Sub-Panel Installation Considerations Reference

Why this matters

A sub-panel install has one detail that, if missed, fails inspection 100% of the time: the neutral and ground must be SEPARATE at the sub-panel. The factory ships every panel as a service panel with the neutral bonded to the enclosure; converting it to a sub-panel means removing the bond. Add the feeder sizing, conduit, and grounding rules around that one detail, and a sub-panel job is straightforward. Miss the bond removal and you've got a parallel-neutral path that will set off every nuisance trip and create voltage on metal surfaces.

Service panel vs sub-panel - what's the difference

A service panel is the first means of disconnect after the meter. Per NEC 250.24(A), the neutral and ground are bonded together at this panel only.

A sub-panel is downstream of the service panel, fed by a feeder. Per NEC 250.142, neutral and ground are KEPT SEPARATE at the sub-panel. The bond exists once, only, at the service.

If you don't separate them at the sub:

• Current carrying neutral and ground in parallel back to the service
• Voltage drop on the neutral creates voltage on the equipment grounds (the EGC)
• Metal-cased appliances can become slightly hot - touch the case and another grounded surface, get a tingle
• Inspector spots it instantly; fails

The 4-wire feeder

Feeder from main service to sub-panel:

• Hot 1 (L1)
• Hot 2 (L2)
• Neutral (N)
• Equipment ground (EGC) (G)

For 120/240V single-phase residential sub-panels, this is 4 conductors. Old code (pre-2008 NEC) allowed 3-wire feeders to sub-panels at detached structures - that's no longer allowed (NEC 250.32 amended). Even existing 3-wire feeders are sometimes grandfathered in but new installs all use 4-wire.

Feeder sizing - quick reference

Per NEC Table 310.16 / 310.12 for residential dwelling-unit feeders at 60 °C / 75 °C termination:

The EGC column comes from NEC Table 250.122, which is sized off the rating of the overcurrent device ahead of the feeder, not off the feeder conductor. That table runs 60A to #10, 100A to #8, and 200A to #6, and it does not reach #4 until 300A. A 125A feeder takes #6 because you round up to the next row in the table, never down. Getting a size larger than the table calls for is legal and harmless; the error that matters is going smaller.

These are starting points. Adjustments needed for:

• Long feeder runs - voltage drop. 100A 100ft #4 cu = ~2% drop, acceptable. 200ft → upsize one or two gauges or use #2 cu. When you upsize the ungrounded conductors for voltage drop, NEC 250.122(B) requires the equipment grounding conductor be upsized proportionally, by the same ratio of circular-mil area. This is the step people skip on the long garage or shop run, and it is exactly the run where a fault has the most conductor resistance to overcome.
• Conduit fill - NEC Chapter 9 Table 1 + Table 4 for max-fill % by conductor count.
• Continuous loads - 125% sizing rule for continuous loads.

Grounding electrode at a detached structure

NEC 250.32: if the sub-panel feeds a detached structure (separate building from the service), a grounding electrode system must be installed at that detached structure. The 4-wire feeder still applies, plus:

• One or more grounding electrodes (ground rod, concrete-encased electrode, etc.)
• Grounding electrode conductor (GEC) sized per NEC 250.66
• Connection to the sub-panel's ground bar

The ground rod at the detached structure is NOT a substitute for the EGC in the feeder. Both are required.

For sub-panels in the SAME structure (e.g., garage sub-panel, basement sub-panel inside the main house), no separate grounding electrode is required.

Conduit and routing

• Run conduit for feeders over short distances or through finished spaces - easier inspection, cleaner install
• EMT typical inside the building; PVC schedule 40 or schedule 80 for outdoor or buried
• PVC schedule 80 (or RMC) required where exposed to physical damage (NEC 352.10)
• Sizing per NEC Chapter 9 - 1-1/4" EMT for 4-wire #4 cu feeder is typical (200A might bump to 1-1/2" or 2" EMT)
• Direct burial options: PVC conduit (schedule 40 minimum, 80 if shallow), buried 18" minimum (NEC 300.5) under residential property - local AHJ may have additional rules

Procedure - sub-panel install

Pre-work:

1. Calculate load (NEC Article 220) to determine sub-panel capacity needed
2. Calculate feeder size from main to sub (including voltage drop)
3. Confirm panel selection (Square D QO, Eaton CH/BR, Siemens) - same brand as main is often easier for breaker stocking
4. Pull permit. AHJ usually wants to see this work.

At main panel: 5. Kill main breaker. Verify dead at all bus terminals before working. 6. Find a 2-pole space for the sub-panel feeder breaker (sized for sub-panel rating - 100A breaker for 100A sub-panel). 7. Snap the new feeder breaker into the main panel.

Pull the feeder: 8. Route conduit / cable from main to sub-panel location. 9. Pull conductors. Black + red = L1 + L2 (re-identify with phase tape at both ends if THHN insulation is black-only). White = neutral. Green = ground. 10. Strip ends.

At sub-panel: 11. Mount the sub-panel enclosure. Working clearance per NEC 110.26 - 36" deep, 30" wide, 78" high in front. Do NOT install in a closet (NEC 240.24(D)), bathroom, or above stairs. 12. CRITICAL: Remove the bonding screw or strap that ties the neutral bar to the enclosure. Each panel manufacturer marks this differently - it's a green screw, a green strap with a "MAIN BOND" label, or a captive jumper that's cut. Some panels have multiple bonds; verify ALL are removed. 13. Install the ground bar - most sub-panels ship with a separate ground bar that bonds directly to the enclosure. Some panels need a ground-bar accessory kit purchased separately. 14. Connect feeder hots to the main lugs (L1 to L1, L2 to L2). Torque per the panel manufacturer's torque tag (typically 250-275 in-lb for 100A residential). 15. Connect feeder neutral to the (UNBONDED) neutral bar. Torque per spec. 16. Connect feeder EGC to the ground bar. Same torque spec range. 17. Install branch breakers as needed.

Detached structure additions: 18. Drive ground rod(s) per NEC 250.53 - single rod typically 8 ft, 5/8" copper-clad. If resistance > 25 Ω, drive a second rod 6 ft from the first. 19. Connect GEC from ground rod(s) to sub-panel ground bar.

Energize: 20. Verify all connections, conductor identification, and especially the bond removal one more time. 21. Restore main breaker. Verify voltage at sub-panel (L1 to L2 = 240V, L1 to N = 120V, L2 to N = 120V). 22. Measure voltage from sub-panel neutral to sub-panel ground - should read 0V or near-0V. If it reads any voltage, you have a bond problem (or a current imbalance on the feeder neutral).

AFCI / GFCI on sub-panel branches

NEC 210.12 still applies to branch circuits regardless of whether they originate at the main or a sub-panel. AFCI required where the branch supplies dwelling rooms. GFCI per 210.8.

Some sub-panels accept only certain breaker brands - verify before quoting AFCI breakers, which carry meaningful per-breaker cost.

Common installation mistakes

Leaving the main bonding jumper in. The one that fails every time. The green bonding screw, the bonding strap, or the factory-installed bond bar tie is still in place and neutral is now grounded twice. Neutral current returns on the EGC and on any metal path between the buildings, metal cases pick up voltage, and GFCIs downstream nuisance-trip. Remove the screw or strap, keep it in the panel envelope, and verify neutral-to-ground is open with the feeder off before you energize.

Landing neutrals and grounds on the same bar. Even with the bond removed, sharing a bar re-creates it. Two separate bars, neutrals on the isolated bar, grounds on the bar bonded to the can. Order the ground bar kit with the panel; most sub-panels do not ship with one.

Two neutrals under one lug. Not permitted, and it is the second thing an inspector looks for. One neutral per terminal. Grounds may double up only where the terminal is listed for it.

Driving a ground rod at a sub-panel inside the same building. A sub-panel in the same structure gets no grounding electrode. Only a separate building or structure needs one, and it still requires the 4-wire feeder with the neutral isolated.

Reusing an old 3-wire feeder. The pre-2008 allowance for a detached structure is gone for new work. If you are replacing the panel, you are pulling a 4-wire feeder.

Sizing the breaker to the panel rating instead of the load and the conductor. A panel labeled 100A does not require a 100A feeder. The overcurrent device protects the conductor. Do the load calculation, size the conductor, then size the breaker to the conductor.

Forgetting voltage drop on a long run to a garage or shop. Meets ampacity, sags under load, and the customer's welder or compressor never runs right.

No handle tie or no listed feeder breaker in the main panel, or backfeeding a main-lug sub-panel with no listed hold-down kit.

Missing working clearance. Not behind a door swing, not over a workbench, not in a closet or bathroom. Get the space right before you mount the can, because moving it later is the whole job over.

References

• NEC 250.32 (grounding at separate buildings and structures)
• NEC 250.142 (uses of equipment grounding conductor)
• NEC 250.24 (grounding of service)
• NEC Table 250.122 (equipment grounding conductor sizing) and 250.122(B) (proportional upsizing where ungrounded conductors are increased in size)
• NEC 310.12 and 310.16 (conductor ampacity)
• NEC 220 (load calculations)
• NEC 110.26 (working clearances)
• NEC 240.24 (overcurrent device location)
• Manufacturer installation manuals for the specific panel