Field guide: installing a subpanel, before you start (edition 6)

Sizing the feeder before anything else

Before you pop a knockout, run the load calc. A subpanel is only as good as the feeder feeding it, and undersized feeders are the number one callback on subpanel jobs. Use NEC 220 Part III or Part IV depending on the occupancy, and don't eyeball it off the main panel's amp rating.

Remember the feeder conductors have to be sized to the calculated load, and the OCPD at the source has to protect them per NEC 240.4. If you're running 100A to a detached shop, that's 1 AWG copper or 1/0 aluminum at 75C terminations, not #4 because "it's what was on the truck."

Voltage drop kills more subpanels than bad terminations. NEC 210.19(A) Informational Note 4 and 215.2(A)(1) IN 2 recommend 3% on feeders, 5% total. On a 150 foot run to a garage, that changes your wire size more often than not.

• Calculate load per NEC 220 before sizing conductors.
• Apply ampacity from NEC Table 310.16 at the correct termination temp (110.14(C)).
• Check voltage drop on anything over 75 feet one way.
• Confirm OCPD at the supply end protects the feeder (240.21).

Four-wire feeder, always

If you learned the trade before 2008, you remember bonding the neutral at a detached structure's subpanel with a three-wire feeder. That's gone. NEC 250.32(B) requires a separate equipment grounding conductor run with the feeder, and the neutral stays isolated at the subpanel.

This trips up guys working on older installations. You pull the cover, see a bonded neutral at the sub, and assume it's code because it's been there 30 years. It's not. If you're touching the panel for any reason beyond like-for-like breaker swaps, separate the neutral and ground and land the EGC on its own bar.

"Every old farm I work on, the barn sub has the neutral bonded and a ground rod doing double duty. I separate them every time now. The inspector will catch it, and more importantly, you don't want neutral current on the EGC and the building steel."

Grounding electrodes at a separate structure

A subpanel in the same building as the service shares the service's grounding electrode system. A subpanel in a separate building needs its own grounding electrode system per NEC 250.32(A), connected to the EGC and the neutral bar... wait, no. EGC only. Neutral stays floating.

Two ground rods 6 feet apart unless you can prove 25 ohms or less with one (NEC 250.53(A)(2) Exception). Use a #6 copper to the rods per 250.66(A), and don't forget to bond any metal water or gas piping in that structure if present (250.104).

• Separate structure gets its own GES (NEC 250.32(A)).
• GEC lands on the EGC bus, not the neutral bus.
• Two rods unless tested, #6 Cu minimum for rod electrodes.
• Bond interior metal piping systems in the served structure.

Panel location and working space

NEC 110.26 is not negotiable. 36 inches of depth, 30 inches of width (or the width of the equipment, whichever is greater), 6.5 feet of headroom. The 30 inches doesn't have to be centered on the panel, but it has to be clear and it has to include the full width of the panel somewhere in that 30.

No storage in the working space. No water heater in front of the sub. No shelving you "can move." If the homeowner stacks totes there after you leave, that's on them, but don't install it that way. Inspectors are increasingly strict on this one.

Also check 240.24: OCPDs between 6'7" floor to center of the top breaker handle, max. And not in bathrooms (240.24(E)), not in clothes closets (240.24(D)), not over steps (240.24(F)).

"I've pulled three subs this year that were installed behind water heaters. Everyone knew it was wrong, nobody wanted to move the tank. Quote it right the first time and include the tank relocation if you have to."

Breaker and bus compatibility

Use listed breakers for the panel. NEC 110.3(B) says equipment has to be installed per its listing, and most panelboards are only listed with specific breaker brands. Classified breakers exist, but check the panel label. Square D QO panels are not listed for Eaton BR, regardless of what fits mechanically.

Bus ampacity matters for the subpanel rating, not just the main breaker. A 100A main-lug-only sub can be fed by a 100A feeder breaker upstream, but the bus has to be rated 100A or more. Don't feed a 125A-rated sub with a 200A breaker thinking "it's protected somewhere."

Before you energize

Walk it before you flip anything. Torque every lug to the manufacturer's spec (NEC 110.14(D) requires a calibrated torque tool as of the 2017 cycle). Confirm neutral and ground are separated. Confirm the EGC is landed and the bonding screw or strap is removed from the neutral bar.

Meg the feeder if it's a long run or it's been sitting in a trench. Check phase rotation on three-phase subs. Label every circuit per 408.4, not "lights" and "kitchen" but actual descriptions. Future-you or the next electrician will appreciate it.

1. Torque to spec with a calibrated tool (110.14(D)).
2. Verify neutral isolation and EGC landing.
3. Confirm bonding screw removed at the sub.
4. Megger long or buried feeders before energizing.
5. Label circuits with specific descriptions (408.4).

A clean subpanel install is 80% planning, 20% pulling wire. Do the calc, pick the right feeder, separate the bars, and you won't see it again until somebody adds a circuit five years from now.