A subpanel is a secondary breaker box fed by a main service panel to distribute power to a specific area or outbuilding without overloading the main bus. When wiring a subpanel, you change your electrical topology from running a dozen individual branch circuits hundreds of feet back to the main house to running one heavy-gauge feeder that breaks out locally. Beginners frequently confuse a subpanel with a main service panel (missing the critical neutral-to-ground bonding difference) or a large junction box (which merely splices wires and lacks overcurrent protection).
The Core Theory: Feeders, Bonding, and Isolation
The defining theoretical concept of a subpanel is the strict isolation of the neutral (grounded) conductor and the equipment grounding conductor. In your main service panel, the neutral and ground buses are bonded together, and the system is grounded to earth via ground rods or a ufer ground. This is the only place in a standard residential system where this bond should exist.
When you run a feeder to a subpanel, modern NEC Article 250.32 requires a 4-wire setup: two ungrounded (hot) conductors, one grounded (neutral) conductor, and one equipment grounding conductor. The neutral and ground buses in the subpanel must remain physically and electrically isolated.
Worked Example: Sizing a 100A Garage Subpanel Feeder
Let us calculate the feeder size for a detached garage subpanel located 120 feet from the main house panel. The garage will host a 40A continuous EV charger, a 15A table saw, and 20A of general lighting and receptacles. We will size for a 100A feeder breaker.
First, we evaluate wire ampacity based on the 75°C column of NEC Table 310.16 (standard for most residential terminals):
- Option A (Copper): 3 AWG Copper THHN/XHHW is rated for 100A at 75°C.
- Option B (Aluminum): 1 AWG Aluminum XHHW is rated for 100A at 75°C. (Note: 2 AWG Aluminum is only rated 90A and cannot be used on a 100A breaker without specific 100% rated equipment, which residential breakers are not).
Next, we must check voltage drop. The NEC recommends a maximum of 3% voltage drop on a feeder. We will test both options assuming an 80A continuous load (the EV charger drawing max current). The formula is: VD = (2 × K × I × D) / CM.
Testing Option A: 3 AWG Copper
- K (Copper) = 12.9
- I (Current) = 80A
- D (Distance) = 120 ft
- CM (Circular Mils for 3 AWG) = 52,620
VD = (2 × 12.9 × 80 × 120) / 52,620 = 4.71V
Percentage: 4.71V / 240V = 1.96%. Passes easily.
Testing Option B: 1 AWG Aluminum
- K (Aluminum) = 21.2
- CM (Circular Mils for 1 AWG) = 83,690
VD = (2 × 21.2 × 80 × 120) / 83,690 = 4.86V
Percentage: 4.86V / 240V = 2.02%. Passes easily.
(2 × 21.2 × 80 × 120) / 66,360 = 6.12V (2.55%). While technically under 3%, you leave almost no margin for startup surges from the table saw while the EV is charging. Always upsize aluminum feeders for long runs.
Where You Meet This in Practice
You will encounter subpanel requirements when expanding a home's electrical footprint beyond what the main panel's physical spaces or bus rating can handle. The most common scenarios include detached garages, finished basements, backyard ADUs (Accessory Dwelling Units), and heavy-load additions like EV chargers or hot tubs.
| Application | Typical Load Profile | Minimum Panel Bus | Feeder Breaker | Recommended Wire (Cu/Al) |
|---|---|---|---|---|
| Shed / Basic Workshop | Lights, hand tools, battery chargers | 60A | 60A | 6 AWG Cu / 4 AWG Al |
| Finished Basement | Bedrooms, bathroom GFCI, AV equipment | 100A | 90A or 100A | 3 AWG Cu / 1 AWG Al |
| Detached Garage + EV | EV Charger (40-50A), welder, compressor | 125A | 100A or 125A | 2 AWG Cu / 1/0 AWG Al |
| Backyard ADU / Studio | Kitchen, HVAC, laundry, lighting | 125A or 200A | 125A+ | 1/0 AWG Cu / 2/0 AWG Al |
The Decision Path: Picking Your Subpanel Hardware
Choosing the right physical panel and feeder breaker requires matching the bus rating to your calculated load, while ensuring the feeder breaker protects the wire. Follow this decision path to lock in your hardware:
- IF you are powering a simple shed with under 40A of expected simultaneous load, THEN buy a 60A main-lug panel and feed it with a 60A breaker using 6 AWG Copper THHN in conduit.
- IF you are adding an EV charger and heavy power tools in a garage, THEN buy a 100A or 125A main-lug panel and feed it with a 100A breaker using 3 AWG Copper or 1 AWG Aluminum.
- IF the subpanel is located inside the same building as the main panel (e.g., a basement finish) and is within sight of the main, THEN you do not need a main disconnect switch at the subpanel; a standard main-lug panel is sufficient.
- IF the subpanel is in a detached structure, THEN NEC 225.32 requires a local disconnecting means. You must either use a panel with a main breaker built-in, or install a separate disconnect switch outside the structure.
The Concrete Pick: For 90% of residential garage and basement projects, the Square D Homeline 100-Amp 12-Space Main Lug Panel (Model HOM12L100PGC) is the default choice. It accepts the standard 1-inch Homeline breakers, provides enough spaces for a 4-circuit EV setup plus lighting, and the 100A bus rating perfectly matches standard residential feeder breakers without requiring expensive upsizing.
Common Mistakes and Code Caveats
Can I reuse an old main panel as a subpanel?
Yes, but you must remove the main bonding jumper (the green screw or metal strap connecting the neutral bar to the panel chassis). You must also install a separate, add-on equipment grounding bar kit (like the Square D HOM816L) to keep the ground wires isolated from the neutral bar. If you cannot find the bonding screw or the add-on ground bar, buy a new panel designed specifically as a subpanel.
Do I need a ground rod for a detached garage subpanel?
Yes. While the equipment grounding conductor runs back to the main panel, NEC 250.32(B) requires a grounding electrode system (typically two 8-foot copper ground rods spaced 6 feet apart, or a single ufer ground in the concrete footer) at the detached structure. This protects the building from lightning strikes and utility line surges, but it does not replace the need for the 4-wire feeder.
What happens if I use a 3-wire feeder?
Prior to the 2008 NEC cycle, a 3-wire feeder (two hots and a neutral, with the neutral bonded to ground at the sub) was legal for detached buildings if there were no continuous metallic paths (like copper water pipes) between the buildings. This is no longer permitted for new installations. If you are wiring a new subpanel today, you must pull 4 wires. If you are troubleshooting an existing 3-wire setup in an older home, it is 'grandfathered' in, but upgrading to 4-wire is highly recommended for safety.
Does the subpanel breaker need to match the panel's bus rating?
No. The feeder breaker in the main panel protects the wire, not the subpanel bus. You can safely feed a 125A-rated subpanel bus with a 100A breaker and 3 AWG wire. The rule is that the panel's bus rating must be equal to or greater than the feeder breaker supplying it. Never feed a 60A rated panel bus with a 100A breaker.
