Quick answer
A subpanel is a smaller breaker panel that gets its power from a breaker in the main electrical panel, adding circuit spaces in a new spot such as a garage, addition or detached workshop.
It does not add capacity to the home. Every amp the subpanel uses still passes through the main panel and the main breaker, so a load calculation decides whether one fits.
Quick facts
The key facts about Subpanel, with sources:
Key takeaways
- A subpanel is a branch of the main panel, fed by one large breaker and one set of feeder wires.
- It gives you more breaker slots and a shorter run to distant loads. It does not raise your service size.
- In a subpanel the neutral and ground bars stay separate; only the main service panel bonds them.
- A detached building fed by a subpanel needs its own grounding electrode.
- Whether a subpanel works depends on a load calculation, not on how many empty slots you need.
What a subpanel is
Think of the main electrical panel as the trunk of a tree. Each branch circuit is a twig. A subpanel is a full limb: one heavy feeder leaves the main panel and splits again into its own set of breakers somewhere else.
The feeder starts at a two-pole breaker in the main panel. That breaker protects the feeder wires and sets the most the subpanel can draw. A 60-amp feeder breaker means the subpanel can never pull more than 60 amps, even if its own bus is rated for 125.
Subpanels come in the same families as main panels. Most are main lug panels with no main breaker, because the feeder breaker upstream already does that job 3. Some use a main breaker anyway, so there is a local shutoff in the garage or shop.
The power path to a subpanel
- Meter
- → main breaker
- → main panel bus
- → two-pole feeder breaker
- → feeder: two hots, a neutral and a ground
- → subpanel bus
- → subpanel branch breakers → loads
How a subpanel works
Power arrives on the feeder. The two hot wires land on the subpanel's busbars or main lugs. The neutral lands on an insulated neutral bar. The ground wire lands on a separate ground bar that is bonded to the metal box.
That split matters. In the main service panel, neutral and ground are tied together at one point. Downstream of that point they must stay apart.
If they were joined again in the subpanel, normal return current would flow on ground wires, metal boxes and pipes.
Schneider Electric's load center instructions say that when the panel is used as a subpanel, the bonding screw is discarded 2.
Inside, the subpanel behaves like any panel. Each branch breaker protects its own circuit. The feeder breaker back at the main protects the whole subpanel.
- Two hot conductorsLand on the bus or main lugs
- NeutralIsolated neutral bar, not bonded
- Equipment groundGround bar bonded to the box
- Grounding electrodeNeeded when the subpanel is in a separate building
- Branch breakersOne per circuit served
Types of subpanels
| Type | What it is for | Notes |
|---|---|---|
| Main lug subpanel | More slots near the loads | No local shutoff; the feeder breaker is the disconnect |
| Main breaker subpanel | More slots plus a local shutoff | The main acts as a switch; the feeder breaker still sizes it |
| Detached building panel | Garage, shop, ADU or barn | Needs its own grounding electrode and a local disconnect |
| Critical loads panel | Circuits backed up by a battery or generator | Fed through a transfer switch or battery system |
| Outdoor (NEMA 3R) subpanel | Pool, hot tub or yard equipment | Rain-tight enclosure rated for outdoor use |
Example: a garage subpanel for an EV charger
An example of how a load enters the sizing. It uses figures from DOE's Alternative Fuels Data Center and the 2023 NEC text, not a quote or a design.
| Item | Figure | Source |
|---|---|---|
| Typical home Level 2 charger draw | Up to 30 amps, about 7.2 kW | DOE AFDC 5 |
| Circuit DOE describes for it | Dedicated 40-amp circuit | DOE AFDC 5 |
| Load the NEC 2023 standard calculation counts | 7,200 VA or nameplate, whichever is larger | NEC 220.57 4 |
| Level 1 charging, for comparison | About 1.9 kW from a 120 V outlet | DOE AFDC 5 |
So one charger adds about 7.2 kW to the home's calculated load whether it hangs on the main panel or a subpanel. The subpanel only moves the breaker closer to the car. If the main has no room for 7,200 VA, the fix is load management or a service upgrade, not a subpanel.
Where subpanels are used
Garages are the most common spot. They collect tools, freezers, chargers and sometimes a heater, all far from the main panel. Basements, additions and kitchens remodeled into new wings are next.
Detached buildings, such as a shop or electrified backyard unit, usually get a subpanel fed by an underground feeder. A full main panel with no open slots is the other big reason; a subpanel next to it can add spaces when tandem breakers are not allowed.
Why add a subpanel
- More spaces without replacing a full main panel.
- Shorter branch runs, which cuts voltage drop to far-off loads.
- A local shutoff in a shop or outbuilding if the subpanel has a main breaker.
- Tidy wiring: one feeder replaces many long home runs.
- Backup planning: a critical-loads subpanel groups the circuits a battery or generator will carry.
What a subpanel can and cannot fix
A subpanel can fix
- A full panel with no free slots.
- Long runs to a garage or outbuilding.
- The need for a local shutoff in a work area.
A subpanel cannot fix
- A home whose calculated load is already near the service size.
- An old or recalled main panel; the subpanel still depends on it.
- A main panel with no space for the feeder breaker itself.
What drives subpanel cost
We do not list a price, because local labor, permits and materials vary too much. The cost drivers are predictable, though.
Feeder length and size. A long run to a detached building, especially underground, often costs more than the panel. Bigger amps mean thicker copper or aluminum. Panel size. More spaces and a main breaker add cost. Main panel condition.
If the main has no room or is a known problem brand, it may need work first. Grounding. A separate building needs its own electrode system. Permit and inspection fees vary by city.
Panels carry a maker's warranty; labor is covered by the electrician's terms. For ranges, use the project cost estimator or see electrical cost guides.
How a subpanel is installed
A licensed electrician does this work. The steps show what to expect.
- A load calculation checks that the service can carry the new loads.
- The electrician picks the feeder breaker size, wire size and panel size.
- A permit is pulled where the city requires it.
- Power is shut off and the feeder breaker is installed in the main panel.
- The feeder is run in cable or conduit; for a separate building, a grounding electrode is installed there.
- The subpanel is mounted with neutral and ground kept apart, then branch circuits are added.
- An inspector checks it before walls are closed or the trench is filled.
Keeping a subpanel safe
- Keep the front clear; garages tend to fill up with shelves and bikes.
- Keep the door closed so dust and sawdust stay out.
- Label each breaker so you know what it feeds.
- Watch for rust in damp garages or outbuildings.
- Test any GFCI breakers in the subpanel monthly with the TEST button.
A wrongly bonded subpanel can shock
Safety and risk
If neutral and ground are tied together in a subpanel, normal current can flow on metal boxes, pipes and appliance frames. That can shock someone or start a fire.
You cannot see this from outside. Never open a panel; if you get a tingle from a tool, appliance or pipe, stop using it and call an electrician.
Warning signs in a subpanel
- The feeder breaker in the main panel trips.
- Lights in the garage or shop dim when a tool starts.
- The subpanel cover feels warm or smells hot.
- A tingle from a metal tool, freezer or water pipe.
- A subpanel added by a past owner with no permit record. Ask for an electrical inspection.
Code rules for subpanels
Subpanels follow the National Electrical Code (NFPA 70) as adopted by your state or city. As of September 2026, 19 states had adopted the 2023 NEC in some form and six the 2026 edition 7.
Feeder sizing. Feeders are sized from a load calculation under NEC Article 220, the same way services are 6. EV loads. The 2023 NEC 220.57 counts EV charging equipment at 7,200 VA or the nameplate rating, whichever is larger 4.
Grounding. Downstream panels keep the neutral isolated; the manufacturer directs that the bonding screw be discarded when a load center is used as a subpanel 2.
A building supplied by a feeder needs its own grounding electrode under NEC 250.32, unless only a single branch circuit with an equipment ground serves it 1. Listing. Use only breakers listed for that panel.
Subpanel vs other fixes
| Option | Adds slots | Adds capacity | Best for |
|---|---|---|---|
| Subpanel | Yes | No | Full panel or distant loads with service room left |
| Tandem breakers | A few, where listed | No | One or two extra circuits in a panel that allows them |
| Panel and service upgrade | Yes | Yes | Homes short on capacity |
| EV load management | No | Shares existing capacity | EV charging on a tight service |
| Backup subpanel | Yes | No | Backup power for chosen circuits |
Subpanel myths
- Myth A subpanel gives the house more power.
- Reality All of its power still comes through the main breaker. Capacity only grows with a service upgrade.
- Myth A 100-amp subpanel can draw 100 amps.
- Reality It can draw only what its feeder breaker allows.
- Myth Neutral and ground are the same wire, so joining them is fine.
- Reality They are bonded only at the service. Joining them again downstream puts current on metal parts 2.
- Myth A garage subpanel does not need a permit.
- Reality Most cities require a permit for a new feeder and panel. Check before work starts.
What changes by state
Which NEC edition your state uses decides details such as the EV load rule in 220.57, which first appeared in the 2023 edition 4. States also set who may install a feeder and panel, and some allow homeowner permits while others do not. Look up your state before planning.
Is a subpanel right for you?
Start with the numbers. Run the load calculator with the loads you plan to add, then check your free slots with the panel space tool.
- If You are out of slots but have capacity left
- Then A subpanel or listed tandem breakers
- If Your calculated load is near the service size
- Then A service upgrade or load management
- If You need power in a detached building
- Then A subpanel with its own grounding electrode
- If Your main panel is a recalled or problem model
- Then Fix or replace the main panel first
Next step
Bring your load numbers and a photo of the panel label to an electrician. Those two things answer most subpanel questions on the first visit.
Questions about Subpanel
What is a subpanel used for?
A subpanel adds breaker spaces in a new location, such as a garage, basement, addition or detached shop. It is fed by one breaker in the main panel. It shortens wiring runs and can give a local shutoff. It does not add capacity, so the main service must still have room for the new loads.
Does a subpanel increase my amperage?
No, a subpanel does not increase the amps available to your home. Every amp it uses passes through the main panel and the main breaker. It only adds spaces and moves breakers closer to the loads. To add capacity you need a service upgrade, or load management that shares what you have.
Why are neutral and ground separated in a subpanel?
Neutral and ground are separated in a subpanel so normal return current stays on the neutral wire. They are joined only once, at the main service.
If joined again downstream, current would also flow on ground wires, metal boxes and pipes, which can shock someone. Manufacturers direct that the bonding screw be left out of a subpanel.
How many amps should my subpanel be?
The right size comes from a load calculation of what the subpanel will feed. The feeder breaker in the main panel sets the real limit, and the subpanel bus must be rated at least that high. An electrician sizes the breaker, wire and panel together so they match the loads and the service.
Do I need a subpanel for an EV charger?
Not usually. A Level 2 charger needs one dedicated circuit, which can often go in the main panel. A subpanel helps when the main is out of slots or the garage is far away. It does not help if the service has no capacity left; then load management or an upgrade is the fix.
Does a detached garage subpanel need a ground rod?
Yes. Under NEC 250.32, a separate building fed by a feeder needs its own grounding electrode system, such as ground rods. The feeder still brings an equipment grounding wire from the main panel, and neutral stays separate from ground in the subpanel. An inspector checks this before the trench is filled.
Can a subpanel have a main breaker?
Yes. A subpanel can have a main breaker that works as a local shutoff. It is common in garages and outbuildings. The feeder breaker in the main panel still protects the feeder and limits how much the subpanel can draw. When used this way, the panel is not bonded like a service panel.
Is it safe to add a subpanel to a full panel?
Adding a subpanel to a full panel is safe if the main panel has room for the feeder breaker and the service has capacity for the new loads. An electrician checks both.
If the main panel is a recalled or problem model, it may need replacing first. A load calculation is the starting point.
Note that the feeder breaker itself needs two spaces in the main panel, so a panel with zero free slots may need circuits combined or moved first.
Do I need a permit for a subpanel?
Most US cities and counties require a permit for a new subpanel and feeder. The inspection checks wire size, breaker size, grounding and the neutral-ground separation.
Rules differ by place, so check with your local building department or ask your electrician to pull the permit.
A permit record also helps at resale, because buyers' inspectors often ask about panels added after the house was built.
Sources
- IAEI Magazine, "Two Buildings: Common Service, Grounding Requirements" (NEC 250.32), retrieved .
- Schneider Electric FAQ FA118284, main breaker kit instructions (service vs subpanel bonding), retrieved .
- Schneider Electric FAQ FA293376, selecting a load center: main lugs vs main breaker, retrieved .
- Leviton Captain Code 2023, NEC 220.53 and 220.57 EVSE load calculations, retrieved .
- U.S. DOE Alternative Fuels Data Center, Charging Electric Vehicles at Home and Work (electricity stations), retrieved .
- IAEI Magazine, "Residential Service Calculations in the National Electrical Code" (2013), retrieved .
- EC&M, "Wisconsin Adopts 2023 National Electrical Code" (9 September 2026), retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Bonding and panel-type facts from Schneider Electric instructions; EV figures from DOE AFDC and the 2023 NEC 220.57 text via Leviton; adoption counts from EC&M. No prices published (G-COST).
Suggest a correction. We fix errors and say what changed.