Field guide: installing a subpanel, rough-in phase (edition 6)

Rough-in is where subpanel jobs live or die. The finish work goes fast if the rough is tight. This is the sixth edition of the field guide, with fixes from reader pushback and recent code cycle notes. Straight from the truck, not the classroom.

Plan the feeder before you touch a knockout

Size the feeder off the calculated load, not the main panel rating. NEC 220 Part III gives you the math. Most residential subpanels land on a 60A or 100A feeder, but do not guess. Run the calc, write it on the permit, and keep a copy in the panel.

Voltage drop kills more subpanel installs than ampacity. Over 100 feet on a 100A feeder and you are looking at #2 copper or #1/0 aluminum minimum, not the #4 the ampacity table suggests. NEC 210.19(A) Informational Note No. 4 recommends 3% max on branch circuits, 5% total. Detached structures are where this bites hardest.

• Load calc per NEC 220, documented
• Feeder ampacity per NEC 310.16, corrected for temp and conduit fill
• Voltage drop verified for the full one-way run
• Equipment grounding conductor sized per NEC 250.122
• Neutral sized for the unbalanced load, not blindly matched to the hots

Mount location and working space

NEC 110.26 is not negotiable. 30 inches wide, 36 inches deep, 6.5 feet high of clear working space in front of the panel. Dedicated equipment space above per 110.26(E). Inspectors in most jurisdictions will pull a tape on this before they look at anything else.

Panel height: operating handle of the highest breaker no more than 6 feet 7 inches off the finished floor, per 240.24(A). In a garage or utility room, mount the panel so the top of the cabinet lands around 72 inches. Gives you breaker headroom and keeps the working space clean.

If you are roughing in before drywall, snap a line at 72 inches and another at 48 inches for the bottom. Panel centers between them. Saves arguing with the GC about finish floor heights later.

Feeder pathway and conduit

Pick the path that minimizes bends and maximizes access for the pull. Four 90s is the practical max before you are fighting the cable, even though NEC 344.26 allows 360 degrees total between pull points. Every kick counts against you, not just the sweeps.

For a detached structure feeder, 225.30 limits you to one feeder per building with a handful of exceptions. Run a disconnecting means at the second structure per 225.31 and 225.32. Grounding at the second building follows 250.32, and since the 2008 code cycle you are running a separate EGC, no more bonded neutral at the remote panel.

1. Confirm conduit type is rated for the environment (PVC below grade, EMT in dry interior, liquid-tight for the last few feet into vibrating equipment)
2. Verify conduit fill per NEC Chapter 9 Table 1, do not eyeball it
3. Support per NEC 358.30 for EMT, 352.30 for PVC, 344.30 for RMC
4. Expansion fittings on PVC runs over 25 feet exposed, per 352.44
5. Bushings on every raceway entry into the panel, 300.4(G) if conductors are 4 AWG or larger

Grounding and bonding at the subpanel

This is the single most-missed item on rough-in inspections. A subpanel is not a service. The neutral bar floats. The ground bar bonds to the enclosure. The bonding screw or strap that came in the panel box goes in the trash, or at least the bottom of your toolbag.

Run four wires to any subpanel in the same structure: two hots, a neutral, and an equipment grounding conductor. NEC 250.142(B) prohibits using the grounded conductor for equipment grounding on the load side of the service disconnect. Separate bars, separate paths, no bonding jumper.

If the panel ships with the neutral bar bonded to the can and you miss it, you just built a parallel neutral path through every piece of metal conduit and every ground rod downstream. That is a fire waiting for a loose neutral.

Circuit rough and box fill

Home-run every circuit to the subpanel, labeled on both ends before the drywall goes up. Sharpie on the jacket fades. Use a real wire marker or a label printer with outdoor-rated tape. Future-you or the next electrician will thank present-you.

Box fill is where a tight rough gets sloppy. NEC 314.16 is straightforward, but it catches crews who forget that every device, every clamp, and every EGC counts. A standard 20 cubic inch single-gang box with a receptacle and 12-2 in, 12-2 out, plus a 12-2 pigtail to another device runs you right up to the limit.

• Two hots, one EGC per cable, each equal to the largest conductor
• Internal clamps: one deduction total, largest conductor
• Each yoke or strap: two deductions for the largest conductor
• All equipment grounding conductors together: one deduction for the largest EGC

Before you call for inspection

Walk every circuit. Verify the feeder is secured within 12 inches of the panel and every 4.5 feet after, per 334.30 for NM or the applicable section for your wiring method. Check that all unused knockouts are closed with listed plugs. Open knockouts are a guaranteed red tag in every jurisdiction.

Meg the feeder if the run is long or if it was pulled through a wet or abusive path. Ringing it out with a multimeter catches the obvious shorts but misses insulation damage that will show up six months in. A thousand-volt megger test on a new feeder is ten minutes of cheap insurance.

1. Feeder terminated, torqued to spec, torque marked
2. Neutral bar isolated, EGC bar bonded
3. Every circuit labeled, both ends
4. Knockouts closed, bushings installed, anti-shorts on MC
5. Working space clear, panel cover ready to hang
6. Permit and load calc inside the panel door