Subpanel Installation: When You Need One and How It Works

If you're adding a workshop, garage, EV charger, or just running out of spaces in your main panel, you've probably been told you need a subpanel. Here's what that actually means, what it costs, and whether it's the right call for your situation.

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What Is a Subpanel?

A subpanel is a secondary distribution panel that connects to your main panel — it doesn't replace it. Think of your main panel as the main water shutoff for a city, and the subpanel as the meter for a specific building on that water line. The main panel feeds the subpanel, and the subpanel distributes power to a specific area or purpose.

Subpanels are fully permitted and code-compliant when installed correctly. They're the standard solution for extending circuits beyond the practical limits of a main panel.

When Do You Need a Subpanel?

You'll typically need a subpanel when:

• Adding a detached building — garage, shed, workshop, ADU, or barn that needs its own power
• Panel is full — no open spaces left in the main panel for new circuits
• Long circuit runs — running a 120V circuit 200+ feet to a detached structure creates excessive voltage drop; a subpanel fed by a larger feeder allows efficient distribution
• Dedicated equipment — EV chargers, hot tubs, large compressor, welding equipment, or other high-draw loads need their own dedicated circuits
• Organizing a messy panel — some homeowners add a subpanel to clean up a crowded main panel and gain better circuit organization

How to Size a Subpanel

Sizing a subpanel means two things: the amperage rating of the subpanel itself, and the size of the feeder conductors that connect it to the main panel.

Subpanel Amp Rating

The subpanel must be rated equal to or greater than the feeder supplying it. Common sizes:

• 100-amp subpanel — sufficient for a standard garage or small workshop with typical power tools
• 125-amp subpanel — good for a larger garage with multiple high-draw tools
• 200-amp subpanel — for shops with welders, compressors, EV chargers, or any detached structure serving as a primary workspace

Rule of thumb: always go bigger than you think you need. Adding spaces never hurts, and a panel that's too small forces you back into the same problem you were trying to solve.

Feeder Sizing

The feeder is the set of conductors running from the main panel to the subpanel. For a 100-amp subpanel, you'll typically run 4 conductors (two hot, one neutral, one ground) using 3-gauge THHN wire or 4-gauge SER cable. For a 200-amp subpanel, 1/0 aluminum or 3/0 copper is common.

This is where many DIYers get into trouble — undersizing the feeder because they don't understand that the feeder must handle the full load of the subpanel, not just the initial draw.

The Installation Process

I'm going to be direct: subpanel installation is not a DIY job for most homeowners. Here's why, and what the process looks like when a licensed electrician does it:

Step 1: Load Calculation

Before any equipment is purchased, your electrician should perform a service load calculation per NEC Article 220. This determines whether your main service has enough capacity to feed the new subpanel without being overloaded. This isn't optional — it's code.

Step 2: Permit and Inspection

Any subpanel installation requires a permit in most jurisdictions. The work must be inspected by the local building department at rough-in and final inspection stages. Skipping the permit might save money upfront — until you try to sell the house and the inspector finds unpermitted work.

Step 3: Running the Feeder

The feeder conductors must be run in appropriate conduit (usually PVC or EMT for residential) and properly supported. If the subpanel is in a detached structure, the feeder must be buried at the correct depth — 24 inches for direct burial cable, 18 inches for conduit. In my area, anything within 18 inches of the surface needs conduit period.

Step 4: Grounding

Subpanels in detached structures require their own grounding system — typically a ground rod driven at the structure plus a grounding electrode conductor connecting to the subpanel grounding bus. The neutral and ground must remain separate in a subpanel (bonded only at the main service). This is a common DIY mistake that creates stray neutral current and is a code violation.

Step 5: Final Connections

At the main panel, the feeder breaker must be sized to protect the feeder conductors (not necessarily the subpanel rating). At the subpanel, terminations are made to the hot buss bars, neutral bar, and ground bar — in the correct positions.

What This Actually Costs

Here's the honest breakdown from what I've seen on job sites:

• Panel and materials — $150–$400 for a quality 100–200 amp subpanel with spaces
• Feeder and conduit — $200–$600 depending on distance and whether it's buried
• Labor — $500–$1,500+ for an electrician to do the full install, depending on complexity and local labor rates
• Permit and inspection — $100–$400 depending on the jurisdiction

Total for a typical 100-amp subpanel in an attached garage: expect $800–$1,800. Detached structures run more due to the buried conduit requirement. Get at least three bids from licensed electricians — the spread can be significant.

Can a Homeowner DIY This?

In most jurisdictions, a homeowner can pull their own electrical permit and do the work themselves — if they pass the code exam and demonstrate competency. But here's what I'd consider before going that route:

• The load calculation is technical and mistakes are expensive
• The grounding requirements for detached structures are specific and often done wrong
• Conduit bends, burial depth, and wire pulling are physical skills that take practice
• If the work fails inspection, you've spent money on materials and still need to hire someone

If you're studying for your 06A and want to learn by doing — this is one of those projects where shadowing a licensed electrician for a day teaches you more than any book. Offer to help, ask questions, watch how they make up the conduit and terminate the panel.

The Bottom Line

Subpanels are the right solution for a real problem — running out of main panel spaces, extending power to detached structures, or supplying high-draw equipment. They're not complicated conceptually, but the code requirements around feeder sizing, grounding, and panel placement are specific for a reason.

If you're tackling this as a DIY project, do your load calculations twice, study the NEC requirements for grounding electrode systems and conductor sizing, and budget for at least two inspections. If you're hiring it out, get the load calculation in writing and make sure the bid includes permit and inspection fees — not just the physical installation.