Adding circuits to a garage, basement, or detached workshop almost always points to one solution — a second panel fed from the main service. Electrical subpanel installation is one of the higher-value DIY-adjacent projects a homeowner can take on, but only if you understand the four pieces that get tripped up most often: feeder sizing, grounding separation, panel rating, and the permit/inspection process. Done right, a 100-amp subpanel install runs $1,800 to $3,400 with a licensed electrician. Done wrong, you have a fire hazard or a failed inspection that nobody wants to touch.
When You Actually Need a Subpanel
A subpanel makes sense in four common situations: when the main panel is full and adding tandem breakers no longer meets code, when a detached structure (garage, shed, ADU) needs its own disconnect, when a large remodel adds significant load far from the main, or when you want a clean shutoff for a specific zone like a workshop or pool equipment area. NEC 225.30 limits feeders to one set of conductors per building — meaning your detached garage must be fed by a single subpanel, not multiple branch circuits.
Before you spec the panel, run a load calc using NEC Article 220. Add up the total branch-circuit demand for the subpanel area and apply the demand factors. A typical detached two-car garage with two 20-amp general circuits, a 30-amp dryer for an EV charger, and lighting comes to about 60 amps calculated load — well within a 100-amp subpanel.
Sizing the Panel and Feeder
Match the subpanel rating to your real future load, not just current need. A 100-amp panel with 20 spaces costs about $115 (Square D HOM2040M100PQCVP) — virtually the same as a 60-amp box, so why limit yourself? For workshops with welders, dust collectors, or future EV chargers, 125-amp or 200-amp panels deserve consideration.
- 60-amp subpanel: 6 AWG copper or 4 AWG aluminum SER feeder, 60A double-pole breaker at main
- 100-amp subpanel: 3 AWG copper or 1 AWG aluminum SER, 100A double-pole at main
- 125-amp subpanel: 2 AWG copper or 1/0 AWG aluminum SER, 125A double-pole at main
- 200-amp subpanel: 2/0 AWG copper or 4/0 AWG aluminum SER, 200A double-pole at main
For runs over 100 ft, upsize the feeder by one gauge to compensate for voltage drop — particularly important when feeding detached buildings. Use copper THHN in conduit for outdoor runs or for runs through unconditioned spaces; SER cable is fine for interior connections inside the same building.
The Grounding Rule That Trips Everyone Up
In a subpanel located in the same building as the main, the neutral and ground bars must be isolated. Remove the green bonding screw that ties them together at the factory. Run a 4-wire feeder (two hots, neutral, ground) so the neutral floats and the ground bar bonds only to the panel enclosure.
For a subpanel feeding a detached building (garage, shed), NEC 250.32 requires a new ground electrode at that building — typically two 8-ft copper-clad ground rods driven 6 ft apart and bonded with 6 AWG copper to the subpanel ground bar. The feeder still runs four conductors back to the main, and the neutral remains isolated in the subpanel.
Tools and Materials Worth Owning
Even if you plan to hire out the install, having the right tools lets you do the rough-in and prep work that saves on the electrician’s hourly bill.
- Klein Tools 5-in-1 multimeter (MM700, $89) or Fluke 117 ($229) for verifying line and load
- Klein 1-3/8 inch cable cutter (63607, $58) for 4 AWG and larger
- Klein insulated lineman’s pliers (D213-9NE-INS, $52)
- Greenlee 7238SB knockout punch set ($429) for clean 1.5 and 2-inch panel knockouts
- Ideal In-Sure Push-In wire connectors for branch circuits ($14 per box of 50)
- Milwaukee M18 ProPex expander or DeWalt DCD460 stud-and-joist drill for running feeders
- Non-contact voltage tester (Klein NCVT-3P, $32) before touching anything
The Installation Sequence
Begin with permits — most jurisdictions require a permit and at least one rough-in inspection before drywall closes anything up. Permit costs run $80 to $300 depending on locality.
Mount the subpanel 4 to 6 ft above the floor on a 3/4-inch plywood backing (fire-rated where required) screwed into wall studs. Maintain 36 inches of working clearance in front, 30 inches of horizontal width, and 6.5 ft of headroom per NEC 110.26.
Punch the feeder knockouts at the top or bottom of both the main and the subpanel, run the feeder through conduit or as SER cable, and land conductors before touching any line-side power. The double-pole breaker that feeds the subpanel goes in the main panel and must not exceed the subpanel’s main lug rating.
Shut off the main breaker, verify with the NCVT-3P, land the two hots on the new breaker, the neutral on the main panel’s neutral bar, and the ground on the main ground bar. At the subpanel end, land hots to the main lugs, neutral to the isolated neutral bar, ground to the ground bar. Pop in branch breakers (20A AFCI/GFCI as required by 2023 NEC), make up branch circuits, label every breaker, and torque every lug to the manufacturer spec (typically 25 in-lbs for branch breakers and 75 in-lbs for main lugs).
Cost Breakdown
Materials for a 100-amp interior subpanel install: panel $115, feeder cable for a 30-ft run $145, branch breakers (eight at $14 each) $112, conduit/connectors $45, wire nuts and miscellaneous $35 — about $452 in parts. Electrician labor for a 6 to 8 hour install runs $850 to $1,400 plus permit, putting the typical total at $1,800 to $2,400.
A 100-amp detached garage subpanel with a 50-ft underground feeder, two ground rods, and trenching pushes the install into the $3,800 to $5,200 range because of the conduit, trenching labor, and additional inspection. Quotes vary heavily by region — labor in Northeast metros runs 30 to 50 percent higher than Sun Belt suburbs.
Common Mistakes That Fail Inspection
The big three are bonding errors (leaving the green screw in a subpanel), feeder undersize (using 4 AWG copper for 100 amps instead of 3 AWG), and using a 3-wire feeder when a 4-wire is required. Other frequent flags include unprotected cable through framing (no nail plates on Romex within 1.25 inches of the stud edge), missing AFCI on bedroom branch circuits, and breakers stuffed into a panel rated for fewer circuits.
One last warning: if the existing main panel is a Federal Pacific Stab-Lok, Zinsco, or Pushmatic, your subpanel install is a great opportunity to budget for a main panel upgrade too. Insurance carriers in roughly 30 states now refuse coverage for these legacy panels because of documented breaker failure rates.
