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Definition of "Free Air" in Ampacity Tables

Definition of "Free Air" in Ampacity Tables

Definition of "Free Air" in Ampacity Tables

For NEC Table 310-17, what is the definition of "free air"?  A major manufacturer of motor control equipment has sized conductors to motor starters inside enclosures (such as a Hoffman NEMA 4 enclosure) based upon Table 310-17, ampacities in free air.  I have always used this table for overhead conductors, such as festooning or other article 225 applications, not for wiring inside of an enclosure.  The difference in ampacities is tremendous.  For 75C wire, 350kcmil has an ampacity of 310A in a raceway (Table 310-16) and 505A in free air (Table 310-17).  Is Table 310-17 the correct table for conductors in an enclosure?

RE: Definition of "Free Air" in Ampacity Tables

Hi Codemaster,

Gee, that's a good question.  There is no actual definition in the code that I can find or recall -- I suppose, though, that since we're dealing with heat removal that the air must be free enough to carry the conductor's generated heat (without raising the ambient above the limit in the table) away from the enclosure by convection alone.  If a starter cabinet is big enough and is ventilated by some louvers at the bottom and near the top, I can see where the ampacity of the conductors would be much higher than if the air were confined in a conduit or electrical duct.  If it's NEMA 4, though, I think it would be iffy unless tested.

I would imagine that the manufacturer has to test the equipment to determine that the ambient remains within table limits and the conductor temperature remains within its temperature limit, in which case they can say they comply with the tables....  Transitioning to the higher ampacity conductor is permitted as long as the distance isn't far (I think it's 10 feet or 10 percent of circuit length).

I hope to see one of our compadres come up with a ruling from somewhere, this is curious to me!

Kindest regards,

Old Dave

RE: Definition of "Free Air" in Ampacity Tables

I cannot imagine that overcurrent devices such as overload relays, fuses or breakers terminating tab310.17-rated cables being able to retain any imaginable accuracy in time-current function.  

Sounds like a gross misapplication by any definition.  Be sure and tell your friends.  

RE: Definition of "Free Air" in Ampacity Tables

I don't know what the manufacturer's base their wire sizes on, but it is very common to find the internal wiring in drives, MCCs and motors to be much smaller than what the NEC requires for the field installed wiring.  I was just working on a size 4 starter this morning where the internal wiring from the bus stabs to the breaker and from the breaker to the contactor were #4 copper.  The 75°C rating of #4 in 310.17 is 125 amps.  The FLA for a 100 hp, 480 volt motor from Table 430.150 is 124.  

RE: Definition of "Free Air" in Ampacity Tables

Good question - this seems to be one of those grey areas. I don't know of an official definition of "free air".

I think a free air rating should be applicable to those situations where the cable is NOT in a raceway. A raceway is defined in the Code so by process of elimination it can be decided if the free air or raceway ampacity applies.

However, if you use a free air rating for something that could place the conductors at an elevated ambient temperature ( such as within a piece of equipment) then you would need to apply temperature derating factors to the free air rating.

RE: Definition of "Free Air" in Ampacity Tables

Conductors inside an enclosure would not meet my definition of "free air".  But I'm not sure this is defined in the NEC anywhere.

But I agree with resqcapt that conductors provided inside a manufactured item do not fall under NEC jurisdiction.  If this enclosure has a UL or FM mark, then it is presumed to have been designed and tested appropriately.  It is very common for wiring inside MCCs, starters, VFDs, etc to be much smaller than would be required for field wiring per NEC.  

It's similar to the oft-encountered situation of seeing the local utility splice your service entrance cable, sized per NEC, to a conductor about half the size.  

RE: Definition of "Free Air" in Ampacity Tables

According to some wire manufacturers (Canada Wire)
to qualify for free air rating ampacity the cables must be at least one diameter apart if installed in air, on tray racks etc. Of course subject to local authority....
ambient, sunlight exposure, etc.

In the past the "free air" rating was also used for direct buried, embedded, etc. cables if spaced more than 6" apart - this, hawever, is no longer accepted by the inspections.

RE: Definition of "Free Air" in Ampacity Tables

Thanks again to everyone for their replies.  I have learned some additional information.  The starter panel is supposed to be designed to UL508A.  A copy of this is around $400.  There are ampacity tables in the standard.  I feel that the standard was not followed so I will probably buy a copy of the standard.  According to some feedback that I got from UL, the internal wiring must be at least 90C wire, the field wiring was tested and certified at 75C, and if it will be connected to a motor, then the wiring should have an ampacity of 125% of FLA.  The temperature rise at the terminals was monitored for UL testing and the allowable temperature rise was decreased for any increase in ambient temperature above the standard for ampacity tables.  I am very interested in seeing the ampacity tables in UL508A and comparing them with the NEC.  

Also a comment in regard to someone else's comment about the size of the utility connection to the service entrance conductors.  Utilities usually follow the NESC (National Electric Safety Code) and here in California G.O. 95 (General Order 95) which is published by the Public Utilities Commission.  The NESC is a IEEE standard I believe.

RE: Definition of "Free Air" in Ampacity Tables

Free air, from what I know, means ambient temperature of 30C give or take. There should be adequate space around the wire to maintain good convection(right term?). Wire placed in raceways, looms, or any other item is not in free air.

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