when can we use table 310.17?
- Thread starter tobbink21
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: when can we use table 310.17?
I can?t think of a better application. It is not as though we would route cables from one enclosure to a different enclosure, with the conductors in ?free air.? Conductors that never leave an enclosure, and that are routed throughout the enclosure without any type of raceway, should be treated as being in free air.
I can?t think of a better application. It is not as though we would route cables from one enclosure to a different enclosure, with the conductors in ?free air.? Conductors that never leave an enclosure, and that are routed throughout the enclosure without any type of raceway, should be treated as being in free air.
- Location
- Massachusetts
Re: when can we use table 310.17?
The fact the NEC does not spell out how many cubic inches an enclosure has to be before the conductors within are in "Free Air" makes me believe that the conductors may not be Free Air inside electrical enclosures.
JMO, I will be glad to hear other opinions.
Bob
Charlie I disagree, IMO the term free air means "not in an enclosure" .
The fact the NEC does not spell out how many cubic inches an enclosure has to be before the conductors within are in "Free Air" makes me believe that the conductors may not be Free Air inside electrical enclosures.
JMO, I will be glad to hear other opinions.
Bob
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: when can we use table 310.17?
I believe I lean toward ?the other? Bob?s view. Table 310.17 is specifically referenced in Section 392.11(B) as it applies to various single conductor applications in cable tray. See especially 392.11(B)(3).
Edit Add I've thought about this even more. I imagine Table 310.17 would be the basis for determining the ampacity of Art 398 conductors also.
[ October 05, 2004, 06:00 PM: Message edited by: rbalex ]
I believe I lean toward ?the other? Bob?s view. Table 310.17 is specifically referenced in Section 392.11(B) as it applies to various single conductor applications in cable tray. See especially 392.11(B)(3).
Edit Add I've thought about this even more. I imagine Table 310.17 would be the basis for determining the ampacity of Art 398 conductors also.
[ October 05, 2004, 06:00 PM: Message edited by: rbalex ]
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: when can we use table 310.17?
My view is that the terms ?free air? and ?enclosure? are not mutually inconsistent.
I readily concede that the issue is not self-evident. But consider this: Table 310.16 applies to conductors in ?raceway.? The Article 100 definition for ?raceway? is fairly extensive, and I think it only leaves out cable tray. But then, 392.11 tells how to amend that table to make it applicable to cable tray.
So, is there any way to get from one enclosure to another, without using some method to which 310.16 applies? I can?t think of any. That leave me wondering when 310.17 would come into play.
As to the wiring internal to an enclosure, I think that 310.16 does not apply. The definition of raceway is limited to an ?enclosed channel.? I would not accept the notion that an ?enclosure? constitutes an ?enclosed channel.? The next logical thought is that, if 310.16 does not apply within an enclosure, what does? My only answer is 310.17.
I too would like to hear other thoughts.
My view is that the terms ?free air? and ?enclosure? are not mutually inconsistent.
I readily concede that the issue is not self-evident. But consider this: Table 310.16 applies to conductors in ?raceway.? The Article 100 definition for ?raceway? is fairly extensive, and I think it only leaves out cable tray. But then, 392.11 tells how to amend that table to make it applicable to cable tray.
So, is there any way to get from one enclosure to another, without using some method to which 310.16 applies? I can?t think of any. That leave me wondering when 310.17 would come into play.
As to the wiring internal to an enclosure, I think that 310.16 does not apply. The definition of raceway is limited to an ?enclosed channel.? I would not accept the notion that an ?enclosure? constitutes an ?enclosed channel.? The next logical thought is that, if 310.16 does not apply within an enclosure, what does? My only answer is 310.17.
I too would like to hear other thoughts.
izak
Senior Member
- Location
- Springfield, MO
Re: when can we use table 310.17?
i always kinda thought 'free air' was outside
conductors supported on a messenger and stuff like that
like triplex
i always kinda thought 'free air' was outside
conductors supported on a messenger and stuff like that
like triplex
valassi
Member
- Location
- Merida, M?xico
Re: when can we use table 310.17?
The 310.16 ampacities are based on the capacity of the medium inside the conduit to dissipate the heat of the conductors, the heat of the current in each one and the heat of the conductors in the proximity, 310.17 adds more "room" to the heat to dissipate it in the air, there are more heat exchange, then I think that going from one enclosure to other is as be on "free air" but for this purpose it is important to check how the conductors are placed, with enough space to dissipate heat.
The 310.16 ampacities are based on the capacity of the medium inside the conduit to dissipate the heat of the conductors, the heat of the current in each one and the heat of the conductors in the proximity, 310.17 adds more "room" to the heat to dissipate it in the air, there are more heat exchange, then I think that going from one enclosure to other is as be on "free air" but for this purpose it is important to check how the conductors are placed, with enough space to dissipate heat.
wirenut1980
Senior Member
- Location
- Plainfield, IN
Re: when can we use table 310.17?
How close together are the conductors run, and how many conductors are we talking about? IMO it comes down to, will all of the conductors be able to dissipate the heat as if they were all in free air. The space is climate controlled, but if you have many conductors running together, with the insulations touching each other, some of the conductors may not truly be in "free air" and will not be able to dissipate heat as well. Bob (iwire) makes a good point: how much space defines free air?
Dave
How close together are the conductors run, and how many conductors are we talking about? IMO it comes down to, will all of the conductors be able to dissipate the heat as if they were all in free air. The space is climate controlled, but if you have many conductors running together, with the insulations touching each other, some of the conductors may not truly be in "free air" and will not be able to dissipate heat as well. Bob (iwire) makes a good point: how much space defines free air?
Dave
petersonra
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- Northern illinois
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- engineer
Re: when can we use table 310.17?
Having said that, there are other issues you might want to consider. if this is a machine control panel, NFPA79 rather then the NEC applies to wiring contained solely within said enclosure, and those wire sizes are generally smaller. UL listed panels also allow smaller wires then you might otherwise think.
If your concern is that whomever built your panel used wires that are too small, I would not overwrought.
If you are worried about your own wiring to the panel I'd be inclined to remember that the vast majority of your wires coming to the panel are control conductors rather than power, and virtually no heat will be generated from them.
Are they really free air inside the enclosure? I would say if they just pass through the enclosure hanging loose you have a good argument. If you end up putting them inside wire duct in the control enclosure I would not consider them free air.
Having said that, there are other issues you might want to consider. if this is a machine control panel, NFPA79 rather then the NEC applies to wiring contained solely within said enclosure, and those wire sizes are generally smaller. UL listed panels also allow smaller wires then you might otherwise think.
If your concern is that whomever built your panel used wires that are too small, I would not overwrought.
If you are worried about your own wiring to the panel I'd be inclined to remember that the vast majority of your wires coming to the panel are control conductors rather than power, and virtually no heat will be generated from them.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: when can we use table 310.17?
Grammatically, Table 310.16 applies to:
Ampacity is defined as:
However, I also respect Charlie B?s position; I agree that,?? that the terms ?free air? and ?enclosure? are not mutually inconsistent.? Since I also believe ampacity may be determined legitimately under ?engineering supervision? per 310.15(C); if the engineer keeps all of the ?conditions of use? in mind while determining the ampacity of a specific conductor, I would endorse it.
edited for grammer
[ October 06, 2004, 11:36 AM: Message edited by: rbalex ]
Grammatically, Table 310.16 applies to:
And Table 310.17 applies to:
I ?leaned? toward Table 310.16 in my first response and still do unless we know we are only talking about single-conductor cables generally separated from each other.
Ampacity is defined as:
Part of the ?conditions of use? is the terminations and Article 110.14(C) still applies.
However, I also respect Charlie B?s position; I agree that,?? that the terms ?free air? and ?enclosure? are not mutually inconsistent.? Since I also believe ampacity may be determined legitimately under ?engineering supervision? per 310.15(C); if the engineer keeps all of the ?conditions of use? in mind while determining the ampacity of a specific conductor, I would endorse it.
edited for grammer
[ October 06, 2004, 11:36 AM: Message edited by: rbalex ]
sandsnow
Senior Member
- Location
- Southern California
Re: when can we use table 310.17?
I ran into this years ago when I questioned the size of tap conductors inside a multi-meter switchboard. Conductors tapped from a large buss to a meter/test block and from there a new set of conductors ran to the line side of a feeder OCPD.
Both sets of conductors were the same size and were smaller than allowed in 310-16. Each conductor had air space all around it. The manufacturer stated that they were in free air and table 310-17 applied. I didn't think so because I had thought as the "Bobs" that you had to be outside to be in free air. The manufacturer was right and was verified by talking to an engineer at UL. The standard for switchboards considers this free air.
Question: Is it the same for enclosed industrail control panels? I don't know. Probably.
My opinion is the 3 wires from C/B to bussbar would be free air.
In your control panel I would have to agree with wirenut1980 that if you ran current carrying conductors close together; say bundled or in panduit tray you would be in 310-16.
I ran into this years ago when I questioned the size of tap conductors inside a multi-meter switchboard. Conductors tapped from a large buss to a meter/test block and from there a new set of conductors ran to the line side of a feeder OCPD.
Both sets of conductors were the same size and were smaller than allowed in 310-16. Each conductor had air space all around it. The manufacturer stated that they were in free air and table 310-17 applied. I didn't think so because I had thought as the "Bobs" that you had to be outside to be in free air. The manufacturer was right and was verified by talking to an engineer at UL. The standard for switchboards considers this free air.
Question: Is it the same for enclosed industrail control panels? I don't know. Probably.
My opinion is the 3 wires from C/B to bussbar would be free air.
In your control panel I would have to agree with wirenut1980 that if you ran current carrying conductors close together; say bundled or in panduit tray you would be in 310-16.
Re: when can we use table 310.17?
Let's understand why we have derating for multiple conductors in a raceway. It is because heat cannot dissipate readily as compared to free air.
In a panel with dozens or hundreds of conductors tyrapped tightly together or panduit channels jammed full, heat dissipation is poor.
However, applications for control wiring as in digital inputs and outputs and 4-20 milliamp analog circuits, there is negligible or no heating of conductors. Heat generated by the electronics is probably the main concern and the affect on the conductors should be addressed in the design of the enclosure.
Let's understand why we have derating for multiple conductors in a raceway. It is because heat cannot dissipate readily as compared to free air.
In a panel with dozens or hundreds of conductors tyrapped tightly together or panduit channels jammed full, heat dissipation is poor.
However, applications for control wiring as in digital inputs and outputs and 4-20 milliamp analog circuits, there is negligible or no heating of conductors. Heat generated by the electronics is probably the main concern and the affect on the conductors should be addressed in the design of the enclosure.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: when can we use table 310.17?
One of the reasons I added the reference to 110.14(C) in my second reply is that it specifically refers to Table 310.16 as the basis for determining ampacities in equipment:
[ October 06, 2004, 11:45 AM: Message edited by: rbalex ]
One of the reasons I added the reference to 110.14(C) in my second reply is that it specifically refers to Table 310.16 as the basis for determining ampacities in equipment:
[ October 06, 2004, 11:45 AM: Message edited by: rbalex ]
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: when can we use table 310.17?
The following has been excerpted from the UL
GENERAL INFORMATION FOR ELECTRICAL EQUIPMENT DIRECTORY 2003
This UL Category Code applies under Section 110.3(B).
The following has been excerpted from the UL
GENERAL INFORMATION FOR ELECTRICAL EQUIPMENT DIRECTORY 2003
Category Code ?AALZ? is the primary UL investigation category. No product is ?listed? specifically to it, but virtually all products are evaluated under it.
This UL Category Code applies under Section 110.3(B).
sandsnow
Senior Member
- Location
- Southern California
Re: when can we use table 310.17?
Bob
Regarding your posting of info from the White Book or maybe the Green Book:
The instructions or info there is for the installer of the equipment not the manufacturer. My understanding has always been the manufacturers guidelines are in the standards. For conductors brought into the equipment via conduit or other means I would agree that we would adhere to the info there.
My understanding of the original post was that this control panel was being fabricated and that the conductor did not leave the enclosure, hence my reference to my experience with the internal switchboard wiring.
If no part of the conductor leaves the enclosure, then the applicable UL standard could be used for a guide to size the conductor.
Bob
Regarding your posting of info from the White Book or maybe the Green Book:
The instructions or info there is for the installer of the equipment not the manufacturer. My understanding has always been the manufacturers guidelines are in the standards. For conductors brought into the equipment via conduit or other means I would agree that we would adhere to the info there.
My understanding of the original post was that this control panel was being fabricated and that the conductor did not leave the enclosure, hence my reference to my experience with the internal switchboard wiring.
If no part of the conductor leaves the enclosure, then the applicable UL standard could be used for a guide to size the conductor.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: when can we use table 310.17?
Larry,
You're understanding is correct. I think the original question was how "we" could apply 310.17 and tobbink21's second reply strongly implied a field installation.
My first reply answered the original question. Field applications of Table 310.17 refer to cable trays and probably "open" knob & tube.
The contents of "listed" assemblies can have whatever UL or any other NRTL is willing to certify.
[Edit Add] The "White Book" is what I referenced but the "Green Book" has the same Info.
[ October 06, 2004, 09:25 PM: Message edited by: rbalex ]
Larry,
You're understanding is correct. I think the original question was how "we" could apply 310.17 and tobbink21's second reply strongly implied a field installation.
My first reply answered the original question. Field applications of Table 310.17 refer to cable trays and probably "open" knob & tube.
The contents of "listed" assemblies can have whatever UL or any other NRTL is willing to certify.
[Edit Add] The "White Book" is what I referenced but the "Green Book" has the same Info.
[ October 06, 2004, 09:25 PM: Message edited by: rbalex ]
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