Let's get the critics out of the way first... I am an electrical engineer with an additional degree in electrical construction, as well as time spent in the trade. I would never design or construct an unsafe environment for anyone and the following question is to serve as an educational tool only. 
My boss came to me and asked a simple question which no one seems to have truly answered. Where does the code explicitly state that single conductors such as THHN, THWN, etc., must be run in conduit?
I know the first person will answer.. NEC 300.3(A) "(A) Single Conductors. Single conductors specified in Table 310.104(A) shall only be installed where part of a recognized wiring method of Chapter 3." And even 310 says the same thing... But as my boss and I discussed after reading through Chapter 3 carefully, what "recognized wiring method", as side from subject to physical damage, prohibits a single conductor to be run say in a wall or other open spaces freely? You can buy single conductors marked "direct burial" in which case conduit is not required... so where is the line drawn? I would like a documented code, standard, something concrete other than "because thats the way we do it" response, please... if possible.
Back story, the topic came up because we are on a job where the building was up graded and we found the service entrance cable in a crawl space under the upgrade, breaching through a layer of item-4 and then diving back down in. No sign of conduit on either end. At the meter, there was a conduit sub up.
My boss came to me and asked a simple question which no one seems to have truly answered. Where does the code explicitly state that single conductors such as THHN, THWN, etc., must be run in conduit?
I know the first person will answer.. NEC 300.3(A) "(A) Single Conductors. Single conductors specified in Table 310.104(A) shall only be installed where part of a recognized wiring method of Chapter 3." And even 310 says the same thing... But as my boss and I discussed after reading through Chapter 3 carefully, what "recognized wiring method", as side from subject to physical damage, prohibits a single conductor to be run say in a wall or other open spaces freely? You can buy single conductors marked "direct burial" in which case conduit is not required... so where is the line drawn? I would like a documented code, standard, something concrete other than "because thats the way we do it" response, please... if possible.
Back story, the topic came up because we are on a job where the building was up graded and we found the service entrance cable in a crawl space under the upgrade, breaching through a layer of item-4 and then diving back down in. No sign of conduit on either end. At the meter, there was a conduit sub up.
