Field guide: installing a subpanel, retrofit version (edition 6)

Sizing the feeder before you touch a wire

Retrofit subpanels live or die by the feeder calc. Before pulling permits, run the load per NEC 220 Part III and confirm the existing service can carry it. A 100A main with 82A of calculated existing load will not feed a 60A subpanel, no matter how clean the install looks.

Check the service conductor ampacity in NEC 310.12 if you are on a single-phase dwelling service. The 83% rule for main feeders does not extend to subpanel feeders, those fall under standard 310.16 ampacity. Copper vs aluminum matters here, #2 AL is 90A at 75C, #4 CU is 85A.

• Existing total demand load (NEC 220.82 or 220.83 for existing dwellings)
• Available spare capacity at the main
• Subpanel feeder size, OCPD, and conductor type
• Voltage drop if the run exceeds 100 feet (aim under 3%)

Picking the location and the panel

Working clearance under NEC 110.26 is non-negotiable. 30 inches wide, 36 inches deep, 6.5 feet high, measured from the face of the enclosure. In a retrofit this is where most installs get rejected, especially in finished basements where the homeowner framed a closet around where the panel needs to go.

Avoid bathrooms (NEC 240.24(E)) and clothes closets (240.24(D)) entirely. Garages and unfinished basements are fair game. If you are in a detached structure, you are now dealing with NEC 225 Part II and a disconnect at the building, which changes the whole job.

If the customer wants the panel in a "clean" spot behind a door or inside a cabinet, walk them through 110.26 with a tape measure on site. Easier to lose the argument before the drywall goes up than after the inspector fails it.

The 4-wire feeder rule

Since the 2008 NEC, separately derived and remote structures aside, any subpanel in the same building as the service needs 4 wires: two hots, a neutral, and an equipment grounding conductor. The neutral and ground must be isolated at the subpanel. This is NEC 250.32(B) and 408.40.

On retrofits in older homes you will find bonded subpanels constantly. The bonding screw or strap gets removed, a separate ground bar gets added, and all EGCs land on the ground bar while neutrals stay on the floating neutral bar. If the existing feeder is 3-wire with no EGC, you are pulling a new feeder. There is no grandfather for this in a retrofit where you are replacing the panel.

• Remove factory main bonding jumper or green screw
• Add supplemental ground bar, bonded to enclosure
• Neutrals and grounds on separate bars, no exceptions
• EGC sized per NEC 250.122 based on feeder OCPD

Running the feeder through existing construction

Fishing a 4-wire feeder through a finished house is where the hours go. SER cable (NEC 338) is the common choice for interior runs, but it must be protected from physical damage and cannot be run in contact with thermal insulation in a way that derates it below its load. NEC 338.10(B)(4)(a) sends you back to 334.80 for ampacity when SER is used in thermally insulated walls, which means the 60C column. That bumps a 4/0-4/0-2/0 SER from 195A down considerably.

For most residential 100A subpanels, 2-2-2-4 AL SER works if you are not buried in insulation. When in doubt, go to PVC or EMT with individual THHN conductors, the ampacity math is cleaner and the inspector will not second-guess it.

On a retrofit, measure twice and drill once. A 1 inch hole through the wrong top plate into a bathroom vent chase costs you a half day and a drywall patch.

Terminations, torque, and labeling

NEC 110.14(D) requires torque per the manufacturer's listed values, and since 2017 inspectors are actively checking for a calibrated torque tool on site. Lugs on the feeder, the main breaker in the existing panel, and every branch breaker in the subpanel all need to hit spec. Write the torque values on the inside of the dead front with a paint pen if the label is not already there.

Aluminum terminations get antioxidant compound per the lug listing. Clean the strands, brush the compound in, torque, and re-torque after 24 hours if the manufacturer calls for it. Most do.

1. Strip to the lug's strip gauge, not by eye
2. Apply anti-ox on AL conductors
3. Torque to spec with a calibrated tool
4. Update the directory, circuit by circuit, with actual room names
5. Label the feeder disconnect location at the subpanel per NEC 408.4(B)

Inspection-ready checklist

Before calling for rough or final, walk the job the way the AHJ will. Working clearance, bonding, neutral isolation, EGC continuity, conductor fill in the feeder raceway (NEC Chapter 9 Table 1 if you pulled in conduit), AFCI and GFCI on required circuits per 210.8 and 210.12, and a legible directory.

Most retrofit subpanel failures come down to three things: the bonding screw left in, the feeder EGC undersized, or working clearance violated by something the homeowner built. Catch those on your own pre-inspection and the AHJ visit takes ten minutes.

• Bonding screw removed, verified with meter (no continuity neutral to ground bar via anything but the main)
• Feeder EGC sized to NEC 250.122
• Working space per 110.26, photographed
• Torque values recorded
• Directory filled out, permanent marker or typed label