Step-by-Step Subpanel Addition Instructions

Adding extra capacity to your home's electrical system with a new subpanel is an involved but doable DIY project. Installing a subpanel requires running new wires, mounting the subpanel box, connecting circuits through breakers, and integrating the new power source properly with your existing main service panel.

With some basic electrical skills and the right precautions, homeowners can upgrade their electrical service to power additional lighting, appliances, electronics, and equipment. Follow this step-by-step guide to successfully introduce a new subpanel that meets code and handles your home's growing electricity needs.

Determine the Need for Adding a New Subpanel

Before going ahead with installing a subpanel, the first step is confirming that an additional panel is necessary. Compare your home's existing electrical capacity versus current or planned demand to decide if upgrading is required.

Assess Current Electrical Load and Capacity

Check the amp rating of your main breaker panel to determine available supply. Calculate existing load by adding up nameplate amps for all circuits. This gives you existing usage. Now calculate potential new load being added from additional appliances or equipment you plan to install. If total projected load nears or exceeds current capacity, adding a subpanel is the solution.

Identify Causes of Tripped Breakers or Blown Fuses

If you are already facing tripped breakers or blown fuses frequently, that's a sign that you need more power. Common causes include running too many appliances on one circuit, faulty wiring, exceeded nameplate load capacity, or drawing more amps than breakers can handle safely. When circuits trip or fuses blow often, this risk overheating wires which is a fire hazard.

Calculate Electrical Needs for New Appliances or Devices

To determine if your planned new devices and appliances will push demand beyond capacity, ensure the main panel's current load plus additional load doesn't exceed its supply rating. Add up wattages of all existing lights, outlets and hardwired equipment. Then add anticipated wattages of any additional loads you plan to power in the future. Compare this total wattage to your panel rating to see if an upgrade is warranted.

Compare Electrical Supply to Electrical Demand

Finally, directly compare your main panel capacity to your home's total electrical requirement with all planned additions included. This will confirm definitively whether your current supply is inadequate for present and future power demand. If your home needs more juice to operate safely, then adding a subpanel is the next logical step.

Choose the Right Subpanel

Once you determine that a subpanel is required, next comes selecting the appropriate subpanel type and specifications.

Decide on Subpanel Amperage Rating

Choose a subpanel enclosure that can handle more capacity than your calculated maximum load. Common ratings suitable for most homes are 60-amps, 100-amps or 200-amps. Ensure it has enough breaker spaces to power your planned circuits. Leave some spaces vacant for future expansion ability.

Select Between Main Lug or Main Breaker Subpanel

Main lug subpanels lack a main breaker, so all power flows through branch circuit breakers. Choose this if using a backfed breaker in the main panel to protect the subpanel feed. For standalone overcurrent protection, pick a main breaker panel instead, which isolates all subpanel circuits.

Determine Number of Circuit Breaker Spaces Needed

Decide how many branch circuits you need to power currently and in the near future. Add up the spaces needed and pick a subpanel sized 30-40% above this number. More spaces than needed allows adding more circuits later easily without replacing the entire subpanel.

Pick Metal or Plastic Enclosure as Per Location

Indoor subpanels are usually plastic for safety and corrosion resistance, while outdoor or garage subpanels are metal to withstand weather elements. Mount indoors unless code requires otherwise. Select the right NEMA enclosure type for projected environmental exposure.

Install the Subpanel Box

With the appropriately sized subpanel acquired, next step is securely installing the subpanel as per NEC codes for clearance space, location and physical mounting.

Turn Off Main Breaker and Verify Power is Off

Shutting off all home electricity via the main breaker is essential before wiring the new subpanel. Use a non-contact voltage tester to confirm power is off. Working on live wires can cause injury or death through electrocution and sparks have the potential to ignite fires.

Select Location for Subpanel as Per Codes

Mount the subpanel on a basement, garage or utility room wall without obstructions. Per code, it requires 30" minimum clear workspace in front. There should also be at least 6.5' height clearance above and 1' side clearance all around. Check local regulations for specifics.

Mount Subpanel Securely on Wall Studs

Attach the panel enclosure securely on wall studs, keeping it plumb and level. Lag screws anchored firmly in studs provide reliable mounting. The box must lie flush on the wall without wobbling. Pay attention to wall material while drilling pilot holes.

Connect Grounding and Bonding Wires

Run a ground wire from the main panel ground bus bar to the subpanel's ground bar using 6 AWG copper wire. Connect all metallic subpanel parts to this ground bar. Install separate neutral and ground bars if not factory-installed already. Keep them isolated as per code.

Connect the Subpanel

Physically linking the new subpanel to the main service panel involves installing new wires between them and making secure electrical connections.

Run Conduit Between Main and Subpanel

House steel conduit between panels and pull THHN/THWN-2 copper individual conductors through it rated for 75degC minimum. Follow the 4-wire system by running hot, neutral, ground and bond wires. Use conduit appropriate for your installation location as per NEC.

Pull Electrical Wires Through Conduit

Use metal fish tape to pull the individual THHN wires through the conduit without scratching insulation or binding conductors. Wires must slide smoothly without getting stuck mid-way. A little lubricant eases this process.

Connect Neutral and Ground Wires

Join neutral wires from main panel and subpanel on respective neutral bars using correct lug terminals. Subpanel's ground bar links to main panel's ground through 6 AWG bonding conductor. Verify ground continuity using a multimeter.

Connect Hot Wires Last

With neutral and ground links complete, connect hot feeds last. For 240V subpanels, join hot 1 bus to hot 1 and hot 2 bus to hot 2. 120V subpanels just need the hot 1 hot wire. Double check connections are tight and secure.

Add and Connect Circuits

Populate the new subpanel with branch circuit breakers and connect new branch wiring to power additional outlets, lighting and equipment.

Install Circuit Breakers in Subpanel

Choose appropriate amperage breakers for each new circuit and snap them securely into subpanel breaker slots. Follow manufacturer specs on maximum breakers allowed based on enclosure type to avoid overloading.

Connect Circuits to Circuit Breakers

Run new 12/2 NM cables from subpanel breakers to new electrical boxes. Use clamp connectors to join wire ends to breaker terminals neatly. Group all wires going to a common circuit on one breaker.

Label All New Circuit Breakers

Identifying new circuits is vital for safety and ease of maintenance. Mark each new circuit breaker clearly describing the location powered, such as "Bedroom Outlets", "Garage Lights" etc. Use durable stick-on labels.

When using a 240V subpanel, balance power draw across the two 120V phases by connecting lights and outlets equally on both poles to avoid overloading one leg. Alternate circuits between hot 1 and hot 2.