When adding circuits to your electrical system, you may need to install a sub panel to provide more capacity. Sub panels allow you to connect additional circuits by tapping power off the main service panel. While convenient, sub panels have specific code requirements to ensure safety. Understanding these requirements is crucial when planning a sub panel installation.
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What is a Sub Panel?
A sub panel, also called a sub feed panel, is an electrical panel that branches off the main service panel. The main service panel is the central hub that connects your home’s electrical system to the utility power lines. This primary panel may not have enough open slots or ampacity for new circuits you need to add. A sub panel provides additional space and amperage capacity while eliminating the need to upgrade the entire main panel.
A sub panel is connected to and fed power from a double pole circuit breaker in the main panel. It functions in a similar way to the main breaker panel, providing overcurrent protection with circuit breakers and wiring terminals for branch circuits. However, the sub panel gets its power supply from the main and does not directly connect to the utility feed.
Reasons to Install a Sub Panel
There are several reasons you may need to install a sub panel for your home:
- The main breaker panel is full and has no room for new circuit breakers.
- The existing main panel does not have enough amperage capacity for added loads.
- Electrical alterations or additions are needed in an area far from the main panel.
- The main breaker panel uses an obsolete fuse-type system.
- Improved organization is desired to divide and isolate circuits.
Adding a sub panel allows you to expand your electrical system without replacing the main service panel. This is a cost-effective option when upgrades are needed for additions, renovations, or modernization.
Sub Panel Requirements
Installation of a sub panel must follow codes to ensure safety. While requirements vary slightly between jurisdictions, there are some general standards:
Location
The sub panel must be installed in an accessible location. It cannot be placed in bathrooms or above stairways. Make sure there is adequate workspace clearance according to codes. The area should be free of extreme temperatures and moisture.
Mounting
Proper mounting is essential for safety and durability. The sub panel must be securely mounted to the supporting structure using corrosion-resistant fasteners. It should be vertically plumb and mounted at the required height per local codes.
Circuit Breakers
All circuits fed from the sub panel must have overcurrent protection. Breakers must match the amperage of the connected wires. Use only approved, listed breakers matching the panel brand. Ensure the total amp rating does not exceed the main breaker size.
Grounding
Sub panels must be properly grounded. An equipment grounding conductor must connect to the ground bus bar in the main panel. Use copper wire sized to regulations. The sub panel’s ground bus must also be connected to the grounding electrode system.
Bonding
The neutral bus bar in the sub panel must be isolated from the enclosure. Remove any factory-installed bonding screw or strap. The ground bus will provide the connection to the panelcase. Only the main service panel should have the neutral bus bonded to the enclosure.
Wiring
Use approved wiring methods and materials to connect the sub panel to the main panel area. Follow required ampacities, insulation ratings, and termination points. Use only copper conductors unless aluminum wiring is specifically allowed per Code.
Feeder Circuit Breaker
The sub panel feeder from the main panel must be protected by a circuit breaker in the main panel sized according to the rating of the wire. A 15 to 20 amp dual pole breaker is commonly used to feed a sub panel. Match the voltage and amperage of the existing system.
Physical Protection
The sub panel must be enclosed in a UL listed cabinet suitable for the environment. Indoor panels require a weatherproof enclosure. Outdoor panels must be raintight. Size the enclosure to allow easy access and proper wire bending space per Code.
Permits
Electrical permits are required for sub panel installations. A licensed electrician must do the work in many jurisdictions. Even if permits are not needed locally, adhere to all applicable electrical codes.
Locating and Sizing a Sub Panel
Choosing the right location is important for optimal sub panel function. Follow these guidelines when planning:
- Pick an accessible area with adequate work space clearance.
- Minimize the distance from main panel for easier wiring.
- Choose a central location to minimize wire distances to new circuits.
- Place at an appropriate height for visibility and access.
- Avoid moisture, extreme temps, or combustible areas.
For capacity, the sub panel must exceed the estimated load to be added. Common sizes are 60 amps, 100 amps, and 125 amps. The feeder breaker, wires, and overcurrent devices must all align with the panel rating. Leave room for future expansion as well.
Wiring a Sub Panel
One of the most important parts of connecting a sub panel is the wiring. This includes the feeder from the main panel and branch circuits in the sub panel. Follow these important guidelines:
Use Copper Conductors
Copper wiring is required for most residential sub panel feeders less than 200 amps, per NEC section 310.106(A). Common sizes are #2 AWG to #3 AWG. Match wire ampacity ratings to the circuit breakers.
Follow Feeder Ampacity
Size the sub panel feeder per NEC guidelines to handle the amperage. For example, a 100 amp sub panel often requires #3 AWG copper feeders. Refer to code ampacity tables for exact specifications.
Install External Shutoff
Code requires a means to disconnect the sub panel without getting behind it. Install a circuit breaker disconnector or fused pull-out switch in the feeder line external to the sub panel.
Use THHN/THWN-2 Wire
Feeders require moisture and heat resistant THHN/THWN-2 insulation. NM cables cannot be used for sub feeds. Use THHN/THWN-2 for both the ground and the insulated conductors.
Follow Conduit Fill Requirements
De-rate conductors as needed when bundled in conduit. Adjust ampacity or conduit size to fill regulations. Use NEC Chapter 9 or conduit fill calculators to determine requirements.
Use Correct Terminations
Use lug connectors properly sized for the conductors. Connect only one wire per lug. Follow torque specs when tightening terminals. Improper connections can cause arcing, heat damage, and loose wiring.
Adding a Ground Rod
Sub panels may need a separate grounding electrode system depending on the feeder length and size. Follow these rules:
- Install ground rod(s) if oversizing feeders per NEC 220.61 requirements.
- Add ground rod if feeder length exceeds 75 feet for #6 AWG or smaller wire.
- Use one 5/8″ x 8′ rod minimum, two rods if soil conditions require.
- Space rods 6 to 8 feet apart if using multiple.
- Connect to sub panel ground bus with #4 AWG copper or larger.
The main service panel grounding electrode system does not extend to distant sub panels. Installing dedicated ground rods ensures proper sub panel grounding.
Sub Panel Installation Cost
Sub panel installation costs typically range from $800 to $2,500 depending on the scope of work:
- Basic 60 Amp Sub Panel – $800-$1,200
- 100 Amp Sub Panel – $1,200-$1,800
- 125 Amp Sub Panel – $1,500-$2,500
- Added Costs: conduit, wiring, breakers – $200-$1,000
- Electrician Cost Per Hour – $50-$100/hour
Complex installations with long wiring runs, conduit work, and heavy amperages can be more. Have an electrician assess your specific needs.
Sub Panel Safety Tips
Improper sub panels can present serious safety hazards. Consider these tips to ensure reliable function:
- Choose high-quality equipment that meets the latest safety standards.
- Install a surge protective device to protect from voltage spikes.
- Use tamper-resistant breakers in readily accessible sub panels.
- Label all circuits clearly for easy identification.
- Keep the area around sub panels clear for access.
- Have a qualified electrician perform all wiring connections.
- Check for overheating on terminals, lugs, and wiring.
- Regularly inspect the sub panel for any damage or faults.
- Contact an electrician immediately if you see discoloration, smells, or buzzing.
Following codes and best practices for sub panels will keep your electrical system upgrade safe, effective, and reliable.
Conclusion
Installing a properly wired sub panel provides needed capacity for electrical expansion. By following the equipment and installation requirements, your upgraded system will safely deliver power wherever it’s needed. If considering a sub panel addition, have a qualified electrician assess your specific electrical needs and oversee the installation work. This will ensure your home’s electrical system meets the latest safety codes and standards.