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:
| Application | Recommended Size | Typical Use |
|---|---|---|
| Basic garage (lights + outlets) | 60A | 4-6 circuits |
| Workshop garage | 100A | 8-12 circuits, includes 240V tools |
| Home addition | 100A | Full rooms with HVAC |
| Heavy workshop + EV | 125A-200A | Welder, compressor, EV charger |
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:
| Subpanel Rating | Copper Wire | Aluminum Wire | Feeder Breaker |
|---|---|---|---|
| 60A | 6 AWG | 4 AWG | 60A double-pole |
| 100A | 3 AWG | 1 AWG | 100A double-pole |
| 125A | 2 AWG | 1/0 AWG | 125A double-pole |
| 200A | 2/0 AWG | 4/0 AWG | 200A double-pole |
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):
| Subpanel / Wire | 50 ft | 75 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 100A / 3 AWG Cu | 1.9% | 2.9% | 3.8% | 5.7% |
| 100A / 1 AWG Al | 1.9% | 2.8% | 3.8% | 5.6% |
| 100A / 1/0 AWG Al | 1.5% | 2.2% | 3.0% | 4.5% |
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
- Strip the outer jacket from the feeder cable
- Connect the black wire to one breaker terminal
- Connect the red wire to the other breaker terminal
- Connect the white (neutral) wire to the neutral bus bar
- 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:
- Connect the black wire to one main lug (or main breaker terminal)
- Connect the red wire to the other main lug (or main breaker terminal)
- Connect the white (neutral) wire to the NEUTRAL bus bar (insulated from the panel enclosure)
- 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):
- Drive an 8-foot ground rod into the earth near the building
- Run a #6 AWG (or #4 AWG for 200A) bare copper grounding electrode conductor from the ground rod to the subpanel ground bar
- 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:
- Connect hot wire(s) to the breaker
- Connect neutral to the neutral bus bar
- 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
- Bonded neutral and ground in subpanel: The number one mistake. Keep them separate.
- Undersized feeder wire: Size the feeder for the subpanel rating, not just current loads.
- No disconnect for detached building: A detached building subpanel needs a main breaker or disconnect switch.
- No ground rod for detached building: Detached buildings need their own grounding electrode.
- Too few spaces: Choose a subpanel with more spaces than you need now.
- 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).
- 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.
Wire Color Reference for Subpanel Wiring
Conductor colors tell you what each wire does at a glance, but the color code is a convention, not a law of physics -- always confirm with a meter before you touch a wire, especially on older circuits that may have been re-colored with tape.
| Conductor | US (NEC) Color | UK/EU (BS 7671) Color | Function |
|---|---|---|---|
| Line / hot (ungrounded conductor) | Black and red (two hots on a 120/240V feeder) | Brown | Carries current from the source to the load |
| Neutral (grounded conductor) | White or gray | Blue | Carries return current back to the source |
| Equipment ground | Green or bare copper | Green with a yellow stripe | Bonds metal enclosures and parts to earth for fault protection |
On a typical subpanel feeder you will see two hots (black and red), a white neutral, and a bare or green ground -- four conductors total for a 120/240V subpanel. If your feeder cable only has black conductors for both hots, NEC allows re-identifying one of them as a hot with red or another non-white, non-green tape or paint at each end, rather than requiring a factory-red conductor.
Permits and Code Compliance for Subpanel Installations
Most jurisdictions require an electrical permit before you install a subpanel, and skipping it can create problems later -- from a denied insurance claim after a fire to a buyer's inspector flagging unpermitted work when you sell the home.
A typical permit process includes:
- Submitting a panel schedule or load summary showing the subpanel's amperage and the circuits it will feed
- A rough-in inspection after the feeder and branch circuits are run but before walls are closed up
- A final inspection once the panel cover, breaker labels, and grounding electrode (if applicable) are in place
If the feeder runs underground to a detached building, call 811 (or your local utility locate service) before you dig so gas, water, and other buried lines get marked. NEC Article 225 governs feeders and disconnecting means for detached structures, and the 36-inch working clearance in front of the finished subpanel (noted earlier under NEC 110.26) is one of the first things an inspector checks. Bring your feeder size, breaker sizes, and grounding method to the permit office in writing -- inspectors approve well-documented jobs faster than ones they have to piece together on site.
Troubleshooting a Subpanel: Common Problems and Fixes
| Symptom | Likely Cause | Fix |
|---|---|---|
| Feeder breaker trips immediately when switched on | Short circuit in the feeder cable, or a hot touching ground/neutral at the subpanel | Disconnect all branch breakers in the subpanel and retest with no load; if the feeder breaker still trips, inspect the feeder cable and main-lug connections for a pinched or miswired conductor |
| No power at the subpanel, but the feeder breaker is on | Loose or corroded connection at the main lugs, or a failed feeder breaker | With power off, check the torque on the lug connections at both ends of the feeder, then test the feeder breaker with a multimeter |
| A branch breaker trips repeatedly | Overloaded circuit, or a fault in the connected device or wiring | Move some loads to another circuit; if the breaker still trips with everything unplugged, the branch wiring itself likely has a fault |
| GFCI or AFCI breaker trips with no obvious load problem | Neutral and ground are contacting each other somewhere on that circuit -- often traced back to a bonded (rather than separate) neutral and ground bus bar in the subpanel | Confirm the subpanel's neutral and ground bus bars are NOT bonded, and check that each branch circuit's neutral returns to the correct bus bar |
| Lights dim or flicker when a large appliance starts up | Undersized feeder wire or a loose feeder connection causing voltage drop | Verify the feeder wire gauge matches the subpanel rating, and re-torque all feeder connections at both panels |
| Subpanel breaker won't reset, or trips instantly when reset | The breaker is damaged internally, or a persistent short/ground fault exists downstream | Disconnect the downstream wiring to isolate the circuit; if the breaker still won't hold with nothing connected, replace the breaker |
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.
Frequently asked questions
What happens if a subpanel is too far from the main panel?
Longer feeder runs increase voltage drop, which can dim lights and slow down motors under load. Beyond about 100 feet, upsize the feeder wire by one gauge or switch from aluminum to copper to keep drop within NEC's recommended limits. Very long runs may also need a larger conduit to ease pulling and reduce heat buildup in the conductors.
Can I use a standard breaker panel as a subpanel?
Yes, many panels are listed for use as either a main panel or a subpanel, but you must configure them correctly. Remove or leave out the bonding screw/strap so neutral and ground stay on separate bus bars, and use a main-lug or main-breaker version as required by whether the subpanel is in the same building as the service or in a detached structure.
What size wire do I need for a 60 amp subpanel?
A 60A subpanel feeder typically uses 6 AWG copper or 4 AWG aluminum conductors, protected by a 60A double-pole breaker at the main panel. Always verify the exact size against the cable's insulation temperature rating and any voltage-drop adjustment needed for the run length before you buy wire.
Is it safe to add a subpanel without shutting off the main breaker?
No. Working inside the main panel to install the feeder breaker exposes you to live bus bars unless the main breaker is off, and the panel should be verified dead with a voltage tester before you touch anything inside it. Only the utility feed lugs above the main breaker stay energized, and those must never be touched by anyone other than a qualified person coordinating with the utility.
How many circuits can a subpanel handle?
It depends on the number of breaker spaces the subpanel has, not just its amperage rating -- a 100A subpanel might have 20, 24, or 30 spaces depending on the model. The combined breaker ratings can exceed the subpanel's main rating since not all circuits run at full load simultaneously, but the feeder breaker itself must never exceed what the feeder wire and main panel bus can safely supply.
Do I need a disconnect switch at a subpanel in the same building as the main panel?
Not necessarily. A subpanel located in the same building as the main panel can typically use a main-lug design, where the feeder breaker in the main panel serves as the disconnect. A separate disconnecting means at the subpanel itself is only required when the subpanel is in a detached structure, such as a garage or shed.