NEC 310.15, Clarity on "free air"

[metzer0331]

I assume that wiring internal to a cabinet, 2'x6'x2' does not count as free air, even though individual wires are routed inside of the enclosure. I see the cabinet as restricting "air flow" and thus you have to use the conductors ampacity as if they were in a raceway.

Any input???

 

[petersonra]

I assume that wiring internal to a cabinet, 2'x6'x2' does not count as free air, even though individual wires are routed inside of the enclosure. I see the cabinet as restricting "air flow" and thus you have to use the conductors ampacity as if they were in a raceway.

Any input???

a cabinet is not a raceway. a raceway is well defined in the code.

is what you are calling a cabinet more properly termed a gutter? or is this an enclosure that has other components such as CBs and motor starters in it? or does it have only wires and wiring devices in it such as distribution blocks.

 

[metzer0331]

a cabinet is not a raceway. a raceway is well defined in the code.

is what you are calling a cabinet more properly termed a gutter? or is this an enclosure that has other components such as CBs and motor starters in it? or does it have only wires and wiring devices in it such as distribution blocks.

It is an enclosure with Circuit Breakers, PLCs, Termanial Blocks and other misc. signal and power components. It just happens to be a large enclosure which the NEC does not define what "free air" is and how it relates to field components.

 

[Smart $]

I assume that wiring internal to a cabinet, 2'x6'x2' does not count as free air, even though individual wires are routed inside of the enclosure. I see the cabinet as restricting "air flow" and thus you have to use the conductors ampacity as if they were in a raceway.

Any input???

The limiting factor in an enclosure is typically termination temperature limitations under 110.14(C). Your circuit ampacity cannot exceed that which will cause the termination temperature to exceed its rating... and when circuit ampacity is coordinated to that temperature rating, the conductor's insulation rating cannot be less than that either.

Thus, because free-air allowed ampacity values are substantially higher, they are rendered moot for the determination of circuit ampacity. Conductor ampacity is the lowest value determined for any section of a conductor, or second lowest when 310.15(A)(2) Exception is applicable. Circuit ampacity (as differentiated from conductor ampacity) is the maximum permitted ampere value determined for all associated equipment. Where conductor ampacity exceeds the circuit ampacity, circuit ampacity is the overriding limitation.

 

[mgookin]

It is an enclosure with Circuit Breakers, PLCs, Termanial Blocks and other misc. signal and power components. It just happens to be a large enclosure which the NEC does not define what "free air" is and how it relates to field components.

Is this something that was made in a panel shop?

BTW: Welcome to the forums!

 

[Smart $]

...

BTW: Welcome to the forums!

Ditto.

Welcome :thumbsup:

 

[petersonra]

:weeping:

It is an enclosure with Circuit Breakers, PLCs, Termanial Blocks and other misc. signal and power components. It just happens to be a large enclosure which the NEC does not define what "free air" is and how it relates to field components.

It would be an industrial control panel then. The innards of an ICP are generally not subject to the NEC IMO. UL508a would be the most common ICP standard and the ampacity values it uses are basically those found in the 60 or 75 deg C column in table 310.15(B)(16), depending on what conductor insulation you use.

 

[Julius Right]

In my opinion-as Smart $ already said-, the ampacity of the part of cable located in a cabinet [MCB, MCC or else] may be considered the same as the ampacity of rest of cable located enclosure outside-unless the length of the inside part is more then 10 ft. or 10% of the cable total length. [According to Art.310.15(2) Exception].

 

[Julius Right]

Cable ?suspended in Free Air? it is not clear defined in NEC. In VDE 276-1000 is defined as ?free air? if no more than 10% of the cable on open cable tray is covered by supporting structure. ?Free Air? it is considered as it depends not only on surrounding air temperature but also on air convection cooling possibility. For instance 4/0 copper conductor 90oC insulated in a raceway [Table 310.15(B)16 at 30 dgr.C ambient air] ampacity is 260 A. In ?free air?- Table 310.15.(B)(20) ampacity 335 A [at 40dgr.C ambient] or 335*1.1=368.5 A at 30 dgr.C.

 

[Smart $]

.... For instance 4/0 copper conductor 90oC insulated in a raceway [Table 310.15(B)16 at 30 dgr.C ambient air] ampacity is 260 A. In ?free air?- Table 310.15.(B)(20) ampacity 335 A [at 40dgr.C ambient] or 335*1.1=368.5 A at 30 dgr.C.

And say for instance that conductor terminates in a 75?C-rated terminal and exits the enclosure directly to uncovered ladder-type cable tray, its circuit ampacity is limited to 230A under 110.14(C).

 

[metzer0331]

Is this something that was made in a panel shop?

BTW: Welcome to the forums!

Yes, it is a custom panel shop.

 

[Smart $]

Yes, it is a custom panel shop.

For wiring which is entirely internal to a custom panel not fabricated for NRTL listing, I'd refer to NFPA 79 first.

 

[petersonra]

Yes, it is a custom panel shop.

IMO, the internal wiring completely inside the cabinet is just not subject to the NEC, nor are there any NEC sections that would actually apply to that wiring. The wire that comes into the cabinet from the outside is subject to the NEC.

How is this any different than an MCC?

As long as the minimal requirements the NEC has for an ICP are met, there would not appear to be anything a mere inspector could do anyway, especially if the ICP was listed. And why would they want to?

Think about a few things.

UL508a allows the use of conductor insulation types inside an ICP that are not allowed by the NEC. AWM comes to mind. Under what authority would an inspector be able to reject the use of AWM inside an ICP?

The plastic duct typically used inside of an ICP is not a NEC raceway. Can the inspector reject an ICP because plastic wire duct was used inside it?

It is common to use cord and cables not allowed by the NEC inside an ICP. Should this result in a red tag?

 

[mgookin]

Yes, it is a custom panel shop.

And what raises the question? Are you building the panel or is someone challenging what's inside it or did you experience an adverse event?