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Tuesday, April 4, 2017
Question:
Question https://www.tfkable.com/sites/default/files/spolki_handlowe/pliki/dlo-90c-and-75-c-ampacity-chart-0617111.pdf
I have been using the above table for a project regarding use of 4/0 DLO cable in an open air setting so we can get a 405 Amp rating. We cannot find a perfect definition of "free air" anywhere in the code and a lot of information online contradicts itself. For a three phase system, can we stack a set of 3 cables in a triangular fashion, and keep a distance of the diameter of a cable between sets, and keep the rating of 405 amps? Where does the open air rating end? The code specifies for 4+ conductors needing derating, but with only 3 conductors should we expect any drop?
Niko Panagiotakopoulos
A
Answer:
Hey Niko thanks for your question. The term "free air" is not defined in the NEC but is usually applied to conductors that are not installed in cable assemblies or raceways. Not to differentiate between air that costs something! Table 310.15(B)(17) contains the term "free air" in the heading and provides ampacities for single conductors used that way. Notice that 4/0 copper has the 405 ampere value in the 90 degree C column which matches the manufacturers table you reference. Table 310.15(B)(20) provides a value of 335 amperes in the 90 degree C column for up to 3 conductors supported on a messenger but the table is based on a 40 degree C ambient temperature.
You don't mention how the conductors are installed but single conductors must generally be installed as part of a recognized wiring method in Chapter 3 as stated in 300.3(A). If the conductors are in a cable tray then 392.80 applies.
Diesel locomotive (DLO) cable was originally intended for use on diesel locomotives but can have other applications and is available with RHH and RHW insulations which are in Table 310.104. The termination provisions limit how the conductors can be applied and use Table 310.15(B)(16) for those values so would limit the 4/0 to 230 amperes in the 75 degree C column unless separate pressure connectors are used as stated in 110.14(C)(2).
After all that DLO conductors are fine stranded and the connectors and terminals must be suitable for that as stated in the last paragraph of 110.14 which references Chapter 9 Table 10.