How to Install a Subpanel

Why this matters

Subpanel installation is a project, 4-6 hours labor, common for: garage workshops, basement finishing, ADU + accessory dwelling, separate building (detached garage, barn), EV + heat pump load aggregation, electrification retrofits where main panel doesn't have space. This is the working procedure.

When a subpanel makes sense

Strong fit:

• Main panel full or close to full
• New loads concentrated in a different area (basement, garage, addition)
• Separate building requiring own service distribution
• Multiple new loads (EV + workshop + heat pump): subpanel near the loads simplifies
• Detached structure with significant electrical needs

Weak fit:

• Single new circuit; just add to main panel
• Whole-house upgrade: replace main panel instead
• Different building with utility wanting separate service (some utilities prefer)

Sizing the subpanel

Step 1: Calculate load at subpanel location:

• All circuits to be moved or added
• Per NEC 220 calculation

Step 2: Plus safety factor + future growth:

• 25% headroom typical
• Match a standard amperage

Common sizes:

• 60A subpanel: small additions, light loads
• 100A subpanel: typical garage workshop, basement
• 125A subpanel: heavier loads (EV + heat pump + workshop)
• 200A subpanel: ADU with own service-like setup

Step 3: Verify main panel can supply:

• Main panel can supply subpanel amperage equal to a single breaker
• If main panel near capacity, may need upgrade FIRST

Materials

• Subpanel: choose brand matching the main panel (Square D, Eaton, Siemens, GE) for compatibility
• Wire: 4-conductor wire (2 hot + neutral + ground separately) - NEVER share neutral + ground in a sub-panel
• Conduit if exposed: PVC or EMT depending on location
• Conduit fittings
• Breakers at main panel + subpanel
• Grounding electrode if needed (separate structure)

Wire sizing

Size the feeder from NEC Table 310.16 at the temperature rating of the TERMINATIONS, not the wire. Per 110.14(C) that is the 60 C column for equipment rated 100 A or less and the 75 C column above that, unless the equipment is marked for higher, which most modern panels are. NM cable and interior SE cable are held to the 60 C column regardless of what the terminals are rated (334.80 and 338.10(B)(4)(a)), so a cable assembly always needs a bigger conductor than the same feeder pulled in raceway.

Copper in raceway, 75 C terminations:

• 60A feeder: 6 AWG (65 A)
• 100A feeder: 3 AWG (100 A)
• 125A feeder: 1 AWG (130 A)
• 200A feeder: 3/0 AWG (200 A)

Copper NM or interior SE cable, 60 C column:

• 60A feeder: 4 AWG (70 A)
• 100A feeder: 1 AWG (110 A)

The exception people misapply. NEC 310.12 lets a dwelling service or feeder use a smaller conductor (4 AWG copper at 100 A, for instance), but only where that conductor carries the ENTIRE load of the dwelling. A garage, basement, or workshop subpanel does not, so the 310.12 table does not reach it. Sizing a subpanel feeder off the service table is the most common way these come in undersized, and it is invisible on inspection day because the panel still works.

For aluminum (cost-effective at longer distances):

• Go up in size from the copper value and read the actual ampacity out of the table; do not assume one gauge covers it
• Use an anti-oxidant compound at connections

Voltage drop: for distances over 50 ft, calculate voltage drop. The NEC's informational notes suggest under 3 percent on the branch circuit and under 5 percent total. Those are recommendations in an informational note, not enforceable limits, so an inspector cannot fail you on them and a customer with dimming lights will not care.

Procedure

Step 1: Plan

1. Subpanel location: accessible, NOT in bathroom or closet, working clearance per NEC 110.26 (30" wide x 36" deep x 6.5 ft tall)
2. Wire route from main panel
3. Permit pulled
4. Customer notified of power outage

Step 2: Pull permit + plan inspection

Document:

• Main panel amperage + room for breaker
• Subpanel amperage
• Wire size + route
• Grounding strategy (per AHJ requirements)

Step 3: Run feeder wire

1. Conduit from main panel to subpanel location
2. Or NM-B cable through stud bays + ceilings (verify cable type allowed)
3. Sized + protected per NEC
4. Continuous run; minimize splices (none for feeder ideally)

Step 4: Install subpanel

1. Mount enclosure level + secure to studs / wall
2. Verify clearance + working space
3. Bring feeder into panel
4. Strip + terminate conductors

Step 5: Terminate at main panel

1. Power OFF the main breaker. Understand what that does and does not do: it kills the bus, and the line side of the main and the service lugs stay energized. There is no way to de-energize those from inside the building
2. Verify with a meter, on a known-live source first and after
3. Drill knock-out + run feeder cable into main panel
4. Install double-pole feeder breaker (60A, 100A, etc.)
5. Land hot conductors on breaker terminals
6. Land neutral on neutral bar
7. Land ground on ground bar
8. Torque per panel manual

Step 6: Terminate at subpanel

For a subpanel inside the same building:

1. Hot conductors to main lugs OR main breaker (depending on subpanel type)
2. Neutral to neutral bar (NOT bonded to ground in subpanel - critical)
3. Ground to ground bar (separated from neutral)
4. SUB-PANEL: neutral + ground bars MUST be separated. NEVER bond in sub-panel.

For a subpanel in a separate structure:

1. Same: neutral + ground separated
2. PLUS: ground electrode at the separate structure (driven rod, foundation, etc.)
3. NEC 250.32 applies (separate structure grounding)

Step 7: Install circuits at subpanel

1. Pull existing circuits OR run new from subpanel
2. Match breaker types to subpanel brand
3. Per NEC space rules
4. Label each circuit

Step 8: Verify + restore

1. Multimeter at subpanel: 240V between hots, 120V each to neutral
2. Continuity verified
3. Restore main panel breaker
4. Test each subpanel circuit
5. Label main panel breaker as "Subpanel" + identify location

Step 9: Customer briefing

1. Walk the customer through what was done
2. Show labels at main + subpanel
3. Provide load summary
4. Discuss future expansion
5. Customer signs work order
6. Inspection scheduled

Acceptance criteria

• 240V at subpanel feeder, balanced
• Neutral + ground separated at subpanel
• Each circuit tested + labeled
• Permit passes inspection
• Customer signed + briefed

Code requirements

• NEC 215 + 220 (feeder + load calc)
• NEC 250.32 (grounding for separate structures)
• NEC 408 (panelboards + switchboards)
• NEC 110.26 (working space)
• AHJ-specific (some jurisdictions stricter)

Common pitfalls

• Neutral + ground bonded in subpanel: NEC violation; trips GFCI/AFCI; possible shock hazard
• Wrong wire size: undersized = voltage drop + heat
• Forgetting ground electrode at separate structure: NEC 250.32 violation
• Missing main feeder breaker: subpanel runs without OCPD at supply end
• Insufficient working space: AHJ fails inspection
• Aluminum without anti-oxidant: corrosion + loose connections
• Not labeling: future technician confusion
• No permit: AHJ + insurance issue

When to call utility

Some scenarios need utility involvement:

• New service (not subpanel - entirely separate service to a structure): utility installs meter + tap
• Service upgrade through utility transformer: utility coordinates transformer
• ADU with separate billing: utility installs separate meter + may require service drop

If unsure: call utility account services + ask. Most respond within 24 hours.

Smart panel + subpanel alternative

Modern smart panels (SPAN, Lumin) can sometimes replace subpanel for load management. Instead of adding a subpanel, a smart panel controls circuit by circuit:

• Premium but flexible
• Manages multiple loads on existing service (no upgrade needed)
• App-based monitoring

For customers with future-electrification plans: smart panel often a better long-term investment than subpanel.

Customer talking points

1. "Your main panel is full; we'll install a subpanel to handle new loads."
2. "Subpanel location: [where]. Working space + permit per code."
3. "Total: $X, includes panel, wire, breakers, install, permit, inspection coordination."
4. "Future: room for additional circuits as you add EV / heat pump / etc."
5. "Alternative: smart panel for $Y if you want app-based monitoring."

References

• NEC Article 215 + 220 + 250 + 408
• IRC Part VIII, Chapters 34 through 43 (Electrical). Chapter 27 is the IBC electrical chapter, not the IRC
• Manufacturer panel manuals
• AHJ subpanel installation requirements
• Manuall internal: Residential Service Panel Upgrade, NEC 2023 Residential Updates Reference