OT - Free Air and NEC Tables

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?

"Free air" means the conductor is not effectively surrounded by anything but air, except at its supports.

About the only 'continuous' support method that can be considered as free air is 'cable tray', and even then there are spacing restrictions that must be maintained between conductors.

There is no way that 'panduit' in a panel, or conduit anywhere, would count as a free air installation.

For the purposes of ampacity and alowable fill I usually treat wires in panduit as if they were in a conduit of corresponding cross section.

Note that UL-508 has standards for wire size etc. that may not always be in agreement with NEC.

I thought that NEC used the term "Conduit or Raceway" I have always interpreted this to include wire in any panduit or cable tray.

I interpret the "Free Air" similarly to Jim Dungar, surrounded by air except at the supports. I would use the short lengths of wire in a control panel connecting a fuse block with a motor starter as a prime example of a Free Air application.

jimbo3123 said:

I thought that NEC used the term "Conduit or Raceway" I have always interpreted this to include wire in any panduit or cable tray.

I interpret the "Free Air" similarly to Jim Dungar, surrounded by air except at the supports. I would use the short lengths of wire in a control panel connecting a fuse block with a motor starter as a prime example of a Free Air application.

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I wouldn't swear to it, but I always felt "free air" is kind of a historical artifact, going back to the days when they would snake wire around in an attic and such with just an occasional staple to keep it in place.

godfrey said:

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?

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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