This is for 24VDC control wiring only.
Depends on the outside diameter. Code does not list 18AWG THHN as a recognized wire size in Table 310.104(A) and I cannot find any manufacturer online that lists a size below 14AWG.
That would be because UL 83 for thermoplastic insulation which governs THHN/THWN-2 will not permit 18 AWG as THHN. When you get down to those sizes you are looking at TFN or TFFN. However, when this is the case you can look at section 402.7 for directions which references Table 1, Chapter 9 where the percentage of fill is unchanged.
Oh my gosh, this is my favorite kind of question. Many years ago, when I was still working with the tools, guys like me and my toolee would ask each other these kinds of trick questions all the time.
Why is it a trick question? Well, to start the two previous answers shed some light on information that was not included in the question that can help to get to an answer, but there is even more.
How long is the conduit? – An installation of conduit or tubing that is 24 inches or less in length that is used to connect between boxes, cabinets,or similar enclosures can be filled to 60% of available cross sectional area. (NFPA 70 2014, page 756, note 4)
What is the application of the 24 vdc “control” wire”? - Such articles as 725, and 800 allow for some complicated interpretations of conduit fill requirements for certain specific applications of low power/low voltage/communication wiring installations.
Is this conduit part of a “Premise” wiring installation or part of a “Machine”? – The NEC specifically deals with premise wiring installations (and this is fully explained in the Scope, Article 90.2). If you were building an industrial machine, and using IMC conduit to protect control wires, you would be better off following UL standards, IEEE Standards, and NFPA 79, Electrical Standard for Industrial Machinery
Is this a complete conduit or tubing installation? – This one will get you into trouble with the AHJ almost every single time. One of the notes to Table 1 of chapter 9, note 2, states that table 1 only applies to “complete” conduit or tubing installations and not to sections of conduit or tubing installed to provide physical protection to conductors, go ahead and try to wrap your head around what that is actually supposed to mean.
Overall it is the kind of question that starts off sounding like an answer should be easy to find, but ends up being the kind of question that needs a great deal more explanation and specific details of the application and installation purpose and methods to reach an answer. And keep in mind that a correct answer might end up actually being not the best answer, or even the answer your Authority having Jurisdiction will accept.
Retrieving data ...