Sub Panel: What It Is, When You Need One, and How to Wire It

A sub panel is a second breaker panel fed from the main one. A single breaker in the main panel — the feeder breaker — supplies a set of conductors running to the sub panel, where they're split into new branch circuits.

It's the standard answer to a very common problem: a panel that's physically full but electrically fine. A sub panel adds breaker positions, not service capacity — that distinction is the single most important thing to understand before buying one.

Sub panel at a glance

What it solves, and what it doesn't

Solves: no free breaker positions. A 60-amp feeder breaker occupying two slots in the main panel can supply a subpanel with a dozen new circuit positions. Net gain: ten circuits.

Doesn't solve: insufficient service capacity. Everything on the subpanel still flows through the main breaker. If your calculated demand is already close to your service rating, a subpanel changes nothing about that — you need a service upgrade or load management.

Which problem you have comes from a load calculation, not from counting slots. Run it with the home electrical load calculator, and see Electrical panel upgrade for the full decision.

When do you need a sub panel?

• Finished basement or attic conversion — a cluster of new circuits in one area
• Detached garage or workshop — one feeder out to the building instead of many individual circuits
• Home addition — new circuits local to the new space
• ADU or in-law suite — often required to keep the unit's circuits together
• A full main panel in a home that doesn't need more service

The economics favor it whenever several new circuits are going to one area. Running one feeder is far less work than running six branch circuits back to a main panel across a finished house.

The neutral-ground separation rule

This is the technical point that makes subpanels different, and the most common defect found in DIY installations.

In your main panel — at the service disconnect — the neutral and the grounding system are bonded together. That single bond is what gives fault current a low-impedance path back to the source so a breaker trips.

In a subpanel, they must be kept separate:

• The neutral bar is isolated from the enclosure — no bonding screw, no bonding strap.
• Grounds land on a separate bar that is bonded to the enclosure.
• The feeder runs four wires: two energized conductors, a neutral, and a separate equipment grounding conductor.

Why it matters: bonding neutral and ground a second time creates parallel paths for normal return current. Some of the current your appliances return every day would then flow on grounding conductors and on metal enclosures, conduit, and pipes. That's not a fault condition — it's continuous, and it's exactly what the grounding system is supposed to be free of so it can do its actual job during a fault.

Older installations sometimes used three-wire feeders with a bonded subpanel neutral. That approach is no longer permitted for new work, including for detached buildings, which now require an equipment grounding conductor run with the feeder.

Detached buildings

A subpanel in a separate structure adds two considerations:

• Grounding electrode. A detached building supplied by a feeder generally requires its own grounding electrode system — typically ground rods — bonded to the equipment grounding conductor at the subpanel.
• A disconnecting means. There must be a way to disconnect the supply at or in the separate building.

Plus, over a long run, voltage drop may drive the conductor size rather than ampacity. A 100-foot run to a garage frequently needs conductors larger than the ampacity table alone would suggest.

What size feeder does a sub panel need?

Two steps, in this order:

1. Calculate the load the subpanel will carry, the same way you'd calculate any feeder.
2. Size the conductors to support the feeder breaker, applying derating for temperature and conductor count.

The feeder breaker in the main panel is what protects the feeder conductors, so those conductors must be rated for it — see what size breaker do I need and the wire and breaker size chart.

A subpanel also needs its own main breaker or main lugs, and whether a main breaker is required depends on the installation — a detached building generally needs a disconnecting means, and some jurisdictions require one regardless.

How much does a sub panel cost?

A subpanel is nearly always cheaper than a service upgrade, because it doesn't involve the utility, the meter base, the service conductors, or the mast. The variables are the feeder run length, whether the route goes through finished space, and how many branch circuits you're adding on the other end.

Compare against electrical panel upgrade cost to see what you're avoiding.

The typical hardware

The standard residential subpanel is a 125 A main-lug load center — 12 spaces / 24 circuits covers a garage, shop or addition with room to grow. The reference part:

Feeder sizing, grounding separation and the main-lug vs main-breaker choice are electrician territory — this is a permitted job. Receptacles and breakers for the new panel are in the outlet picks.

Code reference

The requirements behind this guide, for looking up in your adopted edition:

• NEC 250.24(A)(5) — grounded conductor bonded only at the service
• NEC 408.40 — panelboard grounding, the basis for separating neutrals and grounds in a subpanel

Code editions and local amendments vary by jurisdiction. Confirm the adopted edition with your AHJ before relying on any specific article.

Where to go next

• Electrical panel upgrade
• Tandem breakers explained — the other answer to a full panel
• What size breaker do I need

More in our electrical panel guides.