I'm sizing conduit between cabinets using NEC tables 310-16 thru 19. I would like a definition of free air. Regarding single insulated conductors in a conduit or panduit, does the percent void count as "free air"? What instance does not cout as free air. Would a multi-conductor cable not have free air?
Be careful with what you are asking. When you say free air, you are talking about insulated conductor(s) designed to be placed on a cable tray, spaced so far apart, and in the open air 310.17 - NEC 2008. Placing the conductor in a conduit or panduit will give you a different ampacity and you would use one of the other tables.
In regards to your question, Free open air is not the same as
the free air in a conduit or panduit. In a conduit or panduit, you have a specific cross sectional area associated with the the conduit or panduit. Each conductor takes up part of that cross sectional area. For conduits 24" or less, you can have a 60% fill capacity, over 24" it's 53% for 1 wire, 31% for 2 wires and 40% if its 3 or more wires chapter 9 tables. For panduit, its 50% NFPA 79.
The air in the conduits/panduit is considered as part of the conduit fill, not free air because that air is trapped inside the conduit / panduit and is subject to the heat dissapation of the conduit, wire, and other outside forces.
Another site for electrical questions is mikeholt.com
regards,
james