Use of free air ampacities

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mm12866

Member
I am working on an installation where I am running wire between relays located in two 3' x 4' cans. The cans are connected by four 3" nipples. The design load on the wiring between the cans is 750 amps.

When sizing the wire, normally I would use the ampacity tables for 3 conductors in a raceway, but I was wondering if I can use the tables for wire in free air? Is the nipple between the cans considered a raceway? I talked to the customer about this and he indicated that their UL listed inverters use free air ampacities for the interior wiring, so he thought it would be OK for me to do the same.

Any insight/opinions/comments would be greatly appreciated.

Thanks
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Welcome to the forum.:)

Conductors installed in a nipple would be considered to be in a raceway so you would need to use the ampacities listed in Table 310.16 for 3 or more conductors in a raceway or cable.

A typical nipple would most likely be Rigid Metal Conduit.

Chris
 

mm12866

Member
I thought that would be the reply... What if instead I cut an 8" x 8" "window" between the two cans? physically the same situation, but no "raceway"?

Thanks
Mark
 

raider1

Senior Member
Staff member
Location
Logan, Utah
IMHO, Conductors in an 3' by 4' enclosure would still not be located in free air and would still need to have their ampacities calculated using Table 310.16.

Chris
 

Chamuit

Grumpy Old Man
Location
Texas
Occupation
Electrician
Free air means hanging out in the breeze not inside anything at all.

As for the inverter being UL listed with free air ampacities, their engineers and designers have to stand behind that. And, it's been tested.

My 2?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Any particular reason why you're trying to be use free air ampacities within the 4" nipples?
 
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