A subpanel — also called a secondary panel, sub-panel, or load center — is a secondary electrical distribution box fed by your main panel. It’s a fundamental concept for any homeowner expanding their electrical system. Here’s everything you need to know.
Contents
⚡ Key Takeaways
- A subpanel extends your electrical capacity without replacing your main panel
- One double-pole breaker in the main panel feeds all circuits in the subpanel
- Subpanels are ideal for detached structures, basement buildouts, and full main panels
- Detached structure subpanels require their own grounding electrode system (ground rods)
- All subpanel work requires a licensed electrician, permits, and inspection
What Is a Subpanel?
A subpanel is a secondary electrical panel that receives power from the main panel via a feeder circuit — typically a double-pole breaker and heavy-gauge wire. Once installed, the subpanel distributes that power to multiple individual circuits, just like a main panel does. Think of it as a branch office: the main panel is headquarters, and the subpanel handles distribution in a specific area or structure.
Why Install a Subpanel?
Main Panel Is Full
If your main panel has no available breaker slots, adding a subpanel is often more cost-effective than replacing the entire main panel. One double-pole breaker slot in the main panel can feed a subpanel with 12, 20, or even 42 circuit slots.
Power to Detached Structures
Running power to a detached garage, workshop, shed, or guest house almost always requires a subpanel. You run one large feeder (lower cost per circuit than running individual circuits) from the main panel to a subpanel in the structure, then branch out to individual circuits from there.
Basement or Large Addition
A large finished basement or home addition with many circuits benefits from a local subpanel. This keeps circuit protection accessible in the area where the circuits are used, rather than requiring trips to the main panel on the opposite side of the house.
How a Subpanel Works
The subpanel is wired as follows:
- Feeder: Heavy-gauge cable (typically 4-conductor: two hots, one neutral, one ground) runs from the main panel to the subpanel
- Main panel connection: The feeder connects to a double-pole breaker in the main panel (sized for the subpanel’s capacity)
- Subpanel connection: The two hot wires connect to the main lugs or main breaker of the subpanel; neutral connects to the neutral bar; ground connects to the ground bar
- Individual circuits: Branch circuit breakers in the subpanel distribute power to outlets, lights, and appliances throughout the area
Critical: Neutral-Ground Bonding in Subpanels
This is one of the most important technical distinctions in subpanel installation. At the main panel, the neutral and ground bars are bonded together (connected). At a subpanel, they must be separated — the neutral bar and ground bar should not be bonded. This separation is required by NEC and is critical for safe operation of the electrical system.
Exception: In a detached structure where the subpanel is the first means of disconnect, the NEC has specific rules about neutral-ground bonding that differ from attached subpanels. A licensed electrician will apply the correct rule for your installation.
Sizing a Subpanel
Subpanel sizing depends on your intended loads:
- 60A subpanel: Appropriate for a small garage, workshop, or outbuilding with basic lighting, outlets, and a small appliance
- 100A subpanel: Suitable for a large garage, finished basement, or addition with HVAC, a Level 2 EV charger, and full room circuits
- 125A–200A subpanel: For large additions, home offices with significant equipment, or when future expansion is anticipated
Subpanel vs. Panel Upgrade: Which Do You Need?
A subpanel is the right choice when your main panel has available capacity (open slots and sufficient amperage) but you need more circuits in a specific area. A panel upgrade is needed when the main panel itself is at capacity, undersized (100A service in a modern all-electric home), or is an aging/recalled model. A licensed electrician can assess which approach is right for your home.
Frequently Asked Questions
What is the difference between a main panel and a subpanel?
The main panel receives power directly from the utility and is the primary distribution point for your home’s electrical circuits. A subpanel is a secondary distribution panel fed by the main panel via a feeder circuit. The main panel typically has a main breaker that disconnects all power; a subpanel may or may not have its own main breaker.
Do I need a subpanel or a panel upgrade?
A subpanel is appropriate when your main panel has capacity for a feed breaker but you need more circuits. A panel upgrade is needed when the main panel is physically full with no room for a feed breaker, or when your home’s service amperage is insufficient for current and planned loads. A licensed electrician can assess which is right for your situation.
Can I install a subpanel myself?
No. Subpanel installation involves your main electrical panel (where bus bars remain live even with the main breaker off), 240V feeder circuits, and critical grounding requirements set by the National Electrical Code (NFPA 70). Permits and inspection are required. Always hire a licensed electrician for subpanel work.
Does a subpanel need its own ground rod?
For subpanels in detached structures, yes — a separate grounding electrode system (typically two ground rods) is required. For subpanels within the same structure as the main panel, the grounding electrode system is shared via the ground conductor in the feeder. Your electrician will apply the correct requirements for your installation.
How much does a subpanel cost to install?
Subpanel installation in an attached location (same structure as main panel) typically costs $500–$1,500. Installation in a detached structure with underground feeder runs costs $1,500–$4,000+ depending on distance and site conditions. Get quotes from licensed electricians for accurate pricing.