A Professional’s Guide to NEC Article 408: Panelboard Sizing and OCPD Rules

Mastering the requirements of NEC Article 408 is fundamental for any journeyman electrician or master electrician responsible for the safe and compliant installation of panelboards. This cornerstone of the nec code book governs everything from panelboard ampacity and bus ratings to overcurrent protection and labeling. A properly sized breaker panel, whether a 100 amp breaker panel or a 200 amp panel, begins with precise NEC Article 220 load calculations to determine the demand. Key among the rules is NEC 408.36, which mandates that a panelboard must be protected by an overcurrent protective device (OCPD) rated no greater than the panelboard itself. Furthermore, modern code requires that the panel’s short-circuit current rating (SCCR) is not less than the available fault current, a critical safety measure to prevent catastrophic failure. Understanding these rules is essential for every aspect of panelboard work, from installation to inspection and maintenance. This guide provides the expert instruction needed for compliant installations.

Understanding the Core Principles of NEC Article 408

For licensed electricians, Article 408 of the National Electrical Code is not just a set of guidelines; it’s the definitive standard for the installation of switchboards, switchgear, and panelboards. These components form the heart of a building’s electrical distribution system, making strict adherence to the code a matter of safety, reliability, and legality. While the article covers broad ground, its rules on sizing, overcurrent protection, and ratings are particularly critical for daily fieldwork. Whether you are performing electrician training for an apprentice or executing a complex commercial installation, a deep understanding of these requirements is non-negotiable.

Historically, the NEC distinguished between a lighting and appliance branch-circuit panelboard and a power panelboard, each with unique rules like the 42-circuit limit. However, since the 2008 NEC, these classifications have been removed, streamlining the requirements for all panelboards. Today, the focus is on universal principles of load, protection, and ratings, regardless of the panel’s specific application.

Panelboard Ampacity: Sizing Starts with Load Calculations

Before selecting a panelboard, you must first determine the total load it will serve. This is not guesswork; it requires a formal continuous load calculation performed according to the methods outlined in NEC Article 220 load calculations. These calculations are the foundation for determining the required panelboard ampacity and the size of the feeder conductors.

Key considerations in this process include:

• Continuous vs. Non-continuous Loads: A continuous load is one where the maximum current is expected to run for three hours or more. The NEC requires that branch circuits and feeders supplying these loads be sized to 125% of the load.
• Demand Factors: Article 220 provides demand factors that can be applied to certain loads (like general lighting and receptacles) because it’s assumed not all connected equipment will operate simultaneously.
• Specific Loads: Fixed appliances, motors, and HVAC systems have their own specific calculation requirements.

Once the total calculated load is known, you can select a panelboard with a sufficient rating. The panelboard bus rating must be equal to or greater than the minimum feeder capacity calculated. For example, after performing load calculations, you might determine that a 200 amp panel is required for a new residential service.

A related concept is the distinction between a load center and a panelboard. While often used interchangeably in residential contexts, panelboards are typically more robust and are the standard in commercial and industrial settings.

OCPD Requirements: The Mandate of NEC 408.36

The single most important rule for panelboard protection is found in NEC 408.36. It states that every panelboard shall be protected by an overcurrent protective device (OCPD) with a rating no greater than that of the panelboard. This OCPD can be located within the panel—known as a main breaker panel—or at any point on the supply side of the panel, such as a feeder breaker protecting a main lug only (MLO) panel.

Choosing between an MLO and a main breaker panel often comes down to application and cost. An MLO panel can be a cost-effective choice for a subpanel, as it relies on the breaker in the main distribution panel for its protection. However, if the panelboard is used as service equipment disconnect, it must have a main breaker (or up to six disconnects in some specific scenarios).

Short-Circuit Current Rating (SCCR) and Available Fault Current

One of the most significant safety advancements in modern code is the emphasis on fault currents. NEC Article 408.6 requires that panelboards have a short-circuit current rating (SCCR) equal to or greater than the available fault current (AFC) at their point of supply. AFC is the maximum current that could flow during a short-circuit event.

Why is this important? If the AFC exceeds a panel’s SCCR, a short circuit can cause a catastrophic explosion, posing a severe risk to personnel and property. The SCCR of an assembled panelboard is determined by its lowest-rated component, which could be the panelboard bus, a branch breaker like an arc fault circuit breaker, or the main breaker.

For all installations other than one- and two-family dwellings, NEC 408.6 also mandates that the AFC and the date the calculation was performed be field-marked on the enclosure. This provides crucial information for anyone servicing the equipment, including a master electrician planning a system modification or a journeyman performing maintenance.

How to Ensure SCCR Compliance: A Step-by-Step Guide

1. Determine the Available Fault Current: The first step is to calculate the AFC at the line-side terminals of the panelboard. This calculation starts at the utility transformer and considers the impedance of all conductors and equipment upstream. This value is often provided by the utility for service equipment.
2. Select a Panel with Adequate SCCR: Choose a panelboard with a listed SCCR that is greater than or equal to the calculated AFC. For example, if the AFC is 18,000A, a standard 10,000A rated panel is not compliant. You would need one rated for at least 22,000A. Recent code changes also clarify rules for using new panelboards in existing enclosures, which you can learn about in our lesson on replacing panelboards in different enclosures.
3. Verify Component Ratings: When installing circuit breakers, ensure their Ampere Interrupting Capacity (AIC) meets or exceeds the system requirements. You can’t just replace a circuit breaker with any model that fits; it must be listed for that panel and have the correct AIC rating. The safety of the entire system depends on it.
4. Consider Series Rated Combinations: A series rated combination is a cost-effective, engineered solution where an upstream OCPD with a high AIC rating is tested to protect downstream breakers with lower AIC ratings. If you use a series-rated system, you must use the exact components specified by the manufacturer and apply the required warning labels per NEC 110.22. Mixing brands in a series-rated system is a code violation.
5. Mark the Equipment: In commercial and industrial settings, apply a field label indicating the AFC and the date of calculation as required by the code. Worker safety is also improved by new requirements for panelboard barriers, a topic we cover in our lesson on panelboard safety barriers.

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Key Considerations for Installation and Inspection

• Bonded vs. Grounded Neutral: In service equipment panels, the neutral conductor must be bonded to the enclosure and the grounding electrode conductor. In subpanels fed by a four-wire feeder, the neutral must be isolated (floating), not bonded. This is a critical rule to prevent objectionable current from flowing on grounding conductors.
• Circuit Directory: Panelboard circuits must be legibly identified as to their purpose or use on a directory located on the face or inside of the panel doors [408.4]. Vague descriptions like “Plugs” are not sufficient; specificity like “Kitchen Counter Receptacles” is required.
• Unused Openings: All unused openings for circuit breakers must be closed with identified closures (filler plates) to maintain the integrity of the dead-front and enclosure [408.7].

Related Resources

• Load Center vs. Panelboard: What’s the Difference?
• How to Replace a Circuit Breaker Safely and Correctly
• How Do 2023 NEC Panelboard Barriers Improve Worker Safety?
• How Are 2023 NEC Panelboard Enclosure Requirements Different?
• How Do 2023 NEC Rules for Replacing Enclosed and Standard Panelboards Differ?

Frequently Asked Questions About NEC Article 408

1. Can the sum of breakers in a panel exceed the main breaker rating?

Yes. The sum of the individual branch breaker ratings in a breaker panel can, and often does, exceed the main breaker or panelboard rating. This is permissible because a proper continuous load calculation per NEC Article 220 assumes diversity—not all loads will be active at once. The panel is protected by its main OCPD, which is sized based on the calculated load, not the sum of the breaker handles.

2. What is the difference between a Main Lug Only (MLO) panel and a main breaker panel?

A main breaker panel includes a factory-installed main OCPD that protects the entire panel and serves as a disconnect. A main lug only (MLO) panel does not have an integral main breaker; its busbars are fed directly from lugs connected to an upstream OCPD, which provides the required protection per NEC 408.36. MLO panels are typically used as subpanels.

3. What does Short-Circuit Current Rating (SCCR) mean for a panelboard?

SCCR is the maximum level of fault current that a panelboard can safely withstand without causing an explosion or fire. As required by NEC Article 408, the panelboard’s SCCR must be equal to or greater than the available fault current at its location. This is a critical safety requirement to ensure the panel can handle a worst-case short-circuit event.

4. Do I still need to worry about the 42-circuit rule for panelboards?

No. The former NEC rule that limited “lighting and appliance” panelboards to 42 overcurrent devices was removed in the 2008 edition of the nec code book. Modern panelboards are now simply limited to the number of circuits for which they were designed, rated, and listed by the manufacturer.