Understanding overcurrent protection

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.

Fortunately, 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.

Ground 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 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

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.

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.

Selecting OCP ratings

In the following examples, the rating of the OCP trip value will be determined along with the ampacity of the conductors used in the circuit. The short-circuit current rating and interrupting rating must also be determined based on the available short-circuit current in the circuit. Calculation of the available short-circuit current is outside the scope of this discussion.

Branch circuits

The requirements for branch-circuit overcurrent protection are found in Section 210.20. The general requirement is to size the OCP for no less than 125% of the continuous load and 100% of the noncontinuous load. The NEC definition of a continuous load is a load where the maximum current is expected to continue for 3 hours or more.

For example, consider a single-phase 120 V circuit feeding an open-office lighting load (continuous) of 1,000 VA and a small cooling unit’s condensate pump load (non-continuous) of 100 VA. The circuit load for the purpose of sizing the OCP is:

OCP sizing load = 1.25 x 1,000 VA + 1.00 x 100 VA

OCP sizing current  = 1,350 VA/120 V

Now select a conductor in accordance with sections 210.19(A) and 310.15. Section 210.19(A) requires the conductor to be sized in the same manner as the OCP-no less than 125% of the continuous load and 100% of the noncontinuous load. In the example above, the circuit conductors (copper heat-resistant thermoplastic (THHN[A1] [A2] )) are routed through the office environment in a conduit containing six current-carrying conductors. By referencing Table 310.15(B)(16), the minimum conductor size allowed is #14. Even though this example is using copper THHN wire, rated for 90°C, the 60°C column must be used due to the requirement of Section 110.14(C)(1)(a). This section requires the use of the 60°C column in Table 310.15(B)(16), because the terminations for equipment rated 100 amps or less is assumed to be rated for 60°C unless listed and labeled otherwise. Further, Section 240.4(D), Small Conductors, requires the OCP for #14 wire to be rated at 15 amps.

The general rule for NEC ampacity selection is found in Section 310.15, which refers to the tables in Section 310.15(B). Section 310.15 contains limiting factors that must be applied to the ampacity table values when determining ampacity for your specific design conditions. Of the factors to be considered, two commonly encountered factors, or deratings, are the ambient temperature and the number of conductors in a raceway. Looking through the tables in 310.15(B), notice that some tables are based on an ambient temperature of 30°C and others are based on 40°C.

Ambient-temperature correction factors for 30°C tables are found in Table 310.15(B)(2)(a). Ambient-temperature correction factors for 40°C tables are found in Table 310.15(B)(2)(b). Adjustments for the number of current-carrying conductors in a raceway are found in Table 310.15(B)(3)(a). There are some conditions under which the derating factors do not apply, as seen in 310.15(B)(3)(a)(2) through (4). For example, the derating factors do not apply to type armored cable (AC) and metal-clad (MC) cables provided the cables do not have an overall jacket, each cable has not more than three current-carrying conductors, the conductors are #2 AWG, and not more than 20 current-carrying conductors are installed without maintaining spacing.

For this example, the wires are routed through an office environment where the highest temperature is expected to be 85°F during times when the cooling systems are turned off. Table 310.15(B)(2)(a) provides the ambient-temperature correction factors, which must be applied to the ampacities given in Table 310.15(B)(16). For an ambient temperature of 85°F, the correction factor for copper THHN 90°C wire is 1.0, so no ampacity adjustment is required.

Next, derating for the number of conductors in the conduit must be considered. In our example, there are six current-carrying conductors routed in the conduit. Table 310.15(B)(3)(a) is used to determine the appropriate derating factor. For four to six conductors in a raceway, the derating factor is 80%. Number of conductors

#14 copper THHN ampacity = 25 amps x 0.8

As discussed above, the 60°C ampacity of 15 amps must be used for the #14 wire in this example despite the higher ampacity calculated.

Feeder circuits

The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. As with branch circuits, the general requirement is to size the OCP no less than 125% of the continuous load and 100% of the noncontinuous load.

Consider a 208 V, 3-phase feeder supplying a panelboard with a noncontinuous load of 10 kVA and a continuous load of 30 kVA. The circuit load for the purpose of sizing the OCP is:

OCP sizing load

= 1.25 x 30,000 VA + 1.00 x 10,000 VA

= 47,500 VA

OCP sizing current

= 47,500 VA/(1.73 x 208 V)

= 132 amps

The next highest standard OCP (see table 240.6(A)) is 150 amps.

Next, select a conductor in accordance with sections 215.2 and 310.15. Section 215.2 requires the conductor to be sized in the same manner as the OCP-no less than 125% of the continuous load and 100% of the noncontinuous load. In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. The conduit will contain three current-carrying conductors.

Referencing Table 310.15(B)(16), the minimum conductor size allowed for an OCP rating of 150 amps is #1/0. As with the previous example, the wire type selected is copper THHN, which is rated for 90°C. In this case, the 75°C column must be used due to the requirement of Section 110.14(C)(1)(a). This section requires the use of the 75°C column in Table 310.15(B)(16), because the terminations for equipment rated 100 amps or higher are required to be rated for 75°C unless listed and labeled otherwise.

In this example, the wires are routed through a boiler room where the highest temperature is expected to be no higher than 120°F. Table 310.15(B)(2)(a) provides the ambient-temperature correction factors, which must be applied to the ampacities given in Table 310.15(B)(16). For an ambient temperature of 120°F, the correction factor for copper THHN 90°C wire is 0.82. Thus, the calculated ampacity for the #1/0 copper THHN wire used in this example is:

Ambient temperature

#1/0 copper THHN ampacity = 170 amps x 0.82 = 139.4 amps

Note that there is an exception to Table 310.15(A)(2) that allows the higher ampacity to be used for cables having different ampacities where the lower ampacity does not exceed 10 ft or 10% of the total circuit length.

Next, derating for the number of conductors in the conduit must be considered. In the example above, there are three current-carrying conductors routed in the conduit. Since the ampacities in Table 310.15(B)(3)(a) already account for up to three current-carrying conductors, no further derating is required.

Once the ampacity is determined, the voltage drop should also be considered. For long circuits, the conductor size may need to be increased to maintain minimum voltage-drop requirements. The NEC has informational notes concerning voltage drop for branch circuits and feeders, but it is not a code rule. However, many authorities having jurisdiction have made voltage drop a code requirement. Also, energy codes require voltage drop to be considered.

After applying the appropriate deratings, the calculated ampacity of the #1/0 wire is adequately protected by the 150-amp OCP selected above. Consideration should be given to load growth. The load and cable ratings calculated above are minimum values. It is common practice to add 20% of the minimum cable rating to be used for future load additions.

Requirements for motor-circuit protections

The requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. Table 240.4(G) requires Article 430 to be used for selection of motor-circuit overcurrent protection. The requirements for motor-circuit overcurrent protection are different than branch and feeder circuits, often leading to confusion. For motor circuits, overload protection is provided by the motor-overload protector (refer to Article 430 Part III).

The motor-overload protector is usually a device, located in the motor starter, that responds to motor current and is set to trip open the motor controller when the motor current exceeds 125% of the nameplate current for 1.15 service factor motors or 115% of nameplate current for motors without a service factor. The OCP used to provide power to the motor controller and motor has to then provide short-circuit and ground-fault protection for the motor circuit. The requirements for determining the maximum rating or setting for motor-branch-circuit short circuit and ground-fault protection can be found in Table 430.52. To use this table, you must know the type of motor used in the circuit and the type of OCP used to protect the circuit.

Consider a 460 V, 3-phase motor branch circuit supplying power to a 100-hp, squirrel-cage motor protected by an inverse-time molded-case circuit breaker. The copper THHN circuit conductors are routed in an area with an ambient temperature not exceeding 104°F, and the number of current-carrying conductors in the raceway is three. For this example, Table 430.52 allows a circuit breaker with a maximum rating of 2.5 times the full-load current of the motor. The motor full-load current used in this calculation is not the nameplate current, but the current value found in Table 430.250.

Motor current

= 124 amps

Max OCP rating

= 2.5 x 124 amps

= 310 amps

Section 430.52(C)(1) has an exception that allows the next higher standard rating to be used. In this case, the maximum OCP rating is then 350 amps. If the motor starting torque and time to reach operating speed is such that the motor will not start, Section 430.52(C)(1), Exception 2, allows the OCP rating to be raised even higher. In the case of an inverse-time circuit breaker for the motor in this example, Exception (c) allows the OCP rating to be increased from 250% to 300%. However, the next higher rating allowance does not apply to Exception (c). An OCP rating of 300% higher than the full-load current is 3 x 124 amps = 372 amps. This rating falls between the standard ratings of 350 amps and 400 amps. In this example, the OCP rating cannot be increased above 350 amps. Typical practice is to use an OCP rating less than the maximum calculated above. Some electrical distribution manufacturers provide slide-rule-type guides to help select motor-circuit ratings. Phone apps are also available that provide the same function as the slide rule. Three different manufacturer slide-rule guides were checked; they all recommend a circuit breaker OCP rating for 200 amps for the following example.

The conductor ampacities for a motor circuit can be determined using Article 430, Part II. Section 430.22 applies to this example in that it is a single motor circuit. The requirement for sizing the conductors is simply 125% of the full-load current specified in Table 430.50.

Motor circuit conductor ampacity

= 1.25 x 124 amps

= 155 amps

Referring to Table 310.15(B)(16), using the 75°C column, the minimum size wire acceptable is #2/0 with a rating of 175 amps. Note that the maximum OCP rating is 350 amps, which is significantly higher than even the 90°C column ampacity of 195 amps. This condition is allowed by the NEC because overload protection is provided by the overload protector in the motor starter, which is set at 125% of full-load nameplate current for a motor service factor of 1.15. The motor-circuit OCP is providing only short-circuit and ground-fault protection.

In this example, the wires are routed through an environment where the highest temperature is expected to be no higher than 104°F. Table 310.15(B)(2)(a) provides the ambient-temperature correction factors, which must be applied to the ampacities given in Table 310.15(B)(16). For an ambient temperature of 104°F, the correction factor for copper THHN 90°C wire is 0.91. The calculated ampacity for the #2/0 copper THHN wire used in this example is:

Ambient temperature

#2/0 copper THHN ampacity

= 195 amps x 0.91

= 177.5 amps

The conductor ampacity derated for ambient temperature is higher than the 75°C ampacity column, so it is acceptable for use in this example.

Air conditioning and refrigerating equipment circuits

As with motors, the requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. Table 240.4(G) requires Article 440 to be used for selection of motor-circuit overcurrent protection for air conditioning and refrigeration equipment.

When determining OCP ratings for motors, the full-load ampere (FLA) values given in Article 430 are used. These values are generally higher than the FLA values found on the actual motor nameplate, resulting in conservatively selected OCP and conductor ratings. In the case of hermitically sealed motor compressors, the motor FLA values found in Article 430 will not be higher than actual motor values due to the cooling effect the refrigerant has on the motor windings. For example, a 1.5-hp motor used in a hermetically sealed compressor might be able to do 2 hp of work because heat is being removed from the motor windings, allowing higher currents to flow without exceeding the winding-conductor temperature rating.

For this reason, the manufacturer must provide data specific to the air conditioning and refrigerating equipment being used. Specifically, the maximum overcurrent-protection (MOP) value must be used to determine the rating of the air conditioning or refrigerant circuit. Also, minimum circuit amperes (MCA) must be used to determine the minimum conductor rating. This data will be found on the equipment nameplate and can also be obtained from the manufacturer in the form of a data sheet. The air conditioning and refrigeration equipment manufacturer must also indicate whether a fuse or circuit breaker can be used to supply the equipment.

Consider an example of an air conditioning unit with a MOP (circuit breaker or fuse) of 50 amps and an MCA of 31.0 amps. In this example, the air conditioning unit is fed using copper THHN wire in a conduit containing three current-carrying conductors. The air conditioning unit is located outdoors with a maximum ambient temperature of 120°F.

In this example, the OCP is simply equal to the manufacturer-supplied MOP value of 50 amps, since 50 amps is a standard OCP rating per Table 240.6(A). Either a circuit breaker or fuse could be used since the manufacturer listed the equipment with both types of OCP device.

The wire size will be based on the manufacturer-supplied MCA value, which in this case is 31.0 amps. Using Table 310.15(B)(16), the 75°C column, the minimum wire size is #8. The #10 wire has sufficient ampacity, but per Section 240.4(D), it must be protected with an OCP with a rating of 30 amps or less. This example requires a 50-amp OCP, therefore a #8 wire must be used. Since there are only three current-carrying conductors in the conduit for this example, derating for the number of conductors is not required. The cable ampacity must be corrected for an ambient temperature of 120°F. Referring to Table 310.15(B)(2)(a), the correction factor for 90°C-rated copper THHN wire with a maximum ambient temperature of 120°F is 0.82.

Temperature derated ampacity = 0.82 x 55 amps

= 45.1 amps

The derated temperature rating is higher than the MCA value of 31.0 amps for the #8 conductor, which is acceptable for this example.

There are several applicable sections of the NEC that set the requirements for the selection of OCPs and conductors for commercial buildings. Several of the NEC sections are modified for all the specific installations and equipment found in commercial buildings. By paying attention to the details, OCP ratings can be selected to provide safe and reliable operations for the lifetime of the equipment. [HEAD]

Continuous and noncontinuous loads

NFPA 70: National Electric Code (NEC) gives little guidance regarding continuous and noncontinuous loads and why this is important. It is important to have a distinction continuous and noncontinuous loads because of heat. Consider a 25-amp load flowing in a circuit. In the noncontinuous case (for example, a large sump pump), the load might be active for less than a minute. In the continuous case (for example, a chiller), the load might be active for 8 hours or longer. Comparing the two cases, the chiller circuit conductors will be at a higher temperature during operation than the sump pump circuit conductors.

The NEC requires a more conservative selection (125%) in the case of continuous loads due to increased heat dissipated by the circuit conductors as compared with noncontinuous loads. Some examples of continuous loads include office lighting, exterior lighting, data center equipment, fixed storage-type water heaters with capacities of less than 120 gal (450 l; as per NEC 422.13), and chilled/hot-water circulating pumps. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. Determining whether a load is continuous or noncontinuous is not always clear. Consider the case of an office storeroom lighting circuit. If designed to current energy code requirements, it should have a vacancy sensor to automatically shut off the lights when there are no occupants detected. This seems like an example of a noncontinuous circuit. What if the sensor failed or the room was temporarily repurposed as an office? Some permit reviewers might require this to be considered a continuous load. In cases where a load is certain to be noncontinuous, size the circuit to 100% of the load. If the load is debatable, be conservative and design for a continuous load.

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Steven Eich is a vice president and electrical technical director at Environmental Systems Design in Chicago. His expertise includes 29 years of designing electrical systems for industrial and commercial projects including high-rise buildings, hospitals, schools, theaters, museums, hotels, convention centers, manufacturing facilities, water treatment plants, and nuclear power facilities.

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