Chapter 22: Sub-Panel Installation

Welcome to Part V. The projects in this section are bigger in every dimension: more wire, more time, more code complexity, and (in most cases) genuinely more risk. We'll cover sub-panels, 240V circuit additions, EV chargers, generators, panel replacement, and solar. Each chapter includes an honest assessment of when DIY makes sense and when calling a pro is the better choice.

Let's start with sub-panels.

A sub-panel is a smaller breaker panel fed from the main panel, used to provide breaker capacity in another location: a detached garage, a workshop, a finished basement, an addition. Adding a sub-panel solves three problems simultaneously: it gives you more breaker space (your main panel might be full), it puts breaker access closer to where you'll be working with the loads, and it reduces voltage drop on long circuit runs from the main panel.

Estimated time: 8–20 hours depending on distance, trenching needs, and complexity. Cost: $300–1500 in materials. Permit required: yes, in essentially all OKC-metro jurisdictions. This is a permitted-work project regardless of homeowner exemption.

What a Sub-Panel Is and Isn't

A sub-panel is a panel that's fed from another panel (the "main"), not directly from the utility's service. It has the same physical structure as a main panel: a feeder cable comes in, connects to a main breaker (usually) or to feed-through lugs, and distributes power to branch breakers.

The big differences between a main panel and a sub-panel:

• Sub-panel doesn't have its own meter. All power flowing through the sub-panel is metered at the main panel.
• Sub-panel's neutral and ground are isolated from each other. This is the rule that trips up DIYers most often. In a main panel, the neutral bar and ground bar are bonded together (usually by a green screw or strap). In a sub-panel, they must NOT be bonded. The neutrals and grounds get their own separate buses, and they're kept electrically separate.
• Sub-panel is fed by a 4-wire feeder. Two hots, a neutral, and a separate ground. The neutral carries unbalanced current (the difference between the two hot legs); the ground is for safety only and shouldn't carry current.
• If you discover an existing 3-wire feeder (two hots and a combined neutral/ground) at an older sub-panel: do not simply replace the panel without addressing the feeder. Older NEC editions allowed 3-wire feeders to detached structures under specific exceptions; recent editions have tightened these rules substantially. Retrofitting onto an existing 3-wire feed requires either pulling a new 4-wire feeder (the right answer) or navigating exception language that may no longer apply in your jurisdiction. This is a stop-and-call moment, not a substitution you can make in the field.

These rules are critical. Getting them wrong creates conditions where ground current flows through the sub-panel feeder's grounding conductor in normal operation, which is a code violation and a safety issue.

Sizing the Sub-Panel

Two questions: how many breakers do you need, and how much amperage do you need?

Breaker count: count the circuits you plan to feed from the sub-panel:

• Detached garage workshop: maybe 4–6 circuits (lights, outlets, garage door, EV, big tool)
• Finished basement: 6–10 circuits (lights, outlets, hardwired devices)
• Addition: 4–8 circuits (lights, outlets, AC unit, etc.)

Add 50% for future expansion. A 12-space sub-panel with current need for 6 circuits is about right.

Amperage: the sub-panel's main breaker (or feed-through) sets the maximum current. Common sizes:

• 30A sub-panel: small, for a couple of circuits
• 50A or 60A: typical for a workshop or moderate detached structure
• 100A: full-featured detached garage or significant addition
• 125–200A: large additions, ADUs, or workshops with heavy loads

Calculate your expected total load and add safety margin. A sub-panel that runs at 80% capacity continuously is appropriately sized; one that runs at 30% is oversized but cheap to install once.

For most home workshops or finished basements, a 60A or 100A sub-panel is the sweet spot.

Materials

For a typical 60A sub-panel installation in a detached garage 50 feet from the main:

• 60A 8-space (or 12-space) sub-panel with main breaker
• 50 feet of 6/3 NM-B for indoor runs, or 6/3 SER for sub-panel feeders, or 6/3 UF-B for direct burial outdoor runs
• 2 ground rods (8 feet long, copper-clad steel)
• Acorn clamps for ground rod connections
• 6 AWG bare copper wire for grounding electrode conductor
• PVC conduit for any exposed sections
• Conduit fittings, sweeps, glue, etc.
• 60A breaker for the main panel (matched to your panel brand)
• Permit
• Cable connectors for both panel ends

Total materials cost: $400–700 for a typical detached-garage sub-panel.

For longer runs or higher amperage, costs scale up:

• 100A sub-panel: 4/3 wire (significantly more expensive than 6/3)
• 200A sub-panel: 2/0 or 3/0 wire (very expensive)
• Long underground runs: deeper trenching, more conduit, more wire

Step-by-Step: 60A Sub-Panel to a Detached Structure

Let's walk through a representative project: 60A sub-panel in a detached garage, 50 feet from the house, fed via direct burial UF-B cable.

STEP 1: PLAN THE ROUTE AND PULL THE PERMIT

1. Lay out the cable route from main panel to detached structure.
2. Identify trench location, depth, and any obstacles (water lines, gas lines, septic field).
3. Call 811 (Oklahoma One-Call) before digging. They mark utility lines for free; you must call at least 2 business days before digging. Skipping this and hitting a buried gas or electrical line is a serious safety incident plus expensive damage.
4. Pull the permit.
5. Submit any required plans (sub-panel size, feeder size, route).

STEP 2: TRENCHING

1. Rent a trencher ($50–100/day) for runs over 30 feet. For shorter runs, hand-digging is feasible but tedious.
2. Trench to 24" depth (UF-B requires 24" minimum cover; deeper is better in OKC clay soil).
3. Smooth trench bottom to remove sharp rocks.
4. Add a layer of sand or fine soil at the bottom (1–2") to bed the cable.
5. If the run crosses under a driveway, sidewalk, or other obstacle: - For driveways: bore under or trench through (driveway repair after). - For shorter obstacles (sidewalks): a long pipe driven horizontally underneath can carry the cable.

STEP 3: INSTALL CONDUIT STUBS

Where the cable transitions from underground to above-ground, install PVC conduit:

1. At the main panel (house) end: vertical PVC conduit rising from the trench up the side of the house, with a 90-degree sweep. The conduit enters the house through a knockout in the panel or an adjacent box.
2. At the sub-panel (garage) end: vertical PVC conduit rising up the side of the garage, with a sweep into the garage wall, terminating at a knockout in the sub-panel.

The conduit protects the cable where it's exposed. Below ground, UF-B is rated for direct burial without conduit; above ground or where exposed, conduit is required.

STEP 4: PULL THE FEEDER

1. Lay the UF-B (or SER) cable in the trench, with extra slack at each end (4–6 feet at each end is reasonable).
2. Feed the cable up through each conduit stub.
3. Inside each panel, allow enough slack for proper routing to the connections.

STEP 5: DRIVE GROUND RODS AT THE SUB-PANEL

Per code, a sub-panel feeding a separate structure typically needs its own grounding electrode (ground rod) system at the sub-panel. (For a sub-panel within the same structure as the main, the main panel's grounding is adequate.)

1. Drive two 8-foot ground rods at least 6 feet apart, at the base of the structure where the sub-panel is.
2. Use a rotary hammer with a ground rod driver attachment, or a sledgehammer (much harder).
3. Ground rods should be driven flush with the soil surface or slightly below.
4. Connect the rods together using 6 AWG bare copper wire and acorn clamps.
5. Run the 6 AWG wire from the ground rod system to the sub-panel's ground bus.

STEP 6: CONNECT THE SUB-PANEL

1. Verify the sub-panel is wired correctly: ground bar isolated from neutral bar. Most sub-panels ship with a "neutral bonding screw" or "bonding strap" installed; for sub-panel use, this MUST be removed. Re-read the panel's instructions to identify the bonding component and confirm it's removed.
2. Mount the sub-panel on the wall at the planned location.
3. Bring the feeder cable into the panel through the conduit stub and a cable connector.
4. Strip the cable's outer jacket so 6–12" of conductors are exposed inside.
5. Make the connections inside the sub-panel: - Black hot wire to one of the main breaker's terminals (or feed-through lug L1). - Red hot wire to the other main breaker terminal (or feed-through lug L2). - White neutral wire to the neutral bus (NOT to the ground bus). - Bare/green ground wire to the ground bus (NOT to the neutral bus). - Ground rod conductor to the ground bus.
6. Tighten all connections firmly.
7. Install the sub-panel's dead front cover.

STEP 7: CONNECT THE MAIN PANEL END

1. Turn off the main breaker at the main panel. Three-test verify the bus is dead.
2. Bring the feeder cable into the main panel through the conduit stub and a cable connector.
3. Strip the cable's outer jacket so 6–12" of conductors are exposed.
4. Install a 60A double-pole breaker in the main panel. (Double-pole because it serves a 240V feeder.)
5. Connect the wires: - Black hot to one terminal of the new 60A breaker. - Red hot to the other terminal of the 60A breaker. - White neutral to the main panel's neutral bar. - Bare/green ground to the main panel's ground bar.
6. Verify connections, install dead front cover.

STEP 8: BACKFILL THE TRENCH

1. Lay caution tape ("warning: buried electric line below") about 8–12 inches above the cable. This warns future diggers.
2. Backfill carefully, avoiding sharp rocks near the cable.
3. Compact the soil to avoid future settling.

STEP 9: ADD BRANCH CIRCUITS TO THE SUB-PANEL

Now the sub-panel has power, but no branch circuits yet. Add the circuits you planned:

For each circuit:

1. Run cable from sub-panel to its destination (lights, outlets, etc.).
2. Install breakers in the sub-panel.
3. Wire branch circuits per Chapter 17's procedure.

STEP 10: INSPECTIONS

Schedule and pass:

• Trench inspection (BEFORE backfilling) if required by your jurisdiction
• Rough-in inspection for any new branch circuits
• Final inspection after everything is energized and operational

For the trench inspection specifically: don't backfill until the inspector approves the cable depth and routing. Some cities are strict about this.

The Most Common DIY Failure: Neutral and Ground Bonding

The single most common code violation on DIY sub-panel installs: leaving the neutral bonded to the ground at the sub-panel.

The sub-panel ships with a bonding screw or strap that connects neutral to ground. This is correct for use as a MAIN panel. For sub-panel use, this bonding must be removed.

Why does it matter? Because if neutral and ground are bonded at both the main and the sub-panel, ground current flows in normal operation, defeating the ground's purpose as a safety conductor. A fault that should be cleared by a breaker may not be cleared, because the imbalance the breaker needs to detect is masked by the parallel ground path.

This is a hidden problem. Everything appears to work. Lights light, outlets power things. But the safety system has been undermined.

If you're installing a sub-panel: read the panel's instructions to identify the bonding component, and confirm you've removed it. Then double-check before energizing. Some inspectors specifically test for this, but some don't, and it's a violation that often slips through inspection only to show up in a later forensic investigation.

Sub-Panel in the Same Building

If your sub-panel is in the same building as the main panel (a finished basement, addition, etc.), the procedure is similar but with a few simplifications:

• No ground rod system at the sub-panel (the main panel's grounding system serves both).
• The feeder is run inside the building (no trenching, less or no conduit).
• The 4-wire feeder rule still applies; neutral and ground are still separated at the sub-panel.

This is a much simpler project, and it's a more reasonable DIY undertaking. A 60A sub-panel in a finished basement, fed by a 6/3 NM-B cable from the main panel 30 feet away, is a one-day project for an experienced DIYer.

When to Hire Out

Strong signals that this project is over your head:

• The main panel is full and adding a sub-panel requires a service upgrade.
• The run involves boring under a driveway.
• The detached structure is more than 100 feet from the main (longer runs have voltage drop calculations and may need larger feeder size).
• The main panel is unfamiliar (not a major brand) or shows signs of damage.
• You don't have a way to safely de-energize the main panel for the connection (some homes have unusual configurations that require utility involvement).

For these, hiring out (or hiring out for the heavy infrastructure with you doing the easier parts like installing branch circuits at the sub-panel) is sensible.