Overcurrent Protection

Overcurrent Protection

Learning Objectives

Upon completing this chapter, you will be able to:

1.1 General Requirements: Article 240

Article 240 is the cornerstone of overcurrent protection. A master electrician must understand that this article governs all conductors and equipment, with specific exceptions found in other articles (e.g., motors, transformers, capacitors).

Location of OCPDs: Overcurrent devices must be located where the conductor to be protected receives its supply, unless specific exceptions apply. The most common exception is for feeder taps (240.21(B)), which allow conductors to be tapped from a feeder without an OCPD at the tap point, provided they meet length and ampacity rules (e.g., 3 ft, 10 ft, 25 ft taps). These tap rules are critical for commercial panelboards and industrial busway installations.

Accessibility: OCPDs must be readily accessible. This means capable of being reached quickly for operation, renewal, or inspections without climbing over obstacles or requiring portable ladders. However, service disconnecting means and OCPDs rated 1000 A or more may be accessible only to qualified persons, allowing them to be located in locked electrical rooms.

Enclosures: Circuit breakers and fuses must be enclosed unless they are part of an approved assembly. The enclosure must be listed for the specific device. A common field issue is the use of a non-identified enclosure for a breaker, which is a violation.

Disconnection of Ungrounded Conductors: A breaker or fuse must be connected in series with each ungrounded conductor. A multi-pole breaker must simultaneously disconnect all ungrounded conductors of the circuit. For a 3-phase, 4-wire system (wye), a 3-pole breaker is required for the ungrounded conductors; the neutral is not switched.

Supervision Point: On site, verify that all OCPDs are properly identified as to their function (e.g., "MAIN," "PANEL A," "MOTOR 3"). The NEC requires every circuit and modification to be legibly identified (408.4). This is a common inspection finding.

1.2 Standard Ratings and the "Next-Size-Up" Rule

Standard Amp Ratings (240.6): The NEC establishes standard ratings for fuses and inverse-time circuit breakers. These include: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 A. Adjustable-trip breakers with a minimum setting are permitted if the setting is within the standard ratings.

Conductor Protection (240.4): Conductors must be protected against overcurrent per their ampacity (from Table 310.16, etc.). The next-size-up rule (240.4(B)) permits the use of the next higher standard OCPD rating, but only if all of the following conditions are met:

Critical Trap: The next-size-up rule is not permitted for motor branch circuits. For motors, the OCPD is based on the motor's full-load current (FLC) from Tables 430.247–250, not the conductor ampacity. The conductors are sized per 430.22, and the OCPD is sized per 430.52, which often results in a device rating higher than the conductor ampacity. This is a deliberate and permitted exception to 240.4.

Supervision Point: When a panelboard is fed with a 400 A feeder, verify the feeder conductors have an ampacity of at least 400 A. A common error is using 500 kcmil copper (380 A at 75°C) with a 400 A breaker, relying incorrectly on the next-size-up rule. The next-size-up rule only applies to the next standard size (e.g., 380 A to 400 A), which is allowed, but if the conductor ampacity is 365 A, a 400 A breaker is a violation.

1.3 Overcurrent Protection of Transformers

Transformers are a major part of commercial and industrial work. The rules are found in Article 450, and they differ significantly based on voltage and whether you are protecting the primary, secondary, or both.

Primary-Only Protection (450.3(B)): For transformers over 600 V nominal, or for transformers 600 V or less, if the primary OCPD is rated at 125% of the transformer's rated primary current, no secondary protection is required. If the primary device is rated at 250% (for 600 V or less) or 300% (for over 600 V), secondary protection is required.

Transformer Rated Current: For a 3-phase transformer, the rated current is calculated as:

I = kVA × 1000 / (E_line-to-line × √3)

Example: A 75 kVA, 480 V 3-phase transformer has a primary current of 75,000 / (480 × 1.732) = 90.2 A. The primary OCPD can be sized at 125% = 112.8 A, so a 125 A breaker is permitted. This provides protection for the transformer and the primary conductors.

Secondary Protection (450.3(B)): When required, the secondary OCPD must be rated at 125% of the transformer's rated secondary current. For a 75 kVA, 208/120 V 3-phase transformer, the secondary current is 75,000 / (208 × 1.732) = 208 A. The secondary OCPD must be rated at 125% = 260 A, so a 300 A breaker is the next standard size.

Supervision Point: For a transformer feeding a panelboard, the secondary OCPD is often the main breaker in the panelboard. Verify that the panelboard main breaker rating is not less than the calculated secondary protection. If the panelboard is rated 225 A, but the transformer requires a 300 A secondary OCPD, the installation is a violation.

1.4 Overcurrent Protection for Services

Service conductors are protected per Article 230. The service disconnecting means must have a rating not less than the calculated load per Article 220. The OCPD must be located at the service point or a readily accessible location nearest to it.

Service Overcurrent Protection (230.90): Each ungrounded service conductor must have an OCPD. The device must be rated not less than the allowable ampacity of the conductor, but the next-size-up rule is permitted. However, the service OCPD must also be rated to carry the computed load.

Supervision Point: For a 1200 A service, the service conductors must have an ampacity of at least 1200 A. If parallel sets of 600 kcmil copper (420 A at 75°C) are used, two sets provide 840 A, which is insufficient. Three sets provide 1260 A, which is acceptable.

Service Disconnect Rating: The service disconnecting means for a single-family dwelling must be at least 100 A (230.79). For commercial and industrial, the rating must be sufficient for the calculated load.

1.5 Motor and Generator Overcurrent Protection

Article 430 is the most complex article for overcurrent protection. A master must understand the distinction between:

Branch-Circuit OCPD (430.52): The maximum rating of the branch-circuit OCPD is based on a percentage of the motor's FLC, depending on the type of motor and the device used.

If the calculated value does not correspond to a standard rating, the next higher standard size is permitted. However, if the motor will not start with this device, the NEC allows for higher ratings under specific conditions (430.52(C)(1) Exceptions), but the conductors must still be protected.

Motor Overload Protection (430.32): The overload device must be sized at 115% of the motor's nameplate full-load current (FLC) for motors with a service factor of 1.15 or more, or a temperature rise of 40°C or less. For all other motors, it must be sized at 125% of the nameplate FLC. The next higher standard size is permitted if the calculated value does not match.

Critical Trap: Do not confuse the motor's nameplate FLC with the FLC from the NEC tables (430.247–250). The tables are used for conductor sizing and branch-circuit OCPD sizing. The nameplate is used for overload protection sizing.

Generators (Article 445): Generators are treated similarly to motors, but the OCPD is sized to protect the generator windings. The OCPD must be rated at 115% of the generator's rated current (445.12). The conductors from the generator to the first OCPD must be sized at 115% of the generator's rated current.

1.6 Overcurrent Protection Coordination

Selective Coordination (240.12): The NEC requires selective coordination for:

Selective coordination means that when an overcurrent occurs, only the OCPD closest to the fault opens, leaving the upstream devices and the rest of the system energized. This is achieved by ensuring that the time-current curves of the devices do not overlap.

Supervision Point: For a commercial building with a legally required standby generator, the feeder breaker feeding the life-safety panel must be selectively coordinated with the branch-circuit breakers in that panel. This often requires the use of current-limiting fuses or zone selective interlocking (ZSI) breakers. Simply using a larger breaker upstream does not guarantee coordination; you must review the manufacturer's time-current curves.

Exam Trap: The NEC does not require selective coordination for normal (non-emergency) power systems. It is a design choice for reliability, but not a code requirement.

1.7 Code Navigation: Where to Find It

1.8 Inspection and Supervision Points

As a master electrician, your responsibility extends to verifying the work of others. Key inspection points include:

1.9 Common Exam Traps