Every home with electrical power has a main electric service panel, sometimes called a circuit breaker board. That panel transfers power from the outside utility lines into the home’s wiring system, and most residential panels are rated at 200, 300, or 400 amps with room for up to 84 circuits. An electrical sub-panel is a secondary circuit breaker panel fed from a 240-volt breaker in the main panel. It looks and operates like the main panel, but it is smaller and holds fewer breakers, which makes it a practical way to expand capacity or create a convenient control point for a remote area such as an addition, basement, or workshop.
Finishing new living space is the most common reason a sub-panel earns its keep. When a project turns an unused room into a coastal French dining space with painted wood panels and rattan accents, that room suddenly needs lighting circuits, receptacle circuits, and possibly a dedicated line for a wall oven or wine cooler. Running every new circuit back to the main panel means long cable runs and crowded breaker slots. A sub-panel placed near the new space keeps the work local, gives you a shutoff right at the room, and leaves the main panel with room to breathe.
How Sub-panels Work
A sub-panel is a smaller version of the main service panel. It contains the same basic parts: a main breaker or main lug, space for branch circuit breakers, a neutral bar, and a grounding bar. The difference is where it gets its power. Instead of being fed by the utility service, a sub-panel is fed by a dedicated 240-volt circuit breaker in the main panel, and the cable between them is called the feeder.
Modern installations use a four-wire feeder: two hot conductors, a neutral, and a separate ground. The neutral and ground must stay isolated inside the sub-panel, which means removing the bonding screw or strap that connects the two bars. Bonding happens only at the first disconnecting means. A bonded neutral in a sub-panel lets normal return current travel on ground paths, which is a shock and fire hazard. This rule has been standard since the 1999 National Electrical Code, and it is one of the first things an inspector checks.
Feeder and Breaker Basics
The feeder breaker in the main panel protects the cable running to the sub-panel, and the sub-panel rating must be at least as large as that breaker. A 60-amp feeder breaker calls for 6 AWG copper or 4 AWG aluminum, while a 100-amp feeder typically uses 2 AWG copper or 1/0 AWG aluminum. These sizes assume a short run with 60 or 75 degree C termination ratings; longer runs need upsizing for voltage drop.
The same feeder logic applies when a home generates its own power. Rooftop arrays and the choice between solar panels and solar shingles affect how much current a solar feed delivers, and many systems tie in through a dedicated breaker or a small sub-panel before connecting to the main service.
- A main breaker that can shut off the whole sub-panel from one handle, or a main lug design that relies on the feeder breaker upstream
- Branch breakers sized for individual circuits, from 15-amp lighting to 50-amp appliance and equipment circuits
- A neutral bar that carries the grounded conductor back to the main panel
- A grounding bar that ties all equipment grounds and the sub-panel enclosure to the system ground
Why You Might Need One
A sub-panel is worth considering when the main panel runs out of open slots, when a new area needs many circuits, or when a remote structure needs local control. It also shortens branch-circuit runs: instead of running every wire from a far corner of the house back to the main panel, you run one feeder to the sub-panel and distribute short branch circuits from there.
Interior projects are a good example. A coastal French dining space finished with painted wood panels and rattan accents can add a dozen or more new circuits for recessed lights, sconces, receptacles, and specialty fixtures. Feeding that load from a sub-panel keeps the main panel from becoming a tangle of tandem breakers and long homeruns.
Common Scenarios
- Finishing a basement or attic into living space
- Outfitting a garage or detached workshop with 240-volt tools
- Adding a home office with dedicated circuits for computers and equipment
- Serving an accessory dwelling unit, studio, or pool house
- Supporting an electric vehicle charger or a large heat pump
- Creating a local shutoff for a solar or battery system
When the Main Panel Is Full
A full main panel tempts some homeowners to install tandem breakers, which fit two circuits in one slot. Tandems are fine in panels that list them on the label, but they do not add capacity: the panel bus still has a fixed amperage rating, and cramming in more breakers can exceed the number of circuits the enclosure is designed to cool. A sub-panel adds physical breaker space without taxing the main bus, because the feeder breaker limits how much current the sub-panel can draw.
Panel Amp Capacities
Sub-panels are sold in standard sizes, with 30, 60, 100, and 200 amps the most common. The 100-amp sub-panel is the workhorse for additions and workshops: it feeds a whole finished area, supports 240-volt appliances, and still leaves room for future circuits. Smaller panels make sense for a single room or a modest shop where the load is light.
| Sub-panel rating | Typical feeder wire (copper) | What it typically feeds | Typical breaker count |
|---|---|---|---|
| 30 amp | 10 AWG | Lighting and receptacles for one finished room | 4 to 8 |
| 60 amp | 6 AWG | Finished basement, garage, or small shop | 8 to 12 |
| 100 amp | 2 AWG | Full addition, workshop with 240-volt tools | 12 to 24 |
| 200 amp | 2/0 AWG | Large shop, ADU, or heavy electric load | 24 to 40 |
Feeder sizes in the table assume a short run with 60 or 75 degree C terminations. Aluminum conductors one or two sizes larger are common on long feeders because they cost less, and every install should follow the ampacity tables in the code and the manufacturer’s instructions.
Capacity planning also matters for generating equipment. The output differences between solar panels and solar roof tiles change how many watts a roof can produce, which changes the size of the breaker and feeder that carry that power to the panel.
Matching the Feeder Breaker to the Panel
The feeder breaker size is set by the wire, and the sub-panel rating must equal or exceed that breaker. A 60-amp feeder with 6 AWG copper can feed a 100-amp sub-panel; the panel simply never uses its full rating. Upsizing the panel is a cheap way to leave room for future circuits without changing the feeder.
Voltage Drop on Long Runs
Distance is the hidden cost of a feeder. A 100-amp feeder running 150 feet shows noticeable voltage drop, which dims lights and stresses motors. The usual target is no more than 3 percent drop on the feeder and 5 percent total to the farthest outlet. For long runs, step up one wire size, or run 240 volts to the sub-panel and step down locally with transformers where equipment allows.
Installing a Sub-panel
A sub-panel install is a permitted electrical job in most jurisdictions, and the sequence is straightforward when the plan is right.
- Calculate the loads the sub-panel will serve, then size the feeder, breaker, and panel
- Pick a dry, accessible location with working clearance in front of the panel
- Run the feeder cable from the main panel, protecting it through studs and along approved paths
- Mount the sub-panel enclosure and secure the cable with an approved connector
- Terminate the feeder in the main panel on a dedicated 240-volt breaker
- Terminate the feeder in the sub-panel, keeping neutral and ground on separate bars
- Install and torque the branch breakers, then label every circuit
- Call for inspection before closing up the walls
The Four-Wire Feed Rule
The sub-panel must receive the grounded neutral and the equipment ground as separate conductors, and the neutral bar must not be bonded to the enclosure or ground bar. The bonding screw shipped with the panel gets removed or left out. When the neutral is bonded in a sub-panel, return current flows on the ground wire and metal conduit, energizing anything it touches.
Wall construction changes where those wires can go. In a space built with glass fibre reinforced gypsum panels, plan box locations and conduit paths before the panels are set, because cutting channels after installation is far more labor-intensive than running cable through open framing.
Permits and Inspection
Most building departments require a permit for a new sub-panel because it involves a feeder, new breakers, and a connection to the service. The inspector verifies bonding, wire sizes, torque values, working clearance, and proper labeling. Keep the panel schedule and load calculation on site; the inspector will often ask to see them.
Capacity Issues and Load Planning
The most common sub-panel problem is not the panel itself but the load placed on it. A standard residential load calculation counts general lighting at 3 volt-amps per square foot, adds 1,500 volt-amps for each small-appliance circuit, and includes fixed appliances at their nameplate ratings. Running that total against the panel and feeder rating shows whether the plan is realistic.
Layout decisions interact with the load plan. In larger additions where glazing panels and curtain wall construction bring in large expanses of glass, window placement determines where receptacles and dedicated circuits can be routed, so the electrical rough-in needs to be coordinated with the framing and glazing schedule rather than treated as an afterthought.
Signs the Sub-panel Is Overloaded
- A breaker that trips repeatedly under normal use
- A warm panel cover or a burning smell near the enclosure
- Lights that flicker when a large appliance starts
- Breakers that feel hot to the touch after a heavy load
- A panel that was already maxed out before the new equipment arrived
Done right, a sub-panel gives a growing home room to add circuits for years. Pairing it with on-site generation, whether you choose solar panels or solar shingles, eases demand on the feeder during peak hours and keeps critical circuits alive when the utility goes down.