NEC Article 240 covers the requirements for selecting and installing Overcurrent Protection Devices (OCPDs), such as fuses and circuit breakers. The article defines proper ratings, placement, applications, and special cases for OCPDs across applications.

What Is an Overcurrent?

An overcurrent occurs when the current exceeds the safe limit of a conductor or equipment. This can happen due to:

• Overload: Too much current under normal operation (e.g., running multiple high-wattage appliances on one circuit).

• Short Circuit: Direct contact between two hot wires or a hot and neutral wire.

• Ground Fault: A hot wire contacts a grounded surface, like a metal box or wet floor.

How OCPDs Protect Circuits vs. Equipment

OCPDs protect circuits by opening when the current reaches a value that would overheat the conductors. This protection works differently for equipment. For equipment, OCPDs stop short circuits or ground faults before significant damage occurs.

In short, circuit protection avoids gradual overheating; equipment protection prevents immediate, catastrophic failure.

Standard Ampere Ratings

OCPDs are manufactured in standard ratings listed in Section 240.6, ranging from 15A to 6000A. When sizing a breaker, you typically:

• Determine conductor ampacity

• Adjust ampacity per 310.15

• Choose the next standard size (if under 800A and other conditions are met)

Small Conductor Protection (240.4(D))

The maximum overcurrent protection is limited for small conductors:

• 14 AWG copper: 15A

• 12 AWG copper: 20A

• 10 AWG copper: 30A

• 12 AWG aluminum: 15A

• 10 AWG aluminum: 25A

These values are fixed and do not allow the “next-size-up” rule. They are in place to prevent overheating in the most commonly used residential and branch circuit conductors. Note that these are the only sizes of conductors mentioned in that article.

Exception: Power Loss Hazard (240.4(A))

In life-safety or mission-critical systems, overload protection may be omitted if shutting down power would create a greater hazard.

Example: Fire pump circuits are required to continue running during emergencies, even when overloaded. While short-circuit protection remains required, overload protection can be excluded to preserve system functionality.

This exception must be applied with caution and is only allowed where a power interruption poses a risk to life or safety.

Next-Size-Up Rule

When conductor ampacity doesn’t match a standard size, you may round up to the next standard breaker rating if:

• The OCPD is 800A or less

• The conductor does not feed multi-outlet branch circuits

• The next size does not exceed 800A

The conductor ampacity must be at least equal to the breaker rating if the OCPD is over 800 A.

Location in Circuit (240.21)

OCPDs must be located where conductors receive power, except in the following situations:

• (A) Branch circuits that meet 210.19:

• Certain fixed appliances, such as cooking equipment (stoves), can have unprotected conductor extensions when the ampacity and installation conditions comply with 210.19.

• (B) No taps from other taps:

• You can’t make a secondary tap from an existing tap conductor.

• (C) Transformer secondary protection via primary OCPDs:

• If the primary OCPD is properly rated and installed, it can protect the secondary side of a transformer.

• (D) Service conductors (230.91):

• These follow their own rules for protection at the first disconnect.

• (E) Busway taps (368.17):

• Taps from busways are allowed under specific tap conductor rules.

• (F) Motors (430.28, 430.53):

• Motors often use separate overload protection and don’t require OCPDs at every location.

• (G) Generators (445.12, 445.13):

• Generator outputs may omit an immediate OCPD if inherent protection is provided and installation complies with Article 445.

Tap Conductor Rules with Examples

Tap conductors don’t have a breaker at their source, so the NEC limits their:

• Length: Common limits are 10 or 25 feet, depending on conditions.

• Size: Must be large enough for the load and a portion of the feeder ampacity.

• Termination: Must end at an OCPD (e.g., breaker or panel) that limits the fault risk.

Example: A 25-ft tap from a 400A feeder must be at least one-third the size (133A min) and run in conduit to a protected panel.

Supplementary OCPDs (240.10)

These are internal fuses or breakers used inside appliances or luminaires. They do not count as branch circuit protection and need not be readily accessible.

Location on Premises (240.24)

OCPDs must be:

• Readily accessible (no ladders or tools needed)

• No higher than 6'7" to the handle top (with a few exceptions)

• Not in bathrooms or clothes closets

• Installed away from corrosive or hazardous environments

Enclosure Requirements (240.33, 240.81)

• Enclosures must be vertical unless impractical

• Handles operated vertically must be ON in the up position

• Enclosures are designed for safe operation with the user standing at the side

Circuit Breakers (240.80–240.88)

• Must be manually operable

• Can include shunt trips or remote operators

• Must be marked SWD or HID for use as lighting switches (120V or 277V)

Ground-Fault Protection (240.13)

Required for services ≥1000A on solidly grounded wye systems >150V to ground. Prevents long-duration faults that cause fires.

Selective Coordination (240.12)

Used in hospitals, emergency systems, and elevators. Only the device nearest the fault should trip to prevent shutdown of larger systems.

Series Ratings (240.86)

Allows downstream breakers rated below fault current if used with listed upstream OCPDs and properly marked. Saves space and cost in some commercial installations.

Arc Energy Reduction (240.67 for Fuses, 240.87 for Breakers)

Applies when OCPDs ≥1200A. Must use one of the following:

• Zone-selective interlocking

• Maintenance switch

• Instantaneous trip

• Differential relaying

• Energy-reducing active devices

This rule focuses on worker safety during arc flash events.

Cables Suitable for Overcurrent Protection Applications

THHN/THWN-2 Copper Wire

• For general-purpose wiring for feeders and branch circuits in dry or wet locations when installed in conduit.

• 600V, up to 90°C

• Paired with breakers and fuses for indoor and outdoor panelboards.

XHHW/XHHW-2 Wire

• Similar to THHN but with better insulation thickness and flexibility; used in conduit or raceways.

• 600V (XHHW) or 1000V (XHHW-2)

• Used for long feeder runs in commercial or industrial settings.

MC Cable (Metal-Clad)

• For feeder and branch circuits in industrial/commercial environments.

• 600V

• Must be listed for the environment (wet-rated MC required outdoors); often used from panel to equipment with circuit breaker protection.

NM-B Cable (Romex)

• For residential branch circuits in dry, indoor locations.

• 600V

• Used with 15A or 20A breakers for lighting and receptacle circuits. Not allowed in wet or exposed installations.

UF-B Cable

• For underground branch circuits (e.g., landscape lighting, outdoor outlets).

• 600V

• Paired with GFCI breakers, not permitted inside conduit where heat cannot dissipate.

Tray Cable (TC, TC-ER)

• This is for industrial branch circuits in cable tray systems or directly from the panel to the equipment.

• Typically 600V

• Must be used per labeling (e.g., TC-ER for exposed runs); not a substitute for MC unless specifically rated.

Service Entrance Cables (SE/USE)

• Used from utility meter to main breaker panel.

• 600V

• Paired with main breakers or fuses per Article 230 and Article 240.