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Free Air conductors????????????

Question asked by david.michel on Jun 15, 2018
Latest reply on Dec 7, 2018 by rcramereleceng

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I have had zero luck with this but several differing opinions

We were trying to judge the ampacity of an existing cable, 3 conductors 500KCM in one jacket or cable.

This cable is attached to building rafters, about 10 ft between rafters and the cable is surrounded by air above below beside and is indoors near roof no other conductors

Some say it is 3 cables in same jacket and free air table does not apply.

Others say its in free air so it is free air rating

I cant find any definitions or specific chapter that deals with this

Help!!!!!!!!!!!

No one else has this question

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milt.werner Jun 27, 2018 3:55 PM
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It has many definitions by many people using the conductor ampacity tables. The only definitions that counts is the AHJ that passes or fails the installation; or the legal recourse for those that were injured or damaged by the choice of conductors.

My best
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- david.michel @ milt.werner on Jun 27, 2018 6:59 PM
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  Thanks for your input again a 1956 install. There is a possibility that we need more ampacity but it would be a $60000 decision. Doing this and not solving the problem would be very bad.
  
  The main thing I learned from this is that it is very unclear issue regarding the code book. I have asked this on several forums and no one has a solid answer with a code section. Generally if you ask a question if you can’t find something in the code book several guys can usually point you in the right direction. Unfortunately for me my questions I put on forums usually no one has a clear answer. I like to think of myself as a pretty good code guy but this one is tough.
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  - milt.werner @ david.michel on Jun 29, 2018 10:23 AM
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    David:
    
    Sounds like a lot of expense and very old electrical wiring. You may want to work with an electrical engineer and maybe do some testing to better analyze the condition of the wiring. There are most likely other mathematical models to determine the ampacity other than the code tables that would be acceptable to the AHJ. The code tables to me have been a cookbook approach to a fundamental engineering solutions based on the theories of wire ampacity.
    
    My best,
    Milt Werner
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tranzz4md Nov 24, 2018 3:57 PM
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Language always seems troublesome in online chats like this. My immediate thoughts were "it's a freakin' cable, not just insulated conductors!" but I didn't state that, and only mention it now to make my point, because other readers certainly had initial thoughts too which all had applicable aspects or questions.

You have a "Cable assembly", which comprises conductors, insulation, spacing, and jacketing and other materials. You may see this as some insulated conductors within a flexible raceway or sleeve, but that is by no means what you're dealing with. I deal with insulated conductors and cable assemblies quite a bit. (Most people I'm around don't refer to individual insulated conductors as "cables" wether they're large and stranded or not.) Where a cable assembly is terminated, it's outer jacketing is cut and terminated, and the individual encased elements are as well, typically by bringing lengths of those individual insulated conductors to separate, individual points of termination. Occasionally, where special cables are used, other elements, such as support cables, control conductors, or multiple jacket elements may also be individually terminated. Some cables have viscous compounds also, and sealing is part of the termination.

You may well realize that, but seem to be ignoring that, or unaware of what it really lets us know: the individual insulated conductors are not cable assemblies, and their characteristics are decidedly different than the cable assembly they are a part of. The electrical insulating, thermal, heat dispersion, mechanical, and even chemical characteristics of those individual conductors within the assembly are different than when outside of that sealed assembly.

It would be a reasonable assumption that the cable as installed was protected at or near its safe operating limit. In 1956 thermoplastics existed, but were relatively new, and their chemistry different than the materials used in today's electrical insulation materials. Nylon was not used on individual insulated conductors as they have been since the 1970s. (The "N" in THHN and THWN refers to the nylon).

500,000 circular mil copper conductors will carry far more than 400 or 500 amperes of electrical current. The materials that surround them must be able to withstand AND DISPERSE the heat generated (by the resistance of the conductor) however. You must realize that such heat handling is notably different in/by a cable assembly (the bunch of conductors and stuff inside one overall jacket) than individual insulated conductors "in free air".

Personally, I dislike the 10' spacing of supports, but the original installers may well have disliked it too, and just got on with the job, noses upturned.

I see no need to decommission the cable, based on the information we've gotten from you. If you need to replace the protective device feeding the cable, you should not increase ampacity at all, and probably attempt to obtain a slightly lower or faster trip characteristic. If you need to "extend" the cable, it can be done, by use of terminals in an enclosure or a "cable splice", but lots of details must be correctly handled.

This long answer doesn't directly answer your original question, I think you've found an answer to that, like it or not.

1956 to 2018. Sounds like a stable, durable location. If you have to re-do it, do it right: XHHW in GRC (or do they like 3 different letters now? The kids might call it RMC for all I know....)
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rcramereleceng Nov 27, 2018 8:08 PM
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Multi-Conductor Cable: 3-wire 500KCM {circa 1956}. Speculation: this is Asbestos Wrap.

Table 310.15(B)(16) is for 3 in Conduit, for the Insulation Types listed (with expansion of) in Table 310.104. The same Insulation Types are listed in Table 310.15(B)(17) for in Free-Air. According to the Code, any Wire leaving the Enclosure requires protection (i.e. Conduit). Furthermore, any Insulation Types listed for these Tables cannot be run in "Tray" or just strung up. So what gives?!?

"In Free Air" is Wire inside the Enclosure (including Wire Duct, Raceway, Cable Tied, etc.). I follow a general rule to run THHN for all Power Drops (Feeds) & Motors. Once I'm in the Panel, I use MTW for it's flexibility. I don't need to upsize (usually), because MTW has a higher Current-Carrying Ampacity {based on Table 310.15(B)(17)} in Free Air - than THHN (3 in Conduit) {based on Table 310.15(B)(16)}. Viola.

Wire with a Jacket is now a "Cable" (single or multi-conductor). Follow Manufacture's Specs for Ampacity. If not available, use Table 400.5(A)(1) (with expansion of) in Table 400.4. If it's not listed - it doesn't exist.

My recommendation is to install new Cable. Decommission the old Cable and leave in-place until it's confirmed that it's not Asbestos. That Cable has to be so brittle that the slightest 'nudge' (from a wayward Lift Truck Driver or even a shoulder bump doing other work ...because it's exposed) will compromise it in same way.
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opes Dec 2, 2018 9:38 AM
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There is a section in NEC that gives the maximum ampacity for conductors in free air. 2017 NEC table 310.15(B)(21)
insulated 500KCM is good for 812ampere based on 40Deg C ambient [104Deg F] with max conductor temperature of 80Deg C [164Deg F]. The problem the way I see it as that the cable is in an attic and near the roof with no air circulation and the ambient temperature of the air is probably greater than 104Deg F. The other issue includes concealment. If any part of the conductor is concealed it cannot be considered as running in free air.
From a design stand point I would not run an exposed multiconductor cable through [assuming] wood rafters not in conduit unless fire rated and I wouldn't qualify the installation as a conductor running in free air.
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- opes @ opes on Dec 3, 2018 8:45 AM
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  To add to my last comment I have specified multiconductor power cables run in cable tray for ease of maintenance and identification but never used exposed aerial mounted cables. NESC and ICEA has something to say about aerial cables. There are many sections in NEC that deals with multiconductor cables but very little on 3 and 4 conductor power cables. The only place I have seen asbestos insulated multiconductor power cable was for 15KV cables within electrical manholes. The code permits running multiconductor power cable assemblies exposed provided they are approved for the purpose and NESC along with ICEA deals with the mounting. However when considering the capacity of cables run in free air environments all factors need to be viewed and include but is not limited to ambient temperature, heat generated by conductor [ loading ], or air flow over the cable. Panel interiors are not what I would call a free air environment. Running a cable in an interior space with no air flow is not what I would call a free air environment either.
  Conductors are capable of higher then rated ampacity by probably a factor of three if you were to calculate it out. NEC is very conservative when it comes to conductor sizing. Get with the cable manufacturer and find out what they say and if necessary hire an electrical engineer to make the right determination.
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rcramereleceng Dec 7, 2018 10:25 AM
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TABLE 310.15(B)(21) 500kcmil =812A is for Covered (Insulated) single Conductor. This question pertains to a Multi-Conductor Cable.

Newfound info expanding on Table 400.5(A)(1). Annex B: Table B.310.15(B)(2)(1).

Ampacities for THHN within an overall covering (Multiconductor Cable), in Raceway in Free Air: 500kcmil = 427A.

… in Raceway in Free Air … hmmmm, covered yet Free.
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Free Air conductors????????????

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