subpanel feeder breaker at the main panel per NEC 408.4(A). Fill out the circuit directory legibly, in pen, with the actual room and load, not "lights." Inspectors notice. So do the next electricians who open this panel in fifteen years.

If the subpanel feeds dwelling unit branch circuits, GFCI and AFCI requirements still apply at the branch breakers per NEC 210.8 and 210.12. A subpanel does not exempt downstream circuits from current code. Bathroom, kitchen, garage, and outdoor receptacles need GFCI. Most habitable rooms need AFCI.

Field tip: take a phone photo of the inside of the panel before you put the dead front back on. It saves a trip back when the inspector asks about a torque mark or wants to see the EGC routing without pulling the cover.

Verify your work under load. Energize, measure voltage L1 to N, L2 to N, and L1 to L2. You should see roughly 120, 120, and 240. Then pull the cover off and thermal scan or hand check after thirty minutes of load. Warm is fine. Hot means a loose termination, and you fix it before the drywaller shows up.