Last month's column discussed getting the most out your conductor ampacity, but the article did not really report the importance that wire terminations have on the final ampere rating of a conductor; many designers and electricians forget to consider the rating of the conductor versus the ability of the combination of the conductor and the conductor connection to provide heat dissipation without damage to the conductor insulation or to the electrical equipment.
Section 110.14 has covered electrical connections for many National Electrical Code (NEC) cycles but was limited to explaining the different characteristics of aluminum and copper connection devices, such as pressure terminals, pressure splicing devices and soldering lugs. These devices must be identified for the material of the conductor and must be properly installed and used. Conductors of dissimilar materials must not be inter-mixed in a terminal or a splicing device, unless the device is identified for this inter-mixing, since corrosion and adverse heating can occur with resultant damage to the conductor, the conductor insulation and any electrical equipment termination.
During the 1993 NEC revision cycle, a proposal to add temperature termination requirements as a new sub-Section 110.14(C) was submitted and accepted. The basic text that was added to the NEC required that the temperature rating associated with the ampacity of a conductor must be selected and coordinated to not exceed the lowest temperature rating of any connected termination, conductor or device. Similar text has been in the Underwriters Laboratories Inc. (UL) standards for many years but was slightly revised when placed into the NEC to conform to the NEC style; however, the original intent has not been changed in the slight rewrite.
The intent is to always size the current of the circuit based on the lowest common denominator of the system components. For example, where installing a conductor with 75°C insulation connected to a 60°C termination device, the ampacity of the circuit will be based on the lowest value between the conductor and the termination device, which is the 60°C device. In that regard, if a conductor with 75°C insulation is connected to a 75°C termination device, the ampacity of the circuit can be based on the 75°C ampacity since both the conductor and the device are the same rating. A higher ampacity rating can either be used for the device or conductor, but the lowest value will always be used.
So the basic rule as previously stated in the UL standards and Section 110.14(C) of the NEC is to use the 60°C ampacity column in Table 310.16 for all equipment that is rated 100 amps or less or for conductors sized 14 AWG through 1 AWG, unless otherwise marked. In addition, use the 75°C column in Table 310.16 for all equipment rated over 100 amps or with conductors larger than 1 AWG, unless otherwise marked. For motors marked with design letters B, C or D, the 75°C rating can be used since the National Electrical Manufacturers Association MG-1 standard requires motor terminations to be 75°C terminations.
For example, an air conditioner (a/c) installation in a wet location might have a 65-ampere load. If the load terminations on the a/c equipment are not marked, 60°C terminals must be assumed. In addition, if the circuit breaker supplying the branch circuit protection was marked as 60/75°—even if 75°C THWN copper conductors are used—6 AWG conductors could not be used since the 60°C column in Table 310.16 only permits 55 amps. This installation would require 4 AWG copper conductors. If the terminations at the a/c were changed to 75°C terminals and the conductor insulation and both sets of terminals are all rated at 75°C, a 6 AWG copper conductor at 65 amps, based 75°C column of Table 310.16, can be used. This example does not take into consideration the high ambient temperature or adjustment factors for the number of conductors in a raceway or cable.
Conductors with temperature ratings higher than terminations can be used for temperature correction, for ampacity adjustment or both as stated in the last sentence of 110.14(C). In the example above, if the temperature where the conductors are installed is 96°F, then a 0.88 correction factor must be taken for the higher temperature. Since the load is 65-amps, then 4 AWG THWN rated at 85 amps would be used. The calculation would be 85 amps times a 0.88 correction equals 74.8 amps.
Understanding conductor ampacity, conductor termination and conductor insulation is extremely important and can be very costly if the design for the project or the installation is improperly done. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at email@example.com.