Determining Current Carrying Capacity of Conductors
The purpose of NFPA 70®, National Electrical Code® (NEC®) is the practical safeguarding of persons and property from the hazards that arise due to the use of electricity. Typically, this means protecting people from hazards like shock and arc flash, as well as property from fire. Fires resulting from improper wiring have historically been a significant threat ever since electrical systems have been installed within buildings. The NEC has established a long history of installation requirements to help prevent fires from occurring within the electrical system. One such requirement is to determine how much electrical current a conductor can carry continuously without exceeding the temperature rating of its insulation, or as the NEC refers to it, a conductor’s ampacity. However, determining ampacity requires the understanding of a number of other factors that come into play based on how a conductor is used and installed. This involves navigating charts, tables, and a number of other requirements to make sure that we calculate the correct ampacity. Depending on which conditions of installation and use exist, we find ourselves using a number of tables found throughout the NEC, but in particular, many of them are located in Article 310. There are a multitude of tables that spell out items such as conductor ampacity, temperature correction factors, and adjustment factors. So, let’s take a look at how these ampacity charts and tables can be used to ensure we select the appropriate conductor for the installation. There are a few questions we must ask ourselves before we begin. First, we need to know what the conductor insulation is rated for since ampacity is a function of the temperature rating of the insulation. Once we have established if we are using 60-, 75-, or 90-degree Celsius rated insulation, we can determine which column from the appropriate ampacity chart we need to be in. For conductors rated up to 2000V, ampacities can be found in Tables 310.16 through 310.21 based on how they are installed and other specific installation criteria. For the purpose of this blog, we will be using Table 310.16 for conductors installed in a raceway or cable with not more than 3 current carrying conductors total and in an ambient temperature of 30⁰C (86⁰F). These parameters are important to know since any deviation will necessitate a modification of the ampacity value in the tables. Once we know the insulation temperature rating, we can then find the corresponding ampacity in the appropriate column of Table 310.16 for the given conductor size (Note: certain types of insulation carry multiple ratings based on the location type, see Table 310.4 for conductor properties). After we have the ampacity value from Table 310.16, then we can apply adjustment and correction factors, if needed. Let’s start off with adjustment factors. First ask, are there more than three current carrying conductors in the raceway or cable, or are multiple cables installed without maintaining spacing for a distance greater than 24 inches? This count applies to total number of ungrounded (hot) conductors, even spares, and grounded (neutral) conductors on a 3 phase, 4-wire WYE system where: the circuit is single phase or, if the major part of the load consists of nonlinear loads [see 310.15(E)]. If the total current carrying conductor count exceeds three, then the ampacity from Table 310.16 must be adjusted in accordance with Table 310.15(C)(1) based on the total number of current carrying conductors. Next, we must look at the ambient temperature of where the conductor will be installed. If the ambient is anything other than what the starting ampacity is in Table 310.16, then we will find temperature correction factors in 310.15 based on deviations from the original chart’s ambient temperature. There are two temperature correction tables: Table 310.15(B)(1) for tables that are based on an ambient temperature of 30⁰C (86⁰F). Table 310.15(B)(2) for tables that are based on an ambient temperature of 40⁰C (104⁰F). Because this blog is written based on Table 310.16, multipliers for temperature correction from Table 310.15(B)(1) should be used, since both charts are based on 30⁰C (86⁰F) ambient temperature. Table 310.15(B)(1) is also divided up by the temperature rating of the conductor insulation. Having already established this, simply find the corresponding multiplier based on the actual ambient temperature of the installation. Once all necessary adjustment and correction factors have been applied, there is still one more component that affects the ability of the conductors to safely carry electrical current continuously without exceeding the temperature rating of the insulation. This final factor is the termination of the conductor to any equipment. Termination points can be a limiting factor as these are common points on the electrical system for heat build up and rely on the conductor material to act as a heat sink to dissipate any build up of heat where the termination is made. For these requirements, we must consult section 110.14(C) for termination temperature limitations. These requirements help us determine the final current carrying capacity of our conductors so that they can safely handle the circuit current without damage to the insulation from excess heat. Section 110.14(C)(1) is split up into two scenarios. The first group is for circuits 100 amps or less or that are marked for the termination of conductor sizes 14 AWG through 1 AWG. The second group is for circuits with above 100 amps or terminations marked for larger than 1 AWG. The requirements for the first group limit the conductor use to conductors with a 60⁰C insulation rating or if conductors with a higher temperature rating are used, the final adjusted ampacity must not exceed that found in the 60⁰C column for the same size conductor, unless the terminations are also rated for a higher temperature in which case the final ampacity shall not exceed the value in the corresponding column. For the second group, above 100A or 1 AWG, the rules simplify a bit. The conductors must be rated for 75⁰C or higher and if the conductor is rated for higher than 75⁰C, the final ampacity must not exceed the corresponding ampacity in the 75⁰C column unless the terminations are identified as being rated for such higher temperatures. When we follow these requirements, the conductors that we install will be less likely to overheat and become a hazard, provided that the conditions of use remain the same. We’ve developed a free flow chart on this topic, including the tables mentioned above, to help you in your next installation. Be sure to download it here.