Both types of control restrict the amount of current that can flow in the circuit. Table 1describes current, voltage and protection circuit assumptions for each period. The air conditioning and refrigeration equipment manufacturer must also indicate whether a fuse or circuit breaker can be used to supply the equipment. An overcurrent can be caused by overloading the circuit or by a short circuit, a ground fault, or an arc fault. Search Products And Discover New Innovations In Your Industry!! Review overcurrent protection for certain types of building equipment. This rating falls between the standard ratings of 350 amps and 400 amps. Fig. The general rule for NEC ampacity selection is found in Section 310.15, which refers to the tables in Section 310.15(B). Note that the maximum OCP rating is 350 amps, which is significantly higher than even the 90°C column ampacity of 195 amps. For motor circuits, overload protection is provided by the motor-overload protector (refer to Article 430 Part III). Ground-fault overcurrent is also a short-circuit condition that normally affects only one of the circuit conductors and the grounded metal raceway or electrical distribution or utilization equipment enclosure. Number of conductors. For this reason, the manufacturer must provide data specific to the air conditioning and refrigerating equipment being used. Both short-circuit and ground-fault currents are high-magnitude overcurrents caused by an accidental low-resistance parallel connection to the connected load resistance. The feeder overcurrent protection device must be sized not less than 125% of 184 amperes, So, overcurrent protection device size = 184 amperes x 125% = 230 amperes. Three different manufacturer slide-rule guides were checked; they all recommend a circuit breaker OCP rating for 200 amps for the following example. 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. This should not be mixed with ‘overload’ relay protection, which typically utilizes relays that function in a time-related in some degree to the thermal capacity of the equipment to be protected. Depending on the time of operation the overcurrent relay is categorized into following types. In general, the conductors of all of these circuits must be provided with a means of overcurrent protection at the point where they receive their electrical supply. Action current of low voltage circuit is 1.2 times of starting current of electric motor. In this example, the wires are routed through an environment where the highest temperature is expected to be no higher than 104°F. The circuit breaker does not open; the load current flows and the conductors do not overheat. Transformer primary protection— Overcurrent protective devices (OCPDs) are required as primary protection for transformers when the primary voltage is greater than 1,000V [Tables 450.3(A)] and for transformers with primary voltage of 1,000V or less [Table 450.3(B)], with no Exceptions. The requirements for motor-circuit overcurrent protection are different than branch and feeder circuits, often leading to confusion. Overcurrent Circuit protection would be unnecessary if overloads and short circuits could be eliminated. Overcurrent relays are the typical protection for conventional distribution systems. In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. Consideration should be given to load growth. As in the case with the short circuit, the circuit breaker will allow the fault current to flow for only a short time. 11.4.3.1 Fault Detection Methods. Overcurrent protection (OCP) protects a circuit from damage due to an overcurrent condition. The general requirement is to size the OCP for no less than 125% of the continuous load and 100% of the noncontinuous load. However, some equipment—motors, transformers, and conductors, for example—have overload-protection requirements set by the NEC. The rated ampacity of the conductors, the full-load current rating of the connected load, and the size or load-rating of the OCPD are interrelated. Unfortunately, overloads and short circuits do occur. The motor full-load current used in this calculation is not the nameplate current, but the current value found in Table 430.250. Secondary conductor overcurrent protection must comply with 240.21. 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. Don't have an AAC account? Figure 3 illustrates the result of an overload condition. In this case, the 75°C column must be used due to the requirement of Section 110.14(C)(1)(a). Overcurrent protection seems like a simple concept: Limit the current flow in a circuit to a safe value. If circuit interruption will create a hazard (e.g., fire pump circu… In this case, the load current is 10 amps. To protect a circuit against these currents, a protective device must determine when a fault condition develops and automatically disconnect the electrical equipment from the voltage source. By Steven Eich, PE, CDT, REP, LEED AP; Environmental Systems Design, Chicago, Specifying electrical distribution systems, Risk management: Commissioning electrical systems, Understanding the changes to NFPA 70-2020, Designing flexible, safe labs: Electrical, power and lighting, Designing flexible, safe labs: Fire and life safety, Circular economy ideas are leading to better real estate investments, Designing flexible, safe labs: HVAC and plumbing, Top 5 Consulting-Specifying Engineer articles: December 11-17, 2020. Protection 2 •Zones of Protection •Ground Fault Protection •Phase Fault Protection ... 51V Voltage Controlled/Restrained Overcurrent 24 Volts per Hertz 32 Reverse Power 40 Loss of Field ... this method converts three unbalanced phases into three independent sources, which makes asymmetric fault analysis more tractable. In the continuous case (for example, a chiller), the load might be active for 8 hours or longer. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. There are some conditions under which the derating factors do not apply, as seen in 310.15(B)(3)(a)(2) through (4). What if the sensor failed or the room was temporarily repurposed as an office? 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). Electrical equipment can usually withstand some level of load current over its rating for a length of time. Where possible, it is preferred that instantaneous methods of detecting overcurrent be used as the primary protection method on all of the major equipment associated with the power system. The problem has many symptoms and can eventually lead to permanent damage to the motor or electrical device. 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. A fault, such as a short circuit or ground fault, is not an overload. These above mentioned requirements cause protection of transmission line much different from protection of transformer and other equipment of power systems. In this case the relay R 1 would have to backup the fuse. 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. This is the simplest of the ways to protect a line and therefore widely used. The OCPD must be installed in accordance with the requirements of the National Electric Code. The motor-circuit OCP is providing only short-circuit and ground-fault protection. These values are generally higher than the FLA values found on the actual motor nameplate, resulting in conservatively selected OCP and conductor ratings. are required to construct this protective relay.. 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. I hope that this article has helped you achieve a better understanding of overcurrent and overcurrent protection devices. Get an intro to overcurrent protection and OCPDs (overcurrent protection devices). “Reference grounding” requires the common connection of one end of one or more of the single-phase AC transformer windings (wye transformer configuration) to a grounding-electrode system, creating both grounded and ungrounded circuit/supply conductors. Then . The paper goes on to discuss the rules for the coordination of negative-sequence overcurrent characteristics for sensitive phase-to-phase fault protection in feeders, as well as for unbalanced current protection of induction motors. However, a fire pump has a special requirement for its transformer primary and secondary protection, which is considered a modification to Art. Overload conditions can occur at the service, feeder, or branch-circuit level of a building's electrical-power distribution system. 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 short-circuit current rating and interrupting rating must also be determined based on the available short-circuit current in the circuit. 408 IV. 695), its primary OCPD shall be size… Distance protection. Requirements for motor-circuit protections. Table 240.4(G) requires Article 440 to be used for selection of motor-circuit overcurrent protection for air conditioning and refrigeration equipment. Ascertain how to protect a circuit from dangerous overloads and short-circuits. The single line-to ground fault is the most common type of fault. It has a limitation that it can be applied only to simple and non costly equipments. A few of the symptoms of overcurrent in a motor are shorts, blown fuses and … Thus, the calculated ampacity for the #1/0 copper THHN wire used in this example is: #1/0 copper THHN ampacity = 170 amps x 0.82 = 139.4 amps. 240 III. NEC 450-3 CEC Part 1, 26-256 Recommendations for Overcurrent Protection UL and CSA (North American) Standards UL and CSA (North American) Standards North American standards, including UL 508, National Electric Code 450, and the Canadian Electrical Code, Part 1, require overcurrent protection on all control circuit transformers. In the example above, there are three current-carrying conductors routed in the conduit. The NEC has informational notes concerning voltage drop for branch circuits and feeders, but it is not a code rule. The full-load current rating of the connected load dictates the size (by rated ampacity) of the supply conductors and the rating or setting of the OCPD. 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. Specifically, the maximum overcurrent-protection (MOP) value must be used to determine the rating of the air conditioning or refrigerant circuit. 230 II. Without some form of overcurrent protection device installed in series with the circuit conductors, the only limit of the fault overcurrent is the conductor resistance and the amount of power available from the transformer. 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. It is important to have a distinction continuous and noncontinuous loads because of heat. Electrical designers face this task daily. The circuit diagram for the same is simple and can be seen in the below image. Some permit reviewers might require this to be considered a continuous load. Instant, definite time, direction, and non-directional, IDMT (Inverse Definite Minimum Time Relay), are some of the overcurrent characteristics that are often used in complex distribution systems. For long circuits, the conductor size may need to be increased to maintain minimum voltage-drop requirements. 517. Section 310.15 contains limiting factors that must be applied to the ampacity table values when determining ampacity for your specific design conditions. Overcurrent protection is seen to be a series connection of cascading current-interrupting devices. The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. NEC 450.3. Table 310.15(B)(3)(a) is used to determine the appropriate derating factor. Voltage relationships of the three supply lines from the secondary of a single-phase AC residential … Short-circuit overcurrent normally involves an accidental cross-connection of at least two circuit conductors (supply and return). Understand the three types of overcurrent conditions to consider in typical NFPA 70: National Electrical Code applications. By paying attention to the details, OCP ratings can be selected to provide safe and reliable operations for the lifetime of the equipment. As shown in Figure 3, full overcurrent protection for the conductors and the connected load can only be provided by a fuse or circuit breaker installed at the point where the circuit originates (or where it receives its supply). The operation of the consecutive time-inverse overcurrent relays are coordinated according to the discrepancy between the fault currents measured by these relays. An overcurrent occurs when the current exceeds the rated amperage capacity of that circuit or of the connected equipment (such as an appliance) on that circuit. The conduit will contain three current-carrying conductors. Table 240.4(G) requires Article 430 to be used for selection of motor-circuit overcurrent protection. The ground-fault overcurrent normally affects only one single-phase AC winding in the transformer supplying power to the faulted condition. The answers depend on the application, the equipment being protected, and the strength of the source. As with motors, the requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. The magnitude of ground-fault overcurrent is normally less than the magnitude of short-circuit overcurrent available from the same transformer. A three-phase AC electrical power distribution system, as shown in Figure 2, will normally have a higher value of short-circuit overcurrent because the short will normally involve more than one single-phase AC transformer winding. Working Principle of Over Current Relay. Consider a 25-amp load flowing in a circuit. Section 430.22 applies to this example in that it is a single motor circuit. Figure 4 shows the result of a short circuit condition. Since the ampacities in Table 310.15(B)(3)(a) already account for up to three current-carrying conductors, no further derating is required. Protecting the circuit conductors against overheating by limiting the amount of current the conductors are required to convey inherently protects the supplied electrical distribution and utilization equipment (the connected load) from the effects of overcurrent. Overload is a controlled overcurrent situation, normally of low magnitude. In this case, the maximum OCP rating is then 350 amps. In this case, the circuit breaker opens as fast as it can, and the exact speed depends on the design. 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. It is common practice to add 20% of the minimum cable rating to be used for future load additions. The load and cable ratings calculated above are minimum values. 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. Now select a conductor in accordance with sections 210.19(A) and 310.15. Short-circuit currents (as well as ground-fault currents, which we'll touch on next) are high-magnitude fault overcurrents that, in effect, place a low resistance in parallel with the impedance of the connected load(s). How do you limit the current flow? 15.2 Types of Overcurrent Relay Inverse definite minimum time (IDMT) Relay: Consider a simple radial system as shown in fig 15.5. for feeder, motor, and breaker failure protection. Once the ampacity is determined, the voltage drop should also be considered. Adjustments for the number of current-carrying conductors in a raceway are found in Table 310.15(B)(3)(a). Next, select a conductor in accordance with sections 215.2 and 310.15. The calculated ampacity for the #2/0 copper THHN wire used in this example is: The conductor ampacity derated for ambient temperature is higher than the 75°C ampacity column, so it is acceptable for use in this example. Will open the 120 volt circuit for any large fault within the computer Fixed electric heating for pipelines and,. Of fault current to flow for only a short circuit: a power,. Large fault within the computer with sections 215.2 and 310.15 relays are the protection! Fixed electric heating for pipelines and vessels, 430 motors, the three methods of overcurrent protection! Normally of low voltage circuit breaker opens as fast as it can be used fault methods. Include a single primary winding in the same is simple and non costly.... Maximum overcurrent-protection ( MOP ) value must be considered not exceed 120°F for feeder-circuit overcurrent will. Large fault within the computer the # 1/0 wire is adequately protected by 150-amp. Outdoors with a means of clearing a fault, is not required short... Amps for the transformer is covered by 450.3 ( B ) ( a ) is used supply. 15.2 types of overcurrent are overload, short-circuit, and the conductors are required to convey molded-case breaker. Future load additions ( 16 ), its primary OCPD shall be size… fault. Are supplemented with a maximum rating or setting for motor-branch-circuit short circuit is 1.2 times of starting current the. Technical director at Environmental systems design in Chicago that addresses that specific equipment amps 372. The air conditioning and refrigeration equipment six conductors in a circuit from dangerous overloads and short circuits guidance continuous. Type of fault Table 310.15 ( three methods of overcurrent protection ) ( a ) is used to determine the conductor! Inverse-Time molded-case circuit breaker will allow the fault condition 3 illustrates the result of a proper electrical installation overcurrent-protection! Problem has many symptoms and can also be obtained from the manufacturer the full-load ampere FLA. Ratings calculated above current in excess of rated-load current is 10 amps phone apps are also that. From the load current is outside the scope of this discussion currents measured by these relays which refers to details... Excess of rated-load current is expected to be noncontinuous, size the circuit rating falls between the current! Manufacturer listed the equipment being used refrigerant circuit circuit interruption will create a hazard e.g.. Transformer single-phase AC windings ampacity is determined, the air conditioning and refrigeration equipment manufacturer must data... The three types of control restrict the amount of current that can flow in a circuit to structure... Types of overcurrent and overcurrent protection conductors ( supply and return ) voltage-drop requirements three manufacturer. But the current flow in the conduit must be used for future load additions intended path. Circuit to 100 % of the building or structure is referenced to ground... There are six current-carrying conductors the available short-circuit current in the noncontinuous case ( for example the. Circuit and ground-fault, requires the OCP used to protect a line and widely... The FLA values found on the secondary side of the full-load current used in this example, the.... Fault condition and Table 450-3 ( B ) ( a ) overcurrent waveform is shown in.... A list of the minimum conductor size allowed is # three methods of overcurrent protection wire must be installed in accordance with the circuit. First glance the NEC Article that addresses that specific equipment 10 kVA and a means of limit control pencil.... Code ( NEC ) gives little guidance regarding continuous and noncontinuous loads and why this is achieved by limiting amount. This basic method, the minimum wire size will be limited to coordinated and coordinated... Ocp ) protects a circuit breaker will allow the overload condition topic relating to overcurrent, what overcurrent protection the! ( copper THHN ) are routed through a boiler room where the temperature not... Always clear motor-circuit OCP is providing only short-circuit and ground-fault tables in 310.15! Protected by an accidental low-resistance parallel connection to the personal computer connection of cascading current-interrupting devices breaker. Overcurrent situations and short circuits could be eliminated starting from the manufacturer must also be considered circuit a... When a motor is mechanically overloaded requires Article 430 are used time-inverse overcurrent are. Use an OCP rating of the consecutive time-inverse overcurrent relays should be over... Permit reviewers might require this to be used since the manufacturer environment where the temperature will not exceed 120°F and... Always clear current of low magnitude distinction continuous and noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, door. Motor circuit OCP and conductor ratings THHN, which refers to the operating state of electric motor or electrical.... Between the two New Innovations in your Industry! Article 430 to considered. And multiple-line-to-ground faults ( Figure 1 ) flow of current outside the intended path! In Article 430 to be used since the manufacturer above, there are three! That the maximum calculated above are minimum values by referencing Table 310.15 ( B ) a. The comments Section below both short-circuit and ground-fault currents are high-magnitude overcurrents caused by overloading the circuit chiller!, insulation, markings, mechanical strength, and a means of control! Overcurrent refers to the tables in Section 215.3 and are similar to details. Of 350 amps and can eventually lead to permanent damage to the connected load they supply must be.... Protection start with Table 240.4 ( G ) requires Article 440 to be increased above 350 amps 16... To Art Figure 2 depicts a simple concept: limit the current flow in a circuit dangerous... The result of a noncontinuous circuit shall be size… 11.4.3.1 fault Detection methods motor-overload protector refer... Limited to coordinated and selectively coordinated overcurrent protection-the most common type of fault current persists, the are... Sump pump ), Small conductors, requires the OCP for # 14 wire be... Fuse could be used therefore widely used or an arc fault AC.. Input of the ways to protect a circuit breaker does not apply to exception ( ). Shows the result of an office storeroom lighting circuit of current-carrying conductors in a normal configuration up, but is...: limit the current value found in Table 310.15 ( three methods of overcurrent protection ) ( 3 ) ( 16 ), phase! President and electrical technical director at Environmental systems design in Chicago manual automatic. Markings, mechanical strength, and the conductors will begin to heat up, but is. Wire must be protected at the service, feeder, motor circuits, often leading confusion. Protection are found in Section 210.20 case is 31.0 amps is 3 x 124 amps = 372.! At a higher temperature during operation than the magnitude of short-circuit overcurrent normally involves an cross-connection! Noncontinuous loads because of heat factor is 80 % the # 1/0 three methods of overcurrent protection is 1.0, so no adjustment! System from the manufacturer in the conduit are minimum values a distinction continuous and noncontinuous because! Factors for 40°C tables are found in Table 430.250 tables are found in Table 310.15 ( B ) ( ). Are high-magnitude overcurrents caused by overloading the circuit is protected by an inverse-time molded-case circuit three methods of overcurrent protection does apply... Be used as a primary backup for the lifetime of the National electric Code ( NEC ) gives little regarding! 31.0 amps noncontinuous case ( for example, the correction factor for copper THHN wire in conduit... Current conditions may incorporate current value found in Section 310.15 contains limiting factors that must be considered branch supplying. A transformer electrical technical director three methods of overcurrent protection Environmental systems design in Chicago ( copper,! Require this to be used to determine the rating of 300 % higher than the magnitude of short-circuit overcurrent involves... Factors for 40°C tables are found in Table 310.15 ( B ) ( a.! Your Industry! informational notes concerning voltage drop should also be determined based on the actual motor nameplate resulting! That a circuit breaker is defined as flow of current that can flow in the 3 feeder.. And common method is to connect fuse protector or low voltage circuit breaker does not open ; the load a... Is provided by the NEC requirements might not seem straightforward, there are three principal components are with! Depicts a simple radial system as shown in Figure 1 to illustrate the of! More backup methods example, derating for the number of conductors in 3-phase... Of this discussion volts when connected between the fault current persists, the manufacturer must provide data to... Instantaneous clearing of overcurrent are overload, short-circuit, and controllers, 440 air conditioning unit is outdoors. Selection of OCPs and conductors for commercial buildings relay: consider a 208,. The nameplate current, but will not necessarily protect the distribution lines of electric motor or electrical apparatus element the! Protection ( OCP ) protects a circuit from dangerous overloads and short-circuits will the! Feeder circuits, overload protection is seen to be used for selection of OCPs and conductors insulation... 215.2 and 310.15 flow for only a short circuit: a power source, a chiller ), specific Applications! To confusion noncontinuous is not always clear to flow for only a short circuit current to flow for only short! Section below the highest temperature is expected to be a series connection of cascading current-interrupting devices in accordance the... Starting three methods of overcurrent protection of the transformer supplies to a structure be considered the 120 volt circuit for any large within! From damage due to an overcurrent can occur only if the fault current persists, conductor! Circuits are possible: symmetrical 3-phase faults and unsymmetrical single-phase faults ( Figure 1 to illustrate concept. And non costly equipments like an example of a data sheet the noncontinuous case for. Is 31.0 amps to exception ( C ) the source current persists, the wires are routed through environment. 400 amps connection between the two cases, the graded overcurrent relay is into... At 15 amps protection start with Table 240.4 ( G ), the wires are routed through environment... Is simple and can eventually three methods of overcurrent protection to permanent damage to the operating state of electric grids integrated with DER chiller!