Field guide: installing a subpanel, step-by-step (edition 3)

Plan the feeder before you pull a single conductor

Start with the load calc. Article 220 governs. Size the feeder to the calculated load, not the panel rating. A 100A subpanel fed by a 60A calculated load still gets a 60A feeder if that is what you size for, but most installs run the feeder to match the subpanel bus rating for future capacity. Confirm with the customer what they plan to add: EV charger, mini split, shop tools. Undersizing now costs a second trip later.

Check the main panel for available breaker space and bus rating. If the main is already maxed on calculated load per 220.83, you are looking at a service upgrade before the subpanel goes in. Do not skip this math. An overloaded service with a new subpanel hanging off it is a callback waiting to happen.

Pick the subpanel location. Working clearance per 110.26 is non-negotiable: 36 inches deep, 30 inches wide, 6.5 feet high, and the space stays clear. No shelving, no water heater, no washing machine in front of it. Dedicated equipment space above the panel per 110.26(E) runs 6 feet up or to the ceiling.

Size the feeder and the conduit

Four conductors. Always. Two hots, one neutral, one equipment grounding conductor. The days of three-wire feeders to detached structures ended with the 2008 code cycle, and bonding neutral to ground at a subpanel has never been correct on the same structure. 250.32(B) spells it out.

For a 100A subpanel, #3 copper THHN or #1 aluminum is the common pull under 310.16 at 75C terminations. Neutral sized to the calculated neutral load per 220.61. EGC sized from 250.122 based on the overcurrent device, not the feeder ampacity. A 100A feeder gets a #8 copper or #6 aluminum EGC. Do not oversize the EGC without reason; it just wastes copper and crowds the lug.

• 100A feeder: #3 Cu or #1 Al, #8 Cu EGC, 1.25 inch EMT minimum
• 60A feeder: #6 Cu or #4 Al, #10 Cu EGC, 1 inch EMT
• 200A feeder: #2/0 Cu or #4/0 Al, #6 Cu EGC, 2 inch EMT

Conduit fill per Chapter 9, Table 1 and Annex C. Do not eyeball it. A 1 inch EMT with four #3 THHN is over fill. Run the numbers.

Mount, land, and separate the bars

Set the panel with the top breaker no higher than 6 feet 7 inches per 240.24(A). Leave the deadfront off until rough inspection. Bring the feeder in through a listed connector, maintain the bending radius per 312.6, and land the conductors with a torque screwdriver. Every lug, every time. Manufacturer torque specs are on the label inside the door. 110.14(D) made this mandatory in 2017 and inspectors are looking.

If you do not own a calibrated torque screwdriver yet, buy one before your next job. A Wera or Wiha with the click mechanism runs under 150 dollars and pays for itself the first time an inspector asks you to demonstrate.

Here is the part that trips up apprentices and more than a few journeymen: the neutral bar floats, the ground bar bonds to the can. Remove the green bonding screw or strap that came installed from the factory. The neutral and ground must be isolated in any subpanel on the same premises as the service disconnect. 408.40 and 250.24(A)(5) are the relevant articles.

Grounding at a detached structure

If the subpanel feeds a detached garage or shop, you need a grounding electrode system at that structure per 250.32(A). Two ground rods 6 feet apart, or a single rod if you can prove 25 ohms or less, which nobody actually measures, so just drive two. Bond the EGC from the feeder to the ground bar, and bond the ground bar to the rods with a #6 copper GEC per 250.66.

The neutral still does not bond to ground at the detached structure. This is the 2008 change that a lot of older electricians still get wrong. Four-wire feeder, isolated neutral, local grounding electrode.

1. Drive two 8 foot copper-clad rods, 6 feet apart minimum
2. Run continuous #6 bare copper from rod to rod to the ground bar
3. Use acorn clamps listed for direct burial
4. Keep the GEC run as straight as practical; sharp bends increase impedance on fault current

Label, test, and close it up

Label every circuit. Not "lights" and "outlets." Specific rooms, specific loads. 408.4(A) requires it and it is the number one inspector complaint on finish. A circuit directory that says "bedroom 2 receptacles" beats "BR" every time.

Take a phone photo of the finished panel with the deadfront off and another with the directory filled in. Send it to the homeowner. It saves a service call in three years when they want to add a ceiling fan.

Megger the feeder before energizing if the run is long or got rained on during rough. Verify voltage L-L and L-N at the lugs before you land the breakers. Check for voltage between neutral and ground at the subpanel: it should read zero or very close. Any significant reading means you missed the bonding screw removal or have a neutral-ground fault downstream.

Torque the deadfront screws, fill out the directory in pen, and walk the homeowner through the main shutoff. Done right, this is a two-day job for one electrician and an apprentice. Done wrong, it is a lifetime of callbacks.