Making the Right Connection


(محمد سميري) #1

Making the Right Connection
Jeffrey Sargent
Requirements contained in Article 110 are general and have to be complied with in all electrical installations to which the provisions apply. For instance, in addition to the factors that impact the temperature of a conductor along its entire circuit length within a cable, raceway, cable tray, or other NEC® Chapter 3 wiring method, there is an important temperature requirement contained in Section 110.14© that impacts temperature and ampacity rating. This particular requirement is relatively new in the NEC, first appearing in the 1993 edition, but has long been a provision associated with the evaluation and listing of electrical distribution equipment. Further, this requirement was specified as a general provision for electrical terminations in distribution equipment by the Underwriters Laboratories, Guide Information for Electrical Equipment, better known as The White Book
Ensuring that the temperature of a conductor does not exceed its insulation temperature rating is a fundamental requirement contained in 310.10. Selecting an allowable ampacity from Table 310.16(the allowable ampacity table applicable to the majority of installations rated 600 volts and less) and applying the necessary ampacity adjustment or ampacity correction—known as derating in electrical industry jargon—are important factors that have to be considered along the entire circuit length from the origin of the circuit to its termination at equipment or to a device. The load current of the circuit, mutual heating from adjacent conductors, ambient temperature, and the type of wiring method are factors impacting the operating temperature of the conductor, and these factors may vary along the length of the circuit depending on how and where the conductors are installed.
The limitations imposed by the termination temperature ratings of electrical equipment directly impact the amount of load current that can be connected to the terminal. The question often asked is “Why can’t I use the ampacity from the 90°C column of Table 310.16?” Before the requirement in 110.14© was added to the NEC, the answer was because of the restrictions imposed by the general listing requirements by product evaluation organizations. Now, Code users can refer to 110.14© for the information on the terminal temperature requirements.
Temperature Limitations
Section 110.14© is entitled Temperature Limitations and starts out by stating:
The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
Let’s look closely at what’s being said here. The first sentence requires that the circuit be considered from the standpoint that the terminal with the lowest temperature rating is going to establish the temperature rating of the entire circuit. From the beginning of a circuit at an overcurrent protective device to the point where it terminates, the equipment or termination with the lowest temperature rating is the determining factor on whether to use the 60°C or 75°C column from Table 310.16 for selecting the load ampacity of the circuit conductors.
The second sentence contains the additional very important provision that permits the use of the ampacity associated with a 75°C or 90°C insulated conductor to be used for derating even if the terminal temperature limitation of the circuit is 60°C. Thus, the conductor insulation types with the higher temperature ratings can be used as a starting point to address conductor heating issues in the circuit wiring method even though the load current is limited by the 60°C rating of the equipment to which the conductors are connected.
We have two separate heating considerations here:
· The temperatures associated with the conductor as it makes its way from origination to its termination
· The temperature rating of the terminal to which the conductor is connected.
The first paragraph of 110.14© is simply saying to look at both of these conductors’ heating considerations and coordinate the requirements of 110.14©with those of 310.15(B). This requirement addresses the heating at electrical connections and provides limitations on connected load based on the insulation rating of the circuit conductors and the load of the circuit. The conductor assists in heat dissipation at the terminal, and use of a conductor ampacity not in compliance with 110.14© can lead to insulation failure along the first several inches of conductor adjacent to the terminal.
Conductor Ampacities
The conductor ampacities from the 60°C, 75°C, and 90°C columns of Table 310.16 are allowable under the following conditions of use specified by that table:
· Not more than 3 current-carrying conductors in a raceway, in a cable, or in the earth
· An ambient temperature not exceeding 86°F
If we change one or both of these conditions of use, the conductor ampacity has to be derated. For example, an 8 AWG copper THHN conductor installed in a dry location has an allowable ampacity from the 90°C of Table 310.16of 55 amperes. The heat resulting from this amount of current is not going to exceed the temperature rating of the 90°C insulation, but when the conductor is connected to a terminal within an enclosure, the size of the conductor is not adequate to aid in dissipating the heat from the terminal, thus the insulation integrity is jeopardized. Here we have the rationale for 110.14©: The thermal performance of conductors connected to electrical equipment rated 600 volts and less is evaluated using conductor ampacities selected from Table 310.16 using the limitations specified in 110.14©(1)(a) and ©(1)(b).
Terminal Temperature Benchmarks
Sections 110.14©(1)(a) and ©(1)(b) establish two general benchmarks:

· 60°C ampacities are the default for equipment intended for connection of circuits rated 100 amperes and less and for equipment marked to be connected to conductors in the range of 14 AWG to 1 AWG.
· The insulation temperature rating of conductors connected to equipment supplied by circuits rated over 100 amperes for conductors larger than 1 AWG is 75°C. This higher insulation temperature rating reflects the increased I2R heating occurring in the higher capacity circuits and equipment.
Modifications to these two general benchmarks are permitted as specified in 110.14©(1)(a) and 110.14©(1)(b).
One of the most widely used modifications of the general requirement for conductors in the size range from 14 AWG to 1 AWG is the permission in 110.14©(1)(a)to connect conductors with higher insulation temperature ratings (75°C and 90°C) to terminals suitable for a maximum conductor operating temperature of 60°C. Under this provision, the conductor with the higher insulation temperature rating is limited to a connected load current selected from the 60°C column of Table 310.16. The actual heating at the terminals results from the connected load of the circuit and not on the ampacity of the conductor as specified in the 75°C or 90°C columns. In addition, the ampacity associated with the higher temperature rating can be used as the starting point for conductor derating, since the circuit load does not exceed the ampacity from the 60°C column. Derating of the conductor ampacity by starting at the 90°C or 75°C ampacity can result in an adjusted or corrected ampacity that is greater than the 60°C ampacity of the conductor. This situation is acceptable, however, as long as the load connected to the terminals does not exceed the 60°C ampacity. If derating results in an ampacity less than is required for the load, a different insulation type or a larger conductor will be necessary.
Applying the Requirement
A circuit breaker and the panelboard in which it is installed both have 75°C temperature ratings. (It is not uncommon for equipment rated less than 100 amperes to be listed and identified for connections using ampacities selected from the 75°C column of Table 310.16.) The 8 AWG copper ungrounded conductors connected to the load terminals of the circuit breaker have 90°C (THHN) insulation. The circuit is installed in electrical metallic tubing (EMT) and will supply a single receptacle outlet for an electric range. Assuming that the EMT contains not more than three current-carrying conductors and is not installed in an area that will expose the conductors to an ambient temperature exceeding 86°C, the conductor ampacity is 55 amperes. However, the temperature rating of the terminals of both the branch-circuit overcurrent protective device and the range receptacle will reduce the allowable load current on the conductors.
Based on the circuit breaker terminals, the 75°C ampacity for an 8 AWG copper is 50 amperes, but the terminals of the receptacle, unless otherwise marked for a higher temperature conductor ampacity, will to limit the load current of the circuit to 40 amperes, the 60°C ampacity for 8 AWG copper. Because the receptacle is not identified for higher temperature connections, the general default to using the 60°C conductor ampacity controls the allowable load current that can be supplied by the 8 AWG copper THHN conductors. If the circuit is arranged such that there are more than three conductors in the raceway and/or the ambient temperature exceeds 86°F, the 55 ampere rating of the THHN conductor can be used as the starting point for derating the conductor.
Coordinating Ampacity Selection and Terminal Temperature Limitations
Conductor ampacity and terminal temperature limitations are two separate considerations that have to be coordinated in order to ensure the circuit will operate within the allowable temperature rating of the conductor insulation and of the terminals to which the conductor is connected. By approaching the following concepts systematically, we ensure compliance with 310.10, 310.15(B), and 110.14©:
· Determine the load current of the circuit
· Determine the temperature ratings of all equipment
· Determine whether ampacity adjustment, correction, or both is necessary
· Select a conductor with an ampacity coordinated to the lowest equipment temperature rating
· Establish whether it is necessary to use a higher temperature–rated conductor insulation in order to allow for ampacity adjustment and/or correction (derating)


(system) #2

ألف شكر لك على الموضوع المفيد


(system) #3

شكرا على الموضوع المفيد للمهندسين المبتدئين