A garage subpanel can solve a real power problem. It gives you room for an EV charger, workshop tools, lighting, a freezer, door openers, and future circuits without overloading an old main panel. But if you are searching how to wire garage subpanel information, understand the dividing line: planning the work is useful for a homeowner, while installing a 120/240-volt feeder and panel is safety-critical electrical work that should be designed, permitted, and completed by a licensed electrician.
A bad subpanel installation may appear to work for years. Then a loose aluminum connection, overloaded feeder, shared neutral problem, missing grounding electrode, or neutral-to-ground bond in the wrong location creates shock, fire, or equipment damage risk. This is especially relevant in Oakland, Berkeley, Piedmont, and other East Bay neighborhoods with older service equipment and crowded panels.
Start With the Load, Not the Panel Size
The first question is not, “What size subpanel should I buy?” It is, “What will the garage actually use?” A panel with 24 or 30 spaces can be a good choice for future capacity, but the feeder breaker and feeder conductors must be sized for the calculated load, not for the number of breaker spaces.
A basic garage with lights, receptacles, and a door opener may need relatively little capacity. A garage with a 40- or 48-amp EV charger, air compressor, welder, dust collector, mini-split, or electric heat is a different project. EV charging is a continuous load, meaning the electrical system must be sized using the applicable continuous-load rules. A charger is not just “one more outlet.”
The existing main service must also have enough capacity. A 100-amp service in an older home can be near its practical limit before anyone adds an EV charger, electric appliances, air conditioning, or a substantial garage feeder. In that situation, a garage subpanel may require a complete service load calculation and possibly a 200-amp PG&E service and panel upgrade first.
How to Wire a Garage Subpanel: The Correct System
For a modern detached garage, the normal arrangement is a four-wire feeder from the main service panel or another properly sized upstream panel. That feeder contains two hot conductors, one insulated neutral conductor, and one equipment grounding conductor.
The two hot conductors supply 240 volts for equipment that needs it and 120 volts when used with the neutral. The neutral carries the unbalanced load from 120-volt circuits. The equipment grounding conductor is there for fault clearing and bonding metal equipment. These conductors have different jobs and cannot be treated as interchangeable.
At the garage subpanel, the neutral bar must be isolated from the metal enclosure. The equipment grounding bar is bonded to the enclosure. The green bonding screw or bonding strap that may come with a panel is used only where the neutral is supposed to be bonded, typically at the service disconnecting equipment. It is not installed in a downstream subpanel.
That separation is one of the most common and most dangerous mistakes found in garage panels. If the neutral and ground are bonded together in a subpanel, normal return current can travel on metal conduit, grounding conductors, appliance frames, and other metal paths. The system may still power lights and tools, but it is not wired correctly.
Detached Garages Usually Need Grounding Electrodes
A detached garage generally needs its own grounding electrode system, commonly ground rods, connected to the garage panel’s equipment grounding system. This requirement does not replace the four-wire feeder. Ground rods do not serve as a substitute for an equipment grounding conductor run with the feeder.
Grounding rules can vary based on the building, existing electrodes, metallic water piping, concrete-encased electrodes, and local code interpretation. An electrician should verify the actual site conditions before deciding how the electrode system will be installed.
Choose the Feeder Route and Wiring Method Carefully
The route from the house to the garage determines much of the job. An attached garage may allow a feeder through framed walls, an attic, basement, crawlspace, or surface-mounted conduit. A detached garage often requires underground conduit and a trench at the required depth.
Conduit must be sized for the conductors, bends, pulling distance, and future serviceability. Underground work also requires the right raceway type, properly rated conductors, protection where conduit emerges from grade, and careful planning around water, gas, communications, sewer, and landscape systems. Call-before-you-dig requirements are not optional.
Wire size depends on the feeder breaker rating, conductor material, insulation rating, terminal ratings, ambient conditions, conduit fill, and derating factors. Copper and aluminum conductors are both used in feeder work, but aluminum must be properly sized, listed for the terminals, prepared as required, and torqued to manufacturer specifications. Loose or incorrectly terminated aluminum is a known source of overheating.
Do not size a feeder by copying a forum post or matching what a neighbor installed. The right answer depends on the panel equipment and the specific job.
Install a Proper Disconnecting Means
A garage needs a clear way to disconnect power to the building. In many subpanel installations, a main-breaker panel serves this purpose even though that breaker is fed from a breaker at the main panel. A main-lug-only panel may be acceptable in some configurations, but it depends on the installation and local requirements.
The panel should be mounted where it is accessible, protected from physical damage, and compliant with required working clearances. It should not be buried behind storage shelves, parked vehicles, or piles of boxes. Garages become cluttered quickly, which is one reason panel location matters.
The panel should also have enough room for the circuits the garage needs now and later. Typical garage circuits may include lighting, general-purpose receptacles, a dedicated refrigerator circuit, door openers, a 240-volt tool circuit, a mini-split, and EV charging. A panel packed full on day one is not a good long-term plan.
Use the Right Breakers and Garage Protection
Modern garage wiring requires more than ordinary circuit breakers. Garage receptacles generally require GFCI protection because the space is often damp, connected to concrete floors, and used with portable tools. Depending on the circuit and current code requirements, AFCI protection may also apply.
A qualified electrician will determine whether protection belongs at the breaker, the first receptacle, or both, and will make sure the devices are compatible with the panel. Multi-wire branch circuits, shared neutrals, and GFCI or AFCI breakers require special care. Incorrect neutral connections cause nuisance tripping and can conceal a more serious wiring error.
Any 240-volt receptacle or hardwired equipment needs a correctly sized breaker, conductors, receptacle or disconnect, and equipment rating. A Tesla or other EV charger circuit must be based on the charger’s nameplate rating and installation instructions. Never install a larger breaker simply because a smaller one trips. Breaker tripping is a warning that needs diagnosis.
Permits, Inspections, and Older East Bay Homes
A garage subpanel is usually permit work. The permit and inspection process confirms that the feeder is protected, grounding and bonding are correct, wire sizing is appropriate, and the installation meets current code. It also creates useful documentation for future buyers, landlords, insurers, and real estate transactions.
Older homes add complications. Federal Pacific/Stab-Lok panels, Zinsco panels, fuse boxes, damaged bus bars, knob-and-tube wiring, and undersized services can make it unsafe or impractical to add a garage feeder without first correcting the main electrical system. A new subpanel cannot compensate for a failing main panel.
Before the work starts, an electrician should inspect the main panel, service rating, available breaker capacity, grounding system, feeder route, garage structure, and intended loads. This avoids the expensive surprise of trenching a route only to find that the existing service cannot support the new equipment.
When to Call an Electrician
Homeowners can help by listing planned garage equipment, clearing access to the main panel and garage, and deciding where charging equipment or shop tools will go. The electrical design, feeder installation, panel termination, torque verification, testing, and inspection correction work belong with a licensed professional.
Williams Electric handles garage subpanels, EV charging circuits, panel upgrades, grounding corrections, and difficult older-panel issues throughout the East Bay. The practical goal is not merely to get power into the garage. It is to install capacity that is safe, code-compliant, serviceable, and ready for the next improvement you make to the property.
