Field guide: installing a subpanel, common mistakes (edition 4)

Bonding and grounding: the #1 failure

Subpanel installs get red-tagged more often for bonding errors than anything else. The neutral bar must float. The ground bar must bond to the enclosure. If the factory bonding screw or strap is still in place on the neutral bar, pull it. This is not optional and it is not a suggestion.

NEC 250.24(A)(5) prohibits a neutral-to-ground connection on the load side of the service disconnect. Every subpanel downstream of that disconnect counts. Separate buses, four-wire feeder, done right.

If you are looking at a subpanel and cannot tell whether the bonding screw is in or out, pull the deadfront and look. Never assume the last guy did it right.

Feeder sizing and conductor selection

Size the feeder to the calculated load per NEC 220, not the panel rating. A 100A subpanel fed from a 60A breaker is fine if the load math works. What is not fine is running #6 copper on a 100A feeder because someone eyeballed it.

Check NEC 310.16 for ampacity and remember the 83% rule under NEC 310.12 only applies to dwelling service and main feeder conductors supplying the entire load. Do not stretch it to cover undersized subfeeders.

• 60A feeder: #6 Cu or #4 Al, with #10 Cu or #8 Al equipment ground
• 100A feeder: #3 Cu or #1 Al, with #8 Cu or #6 Al equipment ground
• Always pull four conductors: two hots, one neutral, one ground
• Confirm terminal ratings, 60C vs 75C, before final sizing

The four-wire rule and why three-wire is dead

Three-wire feeders to detached structures were permitted under older code cycles, but NEC 250.32(B) now requires a separate equipment grounding conductor for any feeder supplying a separate building or structure. If you are retrofitting, the old three-wire setup with a bonded neutral at the sub is out. Bring a ground, float the neutral.

One exception remains for existing installations where no metallic path exists between buildings and no ground fault protection is installed, but it is narrow and shrinking. If you touch the feeder, bring it up to current code.

Breaker compatibility and listings

Do not mix breakers across manufacturers unless the panel is listed for classified replacements. A Square D QO panel takes QO breakers. A Homeline panel takes Homeline. Stab-in clones from the bargain bin void the UL listing and give the inspector an easy fail under NEC 110.3(B).

Watch for tandem breaker limits. Most panels have a specific number of slots rated for tandems, marked on the label. Loading every slot with a tandem because you ran out of space is a common red tag.

• Check the panel label for tandem-rated positions
• Match the breaker brand and series to the panel listing
• AFCI and GFCI requirements follow the circuit, not the panel, per NEC 210.8 and 210.12

Working space and physical install

NEC 110.26 still applies to subpanels. 30 inches wide, 36 inches deep, 6.5 feet of headroom, clear. The homeowner's shelving unit in front of the sub is your problem the moment you energize it. Document it and get it moved before you close up.

Mount height matters too. The top breaker handle cannot exceed 6 feet 7 inches above the floor per NEC 240.24(A). For a typical 40-space panel that puts the bottom of the can around 4 feet, give or take. Dry locations only unless the enclosure is rated otherwise.

If the panel is in a closet, check 240.24(D). Clothes closets are prohibited. A utility or mechanical closet can work if it is not a clothes storage space.

Terminations, torque, and finish work

Torque every lug to the manufacturer's spec. NEC 110.14(D) now requires it to be verified with a calibrated tool, not feel. A calibrated torque screwdriver costs less than one callback for a burned lug. Write the values on the panel schedule if the inspector wants proof.

Strip length matters on aluminum feeders. Too short and you have high resistance at the lug. Too long and you have exposed conductor. Follow the strip gauge on the lug or in the panel documentation. Apply listed antioxidant on aluminum unless the lug is specifically listed as not requiring it.

1. Verify torque on every feeder and branch lug with a calibrated tool
2. Label the panel schedule clearly, circuit by circuit
3. Install the deadfront with all screws, no missing fasteners
4. Fill unused openings with listed knockout seals, not tape
5. Megger or at minimum insulation-test the feeder before energizing if the run is long or questionable

A clean subpanel install takes maybe an hour longer than a sloppy one. That hour is cheaper than a return trip, a red tag, or an arc flash six months later when a loose lug finally lets go.