Running out of breaker slots in your main panel is one of those problems that sneaks up on you. You add a home office, wire a hot tub, upgrade to an electric dryer, and suddenly there is no room for the next circuit. A sub panel solves this by extending your electrical system with additional breaker capacity in a second enclosure, fed from the main panel through a dedicated feeder circuit.
What a Sub Panel Does
A sub panel is a smaller electrical distribution panel that receives power from your main panel and distributes it to branch circuits. It contains its own bus bars, breaker slots, and neutral/ground bars, but it does not have its own main breaker that disconnects utility power — that function belongs to the main panel.
Think of the sub panel as a satellite office for your electrical system. It takes a single large circuit from the main panel and breaks it into multiple smaller circuits closer to where the power is needed. This reduces the length of individual circuit runs, saves wire, and simplifies troubleshooting.
When You Need a Sub Panel
Several situations make a sub panel the right choice:
- Main panel is full. If every breaker slot is occupied and you need additional circuits, a sub panel adds 6 to 42 new slots depending on the size you choose.
- Detached structures. Garages, workshops, pool houses, and accessory dwelling units (ADUs) need their own panel to distribute power locally. Running individual circuits from the main panel to a detached building is impractical and code-prohibited for most situations.
- Long circuit runs. If a basement renovation or an addition is more than 75 feet from the main panel, a sub panel installed closer reduces voltage drop and wire costs.
- EV charger or major appliance addition. Adding a Level 2 EV charger (40 to 60 amps) or a workshop with multiple 240-volt tools may require a sub panel if the main panel lacks available space for high-amperage breakers.
Sizing the Sub Panel
Sub panel sizing involves two decisions: the amperage of the feeder circuit and the number of breaker spaces. Common residential sub panel sizes are 60, 100, and 125 amps. A 60-amp sub panel handles a garage with lights, outlets, and a small compressor. A 100-amp sub panel supports a full basement or ADU with appliances. Choose 125 amps for workshops with welders, large air compressors, or multiple 240-volt tools.
For breaker spaces, buy more than you think you need. A 20-space panel costs only $20 to $40 more than a 12-space panel, and running out of spaces again defeats the purpose. The sub panel amperage cannot exceed what the main panel can spare. If your main panel is 200 amps and existing loads draw 120 amps, you have roughly 80 amps available for a sub panel feeder — though a load calculation by an electrician gives you the precise number.
Wire and Conduit Requirements
The feeder wire from the main panel to the sub panel must be sized for the breaker protecting it. Standard sizing:
- 60-amp feeder: 6 AWG copper or 4 AWG aluminum (four-wire: two hots, one neutral, one ground)
- 100-amp feeder: 3 AWG copper or 1 AWG aluminum
- 125-amp feeder: 1 AWG copper or 1/0 AWG aluminum
Aluminum feeder wire is common for sub panel runs because it costs 40 to 60 percent less than copper at these gauges and performs well in feeder applications. The National Electrical Code (NEC) requires a four-wire feeder to any sub panel: two hot conductors, one neutral, and one equipment grounding conductor. In the sub panel, neutral and ground buses must be separate — they are only bonded together at the main panel.
Installation Overview
Sub panel installation is not a beginner electrical project. It involves working inside the main panel with live bus bars, which presents a serious shock and arc-flash hazard. A licensed electrician typically completes the job in 4 to 8 hours. The general process:
- Mount the sub panel enclosure in the desired location, following NEC clearance requirements (36 inches of clear space in front, 30 inches wide, ceiling height to at least 6.5 feet)
- Run the feeder conduit or cable from the main panel to the sub panel
- Install a two-pole breaker in the main panel sized for the feeder
- Terminate the feeder conductors in the sub panel, connecting hots to the bus bars, neutral to the neutral bar, and ground to the grounding bar
- Install individual branch circuit breakers in the sub panel and run circuits to loads
- Label every breaker clearly in both panels
Cost Breakdown
Total installed cost for a residential sub panel runs $800 to $3,000 depending on size, distance from the main panel, and local labor rates. Here is a typical breakdown for a 100-amp sub panel installed 30 feet from the main panel:
- Sub panel enclosure: $80 to $200
- Feeder wire (30 feet of 1 AWG aluminum): $100 to $180
- Breaker in main panel: $30 to $60
- Conduit and fittings: $50 to $120
- Electrician labor (4-6 hours): $400 to $900
- Permit and inspection: $75 to $250
Longer feeder runs, underground burial to a detached structure, or upgrading the main panel to free up capacity add to the total. A detached garage sub panel with a 50-foot underground feeder typically costs $2,000 to $4,000 all in.
Code Requirements and Permits
A building permit is required for sub panel installation in all US jurisdictions. The inspector will verify correct wire sizing, proper neutral-ground separation, adequate clearances, and correct breaker ratings. Key NEC rules to know:
- Neutral and ground must be separated in the sub panel (NEC 250.24)
- The sub panel must be accessible — not behind drywall or in a cramped crawl space
- All circuits must be labeled accurately at both panels
- AFCI and GFCI protection requirements apply to branch circuits in the sub panel just as they would in the main panel
Hiring a licensed electrician who pulls the permit under their license ensures the work meets code and protects your homeowner’s insurance coverage. This is one of those projects where cutting corners to save money creates real safety and legal risks.
