Overcurrent protection for panelboards

What the code requires

Each panelboard generally needs overcurrent protection rated no higher than the panelboard's own rating. The protective device can be inside the panelboard or anywhere on the supply side ahead of it. A panelboard with snap switches rated 30 A or less must be protected at 200 A or less, and delta breakers are not allowed in panelboards.

Where a panelboard is supplied through a transformer, section 408.36(B) requires its overcurrent protection to be on the secondary side of the transformer. Protection on the primary side counts instead only where it meets 240.21(C)(1).

Under 408.36(D), a plug-in breaker or plug-in main lug assembly that is back-fed, meaning power flows into the panel through it rather than out, and that terminates field-installed supply conductors must be held in place by an additional fastener that takes more than a pull to release. A breaker marked Line and Load is not made to be back-fed, and 110.3(B) requires following that marking.

On the job

• A back-fed plug-in breaker installed without the required additional fastener, often called a hold-down kit, is a correction.
• A panel protected by a breaker larger than its bus rating does not comply.
• Connections for solar and other power sources also fall under Article 705 (see 705.12), so ask your inspector how those rules apply to a back-fed breaker.
• Separate from 408.36, NFPA TIA 23-12 (effective Sept 14, 2023) replaced the 408.6 fault-current marking's reference to 110.21(B)(3), an item that does not exist in the 2023 code, with a plain requirement that the marking be durable enough for its environment. California may print the code without later TIAs, so confirm with your building department.

Exceptions that matter

• Outside service equipment, a panelboard protected by two main breakers or two sets of fuses with a combined rating no higher than the panelboard needs no individual protection; such a panelboard may hold no more than 42 overcurrent devices, counting each pole of a 2- or 3-pole breaker.
• An existing panelboard used as service equipment for an individual residential occupancy does not need individual protection.

Related sections

• NEC 240.4
• NEC 240.21(C)
• NEC 110.3(B): Install equipment per its listing instructions
• NEC 408.6
• NEC 705.12: The 120% rule for solar and battery breakers

Where this comes from

• Mike Holt: NEC requirements for switchboards and panelboards (EC&M, Dec 2024)
• Mike Holt: Summary of the 2023 changes to the NEC
• NFPA: TIA 23-12 to NFPA 70 (2023)
• Read the official NEC free at NFPA