Subpanel Wiring Diagram: How to Wire a Subpanel from Main Panel

A subpanel (also called a sub-panel or subdistribution panel) extends your home's electrical system to a garage, workshop, addition, or any area far from the main panel. Instead of running individual circuits back to the main panel, you run a single large feeder to the subpanel and distribute circuits locally. This guide covers subpanel sizing, feeder calculations, NEC requirements, and step-by-step wiring.

When Do You Need a Subpanel?

• No more breaker slots in the main panel -- a subpanel provides additional circuits
• Remote buildings (garage, shed, barn) -- avoids voltage drop from long individual runs
• Home additions -- adds capacity for new rooms
• Workshops -- centralizes circuit protection near the tools
• EV charger + workshop -- combines multiple new circuits in one location

Choosing the Right Subpanel Size

Amperage Rating

The subpanel amperage should match your current and future needs:

Number of Spaces

Choose a subpanel with more spaces than you currently need:

• 12-space: Minimum for a garage (holds 6 double-pole or 12 single-pole breakers)
• 20-24 space: Recommended for workshops and additions
• 30+ space: For large additions with many circuits

Main Breaker vs Main Lug

• Main breaker subpanel: Has a master breaker in the subpanel that serves as the local disconnect. Required for detached buildings (NEC 225.31).
• Main lug subpanel: No master breaker -- the feeder breaker in the main panel serves as the disconnect. Acceptable for subpanels in the same building as the main panel.

Feeder Wire Sizing

The feeder cable from the main panel to the subpanel must be sized for the subpanel's amperage:

Aluminum vs Copper

Aluminum feeder wire is commonly used for subpanel feeders because:

• Much less expensive than copper for large gauges
• Lighter and easier to pull
• Perfectly safe when installed correctly with anti-oxidant compound and proper connectors
• Must use connectors rated for aluminum (marked AL-CU)

Voltage Drop for Long Runs

For feeders longer than 50 feet, check voltage drop. NEC recommends maximum 3% drop on branch circuits and 5% total (feeder + branch):

For runs over 100 feet, consider upsizing the feeder wire by one gauge.

Feeder Cable Options

NM-B (Romex) -- Interior Runs Only

• For runs through interior walls, attic, or crawl space within the same building
• Available in 6/3, 4/3, 3/3 with ground
• Not suitable for outdoor, underground, or exposed runs

SER (Service Entrance Round) Cable

• For exposed or concealed interior runs
• Available in sizes suitable for subpanels
• Must be protected from physical damage

Individual THHN/THWN Wires in Conduit

• Most versatile option
• Required for underground and exposed runs
• Can use aluminum wire (significant cost savings)
• Conduit protects wires and allows future upgrades
• Typical conduit: 1-1/4 inch PVC for 100A aluminum

UF-B (Underground Feeder)

• Direct burial rated (24-inch minimum depth)
• For runs to detached buildings without conduit
• More difficult to pull than individual wires in conduit

Step-by-Step Subpanel Installation

Step 1: Turn Off Main Power

Turn off the main breaker at the main panel. Verify with a voltage tester that no power is present on the bus bars. Note that the utility feed lugs above the main breaker remain energized -- do not touch them.

Step 2: Install the Feeder Breaker

Install the appropriately sized double-pole breaker in the main panel. This breaker protects the feeder cable.

Step 3: Run the Feeder Cable

Route the feeder cable from the main panel to the subpanel location:

• Through walls, attic, or crawl space for interior runs
• Through conduit for exposed, underground, or exterior runs
• Secure the cable every 4.5 feet with appropriate supports
• Leave sufficient wire length at each panel for connections (at least 6 inches beyond the panel opening)

Step 4: Mount the Subpanel

• Mount at a comfortable working height (panel center between 54 and 66 inches from the floor)
• Ensure at least 36 inches of clear working space in front of the panel (NEC 110.26)
• Mount securely to studs or a plywood backer

Step 5: Connect at the Main Panel

1. Strip the outer jacket from the feeder cable
2. Connect the black wire to one breaker terminal
3. Connect the red wire to the other breaker terminal
4. Connect the white (neutral) wire to the neutral bus bar
5. Connect the green/bare (ground) wire to the ground bus bar

Step 6: Connect at the Subpanel

This is where the critical neutral-ground separation happens:

1. Connect the black wire to one main lug (or main breaker terminal)
2. Connect the red wire to the other main lug (or main breaker terminal)
3. Connect the white (neutral) wire to the NEUTRAL bus bar (insulated from the panel enclosure)
4. Connect the green/bare (ground) wire to the GROUND bus bar (bonded to the panel enclosure)

CRITICAL: Do NOT bond the neutral and ground bus bars in the subpanel. Remove the bonding screw or strap if one is installed. The neutral-ground bond exists ONLY at the main panel (or service entrance).

Step 7: Grounding Electrode (Detached Buildings)

If the subpanel is in a detached building (garage, shed, barn):

1. Drive an 8-foot ground rod into the earth near the building
2. Run a #6 AWG (or #4 AWG for 200A) bare copper grounding electrode conductor from the ground rod to the subpanel ground bar
3. This is in ADDITION to the equipment ground in the feeder cable

Step 8: Install Branch Circuit Breakers

Install breakers in the subpanel for each branch circuit:

• 15A single-pole for lighting (14 AWG)
• 20A single-pole for outlets (12 AWG)
• 20A or 30A double-pole for 240V loads (12 or 10 AWG)
• 50A double-pole for EV charger (6 AWG)

Step 9: Run Branch Circuits

Run NM (Romex) cable from the subpanel to each outlet, light, and device:

1. Connect hot wire(s) to the breaker
2. Connect neutral to the neutral bus bar
3. Connect ground to the ground bus bar

Step 10: Inspection

Have the work inspected by your local building department. This is typically required for subpanel installations.

Neutral-Ground Separation: Why It Matters

In the main panel, neutral and ground are bonded together. This establishes the ground reference point for the entire electrical system.

In a subpanel, they must be separate because:

• If they were bonded, neutral current would flow on both the neutral wire AND the ground wire back to the main panel
• Current on the ground wire is dangerous because it energizes metal parts that should be at zero potential
• It also creates a shock hazard and can interfere with GFCI/AFCI protection

If you have a subpanel with a bonded neutral-ground, remove the bonding screw or strap. This is one of the most common subpanel installation errors.

Common Subpanel Wiring Mistakes

1. Bonded neutral and ground in subpanel: The number one mistake. Keep them separate.
2. Undersized feeder wire: Size the feeder for the subpanel rating, not just current loads.
3. No disconnect for detached building: A detached building subpanel needs a main breaker or disconnect switch.
4. No ground rod for detached building: Detached buildings need their own grounding electrode.
5. Too few spaces: Choose a subpanel with more spaces than you need now.
6. Wrong cable type: Use the correct cable for the installation method (NM for interior, UF-B for direct burial, THHN in conduit for conduit runs).
7. Exceeding main panel capacity: The sum of all breakers can exceed the main panel rating (NEC allows this because all circuits are never fully loaded simultaneously), but the feeder breaker must not exceed the bus bar rating minus existing load capacity.

Creating Subpanel Wiring Diagrams

Plan your subpanel installation with CircuitDiagramMaker. Draw the main panel, feeder cable, subpanel, and all branch circuits. Label wire gauges, breaker sizes, and the neutral-ground separation. Export as a PDF for your permit application and installation reference.

Use the AI circuit generator -- describe "100 amp subpanel in garage with EV charger, outlets, and lighting circuits" for a complete diagram.

Conclusion

A subpanel is the most efficient way to add multiple circuits to a garage, workshop, or addition. The key points are: size the feeder wire for the subpanel rating, keep neutral and ground separate in the subpanel, add a grounding electrode for detached buildings, and get the work inspected. With proper installation, a subpanel provides reliable, expandable electrical service for decades.

Design subpanel wiring diagrams with CircuitDiagramMaker -- free online tool with panel, breaker, and circuit symbols.