Understanding overcurrent protection

Electrical engineers can use this guide to understand NFPA 70: National Electrical Code requirements for overcurrent protection.

08/17/2017


Learning objectives 

  • Understand the three types of overcurrent conditions to consider in typical NFPA 70: National Electrical Code applications.
  • Ascertain how to protect a circuit from dangerous overloads and short-circuits.
  • Review overcurrent protection for certain types of building equipment. 

Overcurrent protection seems like a simple concept: Limit the current flow in a circuit to a safe value. Electrical designers face this task daily. 

But there is much more to it. How do you limit the current flow? What is a safe value? The answers depend on the application, the equipment being protected, and the strength of the source.

Figure 1: There are four types of short circuits: 3-phase fault, line-to-ground fault (the most common), line-to-line ground fault, and line-to-line fault. A 3-phase fault usually results in the highest fault current. Courtesy: Environmental Systems DesignFortunately, the NFPA 70: National Electric Code (NEC) gives requirements for most of the applications that electrical engineers and designers encounter in their work. Though at first glance the NEC requirements might not seem straightforward, there is solid reasoning behind the overcurrent-protection code rules. Overcurrent protection (OCP) protects a circuit from damage due to an overcurrent condition. There are three types of overcurrent conditions to consider in typical NEC applications:

Overload: NEC 2017 defines overload as operation of equipment in excess of normal, full-load rating or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, such as a short circuit or ground fault, is not an overload.

Overload conditions are usually not as time-critical as short circuits and ground faults. Electrical equipment can usually withstand some level of load current over its rating for a length of time. Information regarding equipment-overload capability often comes from the manufacturer. However, some equipment—motors, transformers, and conductors, for example—have overload-protection requirements set by the NEC.

Short circuit: A short circuit is defined as flow of current outside the intended current path. In a 3-phase circuit, two types of short circuits are possible: symmetrical 3-phase faults and unsymmetrical single-phase faults (Figure 1). Symmetrical faults result in the same current flow in each phase during the fault condition. Unsymmetrical faults have different fault currents in each phase. Symmetrical 3-phase faults rarely occur, but their analysis is useful in understanding a system's response to a fault and usually results in the worst-case fault levels. Unsymmetrical faults are more common and usually result in less fault current than a symmetrical 3-phase fault.

Figure 2: Example of a circuit with no fault. The current can flow indefinitely without tripping the overcurrent protection. Courtesy: Environmental Systems DesignGround fault: A ground fault is a specific type of short circuit involving at least one of the phase conductors encountering a grounded conductor or surface. Ground faults include a single line-to-ground fault and multiple-line-to-ground faults (Figure 1). The single line-to ground fault is the most common type of fault.

The different types of faults are shown in Figure 1 to illustrate the concept of overcurrent protection.

What happens during an overload or fault condition? Figure 2 depicts a simple single-phase circuit operating in a normal configuration. In this case, the load current is 10 amps. The circuit is protected by a 15-amp circuit breaker. The circuit breaker does not open; the load current flows and the conductors do not overheat.

Figure 3 illustrates the result of an overload condition. In the overloaded circuit, the load current is nearly 20 amps. The circuit breaker will allow the overload condition to continue for approximately 2.5 minutes before opening the circuit. The conductors will begin to heat up, but will not be damaged.

Figure 3: Example of a circuit with an overload. The overload current can flow for 2 to 3 minutes before the overcurrent protection will open the circuit. The current can flow indefinitely without tripping the overcurrent protection. Courtesy: Environmental Systems DesignFigure 4 shows the result of a short circuit condition. The fault current is approximately 10,000 amps. The circuit breaker will allow the short circuit current to flow for only a short time. If the fault current persists, the insulation will melt and the conductors themselves will be damaged.

Figure 5 shows a ground-fault condition. In this example, the ground-fault path adds approximately .012 ohms of resistance in parallel with the load resistance, resulting in a much lower circuit resistance. The fault current is approximately 5,000 amps. As in the case with the short circuit, the circuit breaker will allow the fault current to flow for only a short time. Again, if the fault current persists, the insulation will melt and the conductors will eventually be damaged. 

How to protect a circuit from dangerous overloads and short circuits

Figure 4: Example of a circuit with a short circuit. The short-circuit current will cause the overcurrent protection to open the circuit immediately. Courtesy: Environmental Systems Design The requirements for overcurrent protection of equipment can be found in the NEC article that addresses that specific equipment. NEC Table 240.3 provides a list of the applicable sections. Sections for articles pertaining to equipment typically found in commercial buildings include: 

  • 230 Services
  • 368 Busways
  • 406 Receptacles
  • 410 Luminaires
  • 422 Appliances
  • 427 Fixed electric heating for pipelines and vessels
  • 430 Motors, motor circuits, and controllers
  • 440 Air conditioning and refrigerating equipment
  • 445 Generators
  • 450 Transformers and transformer vaults
  • 460 Capacitors
  • 517 Health care facilities
  • 620 Elevators
  • 660 X-ray equipment
  • 695 Fire pumps
  • 700 Emergency systems.

The general requirement for overcurrent protection of conductors is provided in Section 240.4, Protection of Conductors. The basic rule for overcurrent protection of conductors—other than using flexible cords, flexible cables, and fixture wires—is to protect the conductor in accordance with the ampacities specified in Section 310.15. Article 310 provides the general requirements for conductors, insulation, markings, mechanical strength, and ampacity rating.

Figure 5: Example of a circuit with a ground fault. The ground-fault current will cause the overcurrent protection to open the circuit immediately. Courtesy: Environmental Systems Design Several articles applicable to commercial buildings modify the general NEC rule for overcurrent protection, as summarized below: 

  • 240.4(A) Power Loss Hazard. If circuit interruption due to an overload condition could create a hazard—for instance, shutting down a fire pump-overload protection is not required. Short-circuit protection is required.
  • 240.4(B) Overcurrent Devices Rated 800 amps or Less. This section allows the next higher standard overcurrent device rating (provided the rating does not exceed 800 amps) to be used, as long as the conductors it is protecting are not used to supply a branch circuit with more than one receptacle for plug-connected loads and the ampacity of the conductor does not correspond with a standard ampere rating. If the overcurrent protective device is adjustable, it must be adjusted to the value equal to or less than the conductor ampacity.
  • 240.4(E) Tap Conductors. The general NEC rule requires the OCP to be located upstream of the conductor being protected. There are, however, special rules allowing the OCP to be placed in other locations of the circuit provided all the conditions of the NEC are met. For instance, household ranges and cooking appliances, fixture wiring, busways, and motors all have special rules allowing taps to be used.
  • 240.4(F) Transformer Secondary Conductors. The NEC, except in two special conditions involving two-wire, single-phase, and delta-delta 3-wire, requires transformer secondary conductors to be protected by a secondary OCP.
  • 240.4(G) Overcurrent Protection for Specific Conductor Applications. The NEC requirements for overcurrent protection for specific applications are found in sections other than 240. For example, the requirements for air conditioning and refrigeration equipment are found in Article 440, parts III and VI. Capacitor circuit conductor OCP requirements are found in Section 460. Motors and motor-control conductor overcurrent-protection requirements are found in Article 430 parts II, III, IV, V, VI, and VII. 

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