Panelboard Overcurrent Protection Calculator

The 408.36 rule, and why it rounds down

NEC 408.36 is short: a panelboard must be protected by an overcurrent protective device having a rating not greater than that of the panelboard, and that device must sit within the panel or anywhere on its supply side. The rating may equal the panel rating - a 225 A panelboard is fine with a 225 A main - but it may never exceed it. The point is that the busbar and the branch devices inside the panel are only rated for so much current, so the device feeding the panel has to clamp the current at or below that rating.

That "not greater than" language makes 408.36 a ceiling, which is why the largest permitted device rounds down to a standard NEC 240.6(A) rating. This is the opposite of sizing a branch or feeder overcurrent device from a load, where 240.4(B) lets you round up to the next standard size when the load falls between two ratings. That next-higher-standard allowance is a conductor-protection rule; it does not apply to protecting a panelboard. On a 199 A panel, the largest permitted breaker is 175 A, not 200 A.

When individual protection is not required

The 2020 NEC keeps one exception that waives an individual protective device on the panel: a panelboard that is not service equipment, protected on its supply side by two main circuit breakers or two sets of fuses whose combined rating is not greater than the panelboard rating. Note the 2020 change - the former exception for a service-equipment panelboard with multiple disconnecting means was deleted when 230.71 was revised to require each of the two-to-six service disconnects to be in its own separate enclosure. For service equipment, the service disconnecting means itself provides the protection; see the Service Disconnect Calculator.

A panelboard supplied through a transformer takes its required overcurrent protection on the secondary side of the transformer (408.36(A)), with a narrow exception for a two-wire secondary. Sizing the transformer's own overcurrent device is a separate calculation - use the Transformer Sizing Calculator for NEC Table 450.3(B).

Back-fed breakers (408.36(B))

A common field question is whether you can back-feed a breaker to supply a panel - for example, feeding a panel from a breaker in another panel, or landing a PV or generator interconnection on a breaker. You can, but NEC 408.36(B) requires a plug-in-type overcurrent device or plug-in main-lug assembly that is back-fed and terminates field-installed ungrounded supply conductors to be secured in place by an additional fastener - a hold-down kit - that requires more than a simple pull to release it. The rule keeps an energized device from being pulled out of the bus while live. Bolt-in devices are already secured. The back-fed device's rating still must not be greater than the panelboard rating.

Worked examples

• 225 A panelboard, 200 A main breaker. 200 A is not greater than 225 A, so the device is permitted. The largest breaker the panel may be protected by is 225 A.
• 100 A sub-panel, 125 A feeder breaker. 125 A exceeds the 100 A panel rating - not permitted. Use a 100 A device or smaller, or a higher-rated panel.
• 105 A panel, breaker choice. The largest permitted breaker is 100 A. You cannot use a 110 A device because 110 A is greater than 105 A - the rating rounds down.
• 650 A panel, fuses. The largest permitted fuse is 601 A (a fuse-only 240.6(A) rating); the largest breaker would be 600 A.

Frequently asked questions

What is the maximum breaker size for a panelboard?

The largest NEC 240.6(A) standard rating at or below the panelboard rating. 408.36 requires the protective device to be rated not greater than the panel, so a 200 A panel takes up to a 200 A device and a 225 A panel up to 225 A. It rounds down, never up.

Does a panelboard always need a main breaker?

No. The protection can be a supply-side device (a feeder breaker ahead of the panel). For a service-equipment panel, the service disconnecting means provides the protection. What matters is that some device not greater than the panel rating protects it.

Is this the same as sizing a breaker for a load?

No. Sizing a breaker or fuse from a load uses 210.20(A)/240.6(A) with the 125% continuous rule and the 240.4 conductor-protection rules, and rounds up. This tool checks a device against a panelboard rating under 408.36 and rounds down. Use the Breaker Size or Fuse Size calculators for the load-based sizing.

Which NEC edition is this?

NEC 2020. The 408.36 rule and the 240.6(A) standard ratings are stable across recent editions; always confirm the edition your jurisdiction has adopted.

Related calculators

• Breaker Size Calculator: size a branch or feeder breaker from a load with the 125% continuous rule and NEC 240.6(A).
• Fuse Size Calculator: size a fuse for a general-use load, including the fuse-only 1/3/6/10/601 A ratings.
• Service Disconnect Number & Rating Calculator: the NEC 230.71/230.79/230.80 rules for the service disconnecting means, which protects a service-equipment panelboard.
• Transformer Sizing Calculator: maximum primary and secondary overcurrent device per NEC Table 450.3(B) for a transformer-supplied panel.