Field guide: installing a subpanel, retrofit version (edition 1)

Plan the load and pick the feeder before you cut anything

Retrofits live or die on load calc. Run Article 220 before you quote the job, not after. Existing service capacity, spare breaker space in the main, and the actual measured demand over a day or two with a clamp meter will tell you whether a 60A subpanel gets it done or you need 100A plus.

Pick the feeder conductor based on the subpanel ampacity, the termination rating (75C column in 310.16 for most modern gear), and the voltage drop over the run. If the sub is feeding a detached structure, you are now in 225.30 territory with its own disconnect rules and one feeder limit.

Four wires, always, on a new sub. Two hots, one neutral, one equipment grounding conductor. Do not reuse the old three-wire feeder from the 1990s kitchen remodel, and do not bond neutral to ground at the sub. That is 250.24(A)(5) and 250.142(B), and inspectors look for it first.

Sizing the enclosure for the space you actually have

Retrofit means wall cavities, not clean new construction. Measure the stud bay depth before you order. A standard 3.5 inch deep panel will not fit flush in a 2x4 wall with drywall on both sides if there is blocking or plumbing behind. Surface-mount is often cleaner than forcing a flush install.

Count your circuits and add headroom. NEC does not mandate spare space, but 20 percent spare breaker positions is the working-electrician standard so the next guy is not tearing it out in five years. Factor handle-tie and 2-pole breaker spots into your count.

Measure the opening twice, then add an inch for the romex connectors and the bend radius on the feeder. I have cut into a wall expecting a clean drop and found a gas line running diagonal through the bay. Always pull the old cover and look before you commit to a location.

Running the feeder in an occupied house

This is where the job lives. Fishing a 4-wire feeder through a finished wall is the difference between a profitable retrofit and a loss. Map the path: basement to attic through a plumbing chase, up an exterior wall with a chase cut, or along the rim joist and into a closet. Check both sides of every wall before you drill.

For the cable itself, SER is the common choice for interior feeders under 310.15 ampacity tables, or individual THHN in EMT if you need to meet 334.10 limits on exposure or physical protection. In a garage or unfinished basement running below 7 feet, 334.15(B) wants protection: running boards, conduit, or guard strips.

• 60A sub: typically 6 AWG copper SER or 4 AWG aluminum, verify with 310.16 and the termination rating
• 100A sub: 3 AWG copper or 1 AWG aluminum, same verification
• Neutral sized per 220.61 if there are significant 120V loads, not automatically downsized
• EGC sized per 250.122 based on the feeder overcurrent device, not the conductor

Support the cable per 334.30: within 12 inches of the panel and every 4.5 feet after. In an attic run perpendicular to joists with access, you can lay it on running boards or through bored holes in the center of the joist per 300.4(A).

Terminating at the main and the sub

At the main, the feeder breaker sizes the whole job. Two-pole, rated for the feeder ampacity, landed on the bus. Torque the lugs to the spec on the inside of the panel door, not to feel. Loose lugs are the number one cause of panel failures on retrofit jobs.

At the subpanel, remove the bonding screw or strap if the panel shipped with neutral bonded to ground. Land the neutrals on the isolated neutral bar and the grounds on the separate ground bar. If there is only one bar, you need to add a ground bar kit. This is not optional.

On older FPE and Zinsco main panels, stop and think. If the customer is paying for a subpanel, they should know the main is a liability. Document it in writing, even if they decline the upgrade. Protects you when something happens later.

Grounding and bonding the right way

Same building, same service: the sub rides on the EGC pulled with the feeder. No new grounding electrode, no ground rod at the sub. 250.32(A) exception covers this.

Separate building or structure: different rules. 250.32(B) requires a grounding electrode system at the second building and an EGC pulled with the feeder. Ground rod, concrete-encased electrode if available, water pipe if qualifying. Bond the EGC to the electrode at the sub, keep neutral isolated.

Metal water and gas piping in the area served by the sub gets bonded per 250.104 if not already bonded at the service. Often overlooked on retrofits where the sub serves a new kitchen or bath addition.

Label, test, and close it out

Panel directory per 408.4(A) with specific descriptions. "Kitchen" is not enough. "Kitchen counter SABC north wall" is enough. Your future self and the next electrician will thank you.

Test before energizing: megger the feeder if you have any doubt about insulation damage during the pull, verify no neutral-to-ground bond at the sub with a continuity check, confirm polarity and voltage at the first device on each new circuit.

1. Torque check every lug, main and sub, with a calibrated driver
2. AFCI and GFCI protection per 210.8 and 210.12 for the covered circuits
3. Working clearance per 110.26: 30 inches wide, 36 inches deep, 6.5 feet high, clear and dedicated
4. Final label on the dead front, inside of door, and on the feeder at the main

Photograph the finished panel with the dead front off before you button it up. Saves a callback when the homeowner calls asking what breaker runs the dishwasher.