I was recently asked an interesting question involving parallel copper, aluminum and copper-clad aluminum conductors installed in parallel. The question was about 310.10(G), and whether cables installed in parallel must be the same for each set of phase conductors for alternating current, for each polarity for direct current, for each set of neutral conductors or for each set of grounded conductors.
Initially, the question was seemingly easy to answer: yes. Then I started thinking about the various applications that could occur in new installations, as well as replacements for existing conductors when one or more of these conductors fail and must subsequently be replaced. What are the ramifications, and are there issues with matching the other conductors within the set? Does the cable manufacturer need to be the same, and does the type of aluminum become an issue? Are there other similar questions that need to be explored and reasonably answered?
I often receive emails from people and, due to the complexity of the question and its answer, will call them for further discussion. This person’s question involved a new installation with parallel aluminum conductors for the phase conductors and parallel aluminum conductors for the neutral. He stated that the paralleled conductors fit the requirements of 310.10(G) since each set of conductors was 1/0 or larger, electrically joined at both ends, the same length, the same conductor material (aluminum), the same size in circular mil area, had the same insulation type and were terminated in the same manner.
His question, however, was if it was an NEC violation since the electrical contractor used different manufacturers for the various conductors in the same phase or neutral. He was concerned that there would be a different impedance due to the probable difference in conductivity between individual conductors within the set.
I assured him that, if the aluminum conductors were all AA-8000 series electrical-grade aluminum-alloy conductor material, the relative conductivity should be the same, even with different manufacturers. There may even be a tiny difference in conductivity by different batch lots from the same manufacturer, but the overall difference will be extremely small and not something to consider for a particular installation.
Section 310.10(G)(3) further states that, where parallel conductors are installed in separate cables or raceways, the cables or raceways must have the same number of conductors in each cable or raceway and the conductors must have the same electrical characteristics for the same phase, polarity, neutral or grounded conductor. Usually, the same electrical characteristic applies to all-copper, all-aluminum or all copper-clad aluminum.
For replacement conductors, always check the type of aluminum used for the other conductors that are not being replaced, since older conductors may be AA-1350, originally used for overhead power distribution by utility companies and used in some branch and feeder circuits for wiring in homes and commercial buildings before the mid-1970s. The older AA-1350 were more prone to failure due to the mechanical properties of the aluminum terminations on devices. Since the older AA-1350 may have been used in parallel installations, replacement of bad or failing individual conductors must be accomplished with appropriately similar or the same aluminum materials. Replacing one of the AA-1350 aluminum conductors with an AA-8000 aluminum conductor would probably have a different conductivity than the other existing conductors, and would thus be a violation of 310.10(G).
Another issue may be the possible difference in conductivity between compact stranded copper or aluminum conductors and standard building wire in these larger sizes of 1/0 and larger parallel conductors. Due to skin affect and other similar conductivity issues, replacing one standard copper or aluminum conductor with a compact stranded conductor in a parallel set may be an issue. Replacing a compact stranded conductor with a standard stranded conductor would not normally happen, since the raceway may have been specifically sized for the compact stranded and not be large enough without the other consideration of the conductivity issue. The entire set of parallel conductors would have to be replaced with the appropriate type of conductor, either all compact stranded or all standard stranded conductors.
A simple answer may not address all the issues with parallel conductor installations, so care must be taken to address all applications. Careful analysis of a new or existing installation will prevent issues with the installation and will help prevent questionable applications.