Derate for more than 3 current - conductors in a raceway or Cable

[Dugswork]

New to the site and first new topic post here. I have the conductors from four 30amp 220v breakers traveling through a nipple less than 24inches to a set of contactors and then back in the same nipple to the large junction box they derive from for a total of 16 #10 THHN conductors. I calculated conductor fill etc. & used 1-1/4 " emt. I have a couple of questions.

• ? Does 310.15{3}{a} "or where single conductors or multiconductor cables are installed without .......longer than 600mm{24in.} mean I do not have to derate the ampacity of the conductors using table 310.15{3}{a} "more than 3 current-carrying conductors in a raceway or cable".
  

• Also I remember in the past someone saying in a class that in a situation like this there would be no induced voltage/amps because the magnetic fields would negate each other because the same wires & loads are "traveling in both directions".

• If I do have to derate amperage capacity of the conductors was there a previous code edition where this was not required in nipple <24" ?

Thanks for your response.

 

[kwired]

The 24 inch nipple adjustment rule still is pretty much the same as it has been for years. It does not matter that the conductors happen to be part of the same circuit or not, it is the amount of heat that is generated within each current carrying conductor that drives the ampacity adjustment rules. At some time the applicable code making panel decided there would not be significant enough heat produced and contained within the nipple that adjustments are necessary in a 24 inch or less nipple.

310.15(B)(3) is the applicable code section.

As mentioned in the other thread where you first brought this up, the magnetic fields issue you bring up likely had to do with metal raceways/enclosures where you do not have an equal current in both directions in a set or multiple sets of conductors in that raceway/enclosure.

If you put a single conductor in a ferrous sleeve and pass a current through it you have a basic core and one turn coil because of the magnetic field around the conductor. If you pass the second conductor of the same (lets stick with simple 2 wire circuit for now) circuit through the same metal sleeve the magnetic fields of both conductors cancel each other out and no induction happens to the sleeve.

I/others can expand on this further if you wish, but maybe is a start in the right direction to what you were asking about.

 

[Dugswork]

Thanks K wired just what I was looking for. The technical explanation for induced voltage/amperage is about as deep as my engineering knowledge gets at this time but having a working knowledge of what the NEC rules are based on helps at times. The phrasing on 310-15 {3} looked like it had changed possibly to where you had to derate the ampacity of the conductors through any length of Conduit and that the 24 inch rule applied only to tightly bound cables - caused me some confusion.

::edited::

 

[Sierrasparky]

Welcome to the Forum:thumbsup:

 

[kwired]

Thanks K wired just what I was looking for. The technical explanation for induced voltage/amperage is about as deep as my engineering knowledge gets at this time but having a working knowledge of what the NEC rules are based on helps at times. The phrasing on 310-15 {3} looked like it had changed possibly to where you had to derate the ampacity of the conductors through any length of Conduit and that the 24 inch rule applied only to tightly bound cables - caused me some confusion.

::edited::

24 inch rule does apply to tightly bound cables as well as raceways. But when dealing with NM cable there has been changes more recently (see 334.80), where if cables are run through holes that are sealed with thermal insulation, caulk or sealing foam you must adjust ampacity even the length of close proximity is less than 24 inches, it has been found when sealed like that the heat doesn't get away, I have seen some testing done with temperature probes in the sealed hole and it is surprising just how much heat does build up in there.

Put NM cables in a tight bundle but not embedded in thermal insulation and you can apply the 24 inch rule before adjustments are necessary, put more than three current carrying conductors (as part of NM cables) through a wall plate only three inches thick and fill it with foam insulation, you must derate the ampacity.

 

[infinity]

Put NM cables in a tight bundle but not embedded in thermal insulation and you can apply the 24 inch rule before adjustments are necessary, put more than three current carrying conductors (as part of NM cables) through a wall plate only three inches thick and fill it with foam insulation, you must derate the ampacity.

Actually the requirement to derate kicks in after you exceed two cables. You could conceivably have 8 CCC's in a two cables and still not require derating.

 

[kwired]

Actually the requirement to derate kicks in after you exceed two cables. You could conceivably have 8 CCC's in a two cables and still not require derating.

You are correct. I have never noticed that before, but at same time never run into an NM cable with enough conductors in it for it to create any issue for me until you get more than two cables in the hole anyway.

 

[Dugswork]

Aha!!! I found the bug in my system!

Aha!!! I found the bug in my system!

Thanks for the clarification Kwired and Infinity......it takes a lot of rooting around to come up with those facts if you have not been confronted with them in a while.

I found out why I was having such trouble with the "less than 24 inches" for derating conductors in a conduit. I use the 2011 Handbook and for several pages in a row those code sections are plastered with blue additional information sections , Informational notes {with their own numbered notes!}, and poor indentation. The Article 310.15[3]{2} {I believe that is the right section designation} on page 328 in the Handbook states specifically the 24 inch/600 mm rule. I just repeatedly scanned over the darn thing evidently. I now have it in ink in my index. :slaphead:

 

[Finite10]

24 inch rule does apply to tightly bound cables as well as raceways. But when dealing with NM cable there has been changes more recently (see 334.80), where if cables are run through holes that are sealed with thermal insulation, caulk or sealing foam you must adjust ampacity even the length of close proximity is less than 24 inches, it has been found when sealed like that the heat doesn't get away, I have seen some testing done with temperature probes in the sealed hole and it is surprising just how much heat does build up in there.

Put NM cables in a tight bundle but not embedded in thermal insulation and you can apply the 24 inch rule before adjustments are necessary, put more than three current carrying conductors (as part of NM cables) through a wall plate only three inches thick and fill it with foam insulation, you must derate the ampacity.

Good points. I always have to remind myself to derate from the 90 degree column. So, #12 derated at 70% for up to 9 conductors is still 21A.
Residential uses that foam in bored holes in outside walls & top top/bottom plates all the time now.
Gotta love the NREC.

 

[charlie b]

Actually the requirement to derate kicks in after you exceed two cables. You could conceivably have 8 CCC's in a two cables and still not require derating.

Allow me to split a hair or two here (though I have few remaining to spare :lol. You would still require derating. But the final derated ampacity might still be high enough so that you don't have to use a different size wire or a different rated breaker. Finite10's post gives one such example.