Field guide: installing a subpanel, for master electricians (edition 4)

Load calc first, everything else second

Before you pick a panel or pull a permit, run the calc. Article 220 Part III for feeders. Existing service plus the new subpanel loads cannot exceed the main service ampacity after demand factors. If the house has a 200A service pulling 160A calculated, you have 40A of headroom for that new garage sub, period.

Master electricians know the calc gets argued twice: once with the AHJ and once with the homeowner who wants a 100A sub for a hot tub and a welder. Write it down, sign it, keep a copy. NEC 220.40 through 220.61 are your working pages.

Verify the existing service with a clamp meter on a hot summer afternoon with the AC running. Nameplate calcs are gospel for permits, measured load is gospel for reality.

Sizing the feeder and the panel

Feeder ampacity per 310.16, adjusted for conditions of use. Most residential subs ride on 60C terminations at the breaker and the lugs, so you use the 60C column for small conductors and 75C for larger ones if both ends are listed for 75C per 110.14(C). Check the lug sticker. Every time.

Grounded conductor sized per 220.61. Equipment grounding conductor per 250.122. And yes, the EGC is based on the overcurrent device rating, not the conductor size you actually pulled. Upsizing the ungrounded conductors for voltage drop triggers 250.122(B), upsize the EGC proportionally.

• 60A sub: #6 Cu THHN, #10 Cu EGC, 1 inch EMT is plenty
• 100A sub: #3 Cu or #1 Al SER where permitted, #8 Cu EGC
• 125A sub: #1 Cu or 2/0 Al, #6 Cu EGC
• Always verify with 310.16 and 250.122 against your actual install conditions

Four wires, not three

Subpanels in separate structures and subpanels in the same structure both get a four wire feeder. Two hots, a neutral, and a separate equipment grounding conductor. The neutral bar floats. The ground bar bonds to the can. The bonding screw or strap that came with the panel stays in the bag.

This is where 90 percent of failed inspections happen. An older electrician who learned the trade before the 2008 cycle will still bond the neutral at the sub out of habit. Check it twice before you energize.

If you see parallel current on the EGC with a clamp meter after energizing, you have a neutral to ground bond somewhere downstream. Find it before you leave the site. That is a fire waiting for the right Tuesday.

Grounding electrode at detached structures

Detached garage, barn, shop. 250.32 applies. You need a grounding electrode system at the separate building, bonded to the EGC that came with the feeder. Two ground rods 6 feet apart unless you can prove a single rod hits 25 ohms or less, which nobody can prove without a fall of potential tester, so drive two.

The electrode conductor sizes per 250.66, not 250.122. Different table, different purpose. A #6 Cu to the rods covers most residential feeders up to 100A.

Same structure sub, no new electrodes required. The EGC from the main panel does the work. Do not drive rods at an attached garage sub and bond them separately, you will create a parallel path and a ground loop that can bite later.

Breaker selection and AFCI/GFCI at the sub

Subpanel protection lives at the sub, not at the main. All 15 and 20A, 120V branch circuits feeding the dwelling rooms listed in 210.12 need AFCI. GFCI per 210.8 applies wherever the outlet lands, bathroom, kitchen, garage, outdoors, basement, laundry. If the sub feeds an outbuilding workshop, every 125V through 250V receptacle 50A or less needs GFCI under 210.8(B) for other than dwelling units, or 210.8(A) for dwelling accessory structures.

Match the breaker brand to the panel listing. A Square D QO panel takes QO breakers. Classified breakers exist but UL listings are panel specific, read the label inside the door before you stock the truck.

1. Lay out the two hot legs to balance 240V loads on opposite phases
2. Keep AFCI and GFCI pigtails organized, label the neutrals at install
3. Torque every lug to the sticker value, use a calibrated screwdriver
4. Photograph the inside of the door after labeling, before you close it

Working space, labeling, and the final walk

110.26 working space. 36 inches deep, 30 inches wide or the width of the equipment, 6.5 feet high. Measure from the face of the panel, not the wall behind it. Storage shelves and water heaters do not belong in that envelope. The AHJ will measure.

Label the sub per 408.4. Every breaker gets a specific description, not "lights" or "plugs." Include the feeder source at the top: "Fed from Main Panel, Breaker 1 and 3, 100A." Add the available fault current and date per 110.24 for non-dwelling work, and increasingly for dwelling work depending on your jurisdiction.

Do the final walk with the panel cover off and a thermal camera after 30 minutes under load. A loose lug shows up as a bright spot before it shows up as a callback.

Close the can, torque the cover screws, hand the homeowner the permit card, and note the panel schedule location in your service records. Next guy in, including future you, will thank you.