You will want to limit the voltage drop in your DC distribution wiring to 5% or less of the 12V. That would be equivalent to a 0.6V drop. An online wire size calculator suggests that for 600 feet at 3A that you would need to use 1 AWG copper wire.
Cost of that size wire suggests that you may want to consider distributing the AC mains voltage instead and converting to 12VDC at the load points. The reason being is that the amount of voltage drop in the wiring follows the amount of current through the wire. 30-40W at a line level voltage results in less than 0.5A down the wire and 5% voltage drop at 120VACrms makes room for a larger allowable voltage drop than the 0.6V at 12V. The net result is that a much more reasonable wire size can be utilized.
If you use optical fiber cables they could be in the same conduit as DC cables. You may want to separate them from AC mains cabling just because it seems like a good idea to keep AC mains by itself.
Note: All code references cited in this answer are from NFPA-70:2014 (the 2014 NEC).
Question #1: It looks like you have it right that the installation is in violation of 312.5(C). The code reference is attached at bottom of my answer.
- 312.5(C): There is less than 18" of EMT used. This rule was evidently introduced in the 2002 NEC because a number of electricians were stuffing a bunch of NM cables through a single hole. I have yet to exactly figure out from code history why 18" was the selected value, but I suspect it is because a generally accepted height that is no longer subject to physical damage is 8' and the maximum height of a breaker is 6'7" (hence the potential minimum height for the top of a service panel with a breaker at this maximum height) per 2014 NEC 404.8 (8' - 6'7" = 17"). It seems like the installer met the spirit of the rule given the low basement ceilings (but not the letter) by giving each cable assembly it's own EMT protection even though the length is only 8" -- this is perhaps why the inspector let it slide.
- 312.5(C)(a): At least one cable is not secured within 12" of exiting the EMT (the orange one on the right).
- 312.5(C)(d): The EMT is not sealed at the outer end (per your description)
- 312.5(C)(e): There is no visible NM sheathing extending into the box and is definitely less than the required 1/4" of sheathing extending into the panel
Question #2: I could see an argument for installing NM cable outside of conduit so long as the cable originates from a punch-out toward the middle of the panel. The reason I say this is that you effectively have EMT providing protection against physical damage on all sides around it. That call would ultimately be up to the inspector's opinion though -- the same goes for all calls on "subject to physical damage" since the NEC doesn't define it.
Question #3: You will note that flexible conduit is not expressly allowed by 312.5(C) and expressly specifies only two allowable methods: securing the cable and specific uses of nonflexible raceways. Also, all flexible conduit options are subject to the same "cannot be used where subject to physical damage" restrictions as NM, so it wouldn't do any good in this case even if it were allowed since the NM cable is being protected because it is subject to physical damage.
Just to show the specific prohibitions on the various flexible conduit options:
ARTICLE 348
Flexible Metal Conduit: Type FMC
348.12 Uses Not Permitted. FMC shall not be used in the following:
(7) Where subject to physical damage
ARTICLE 350
Liquidtight Flexible Metal Conduit:
Type LFMC
350.12 Uses Not Permitted. LFMC shall not be used as
follows:
(1) Where subject to physical damage
ARTICLE 356 Liquidtight Flexible Nonmetallic
Conduit: Type LFNC
356.12 Uses Not Permitted. LFNC shall not be used as
follows:
(1) Where subject to physical damage
ARTICLE 360
Flexible Metallic Tubing: Type FMT
360.12 Uses Not Permitted. FMT shall not be used as
follows:
(5) Where subject to physical damage
ARTICLE 362
Electrical Nonmetallic Tubing: Type
ENT
362.12 Uses Not Permitted. ENT shall not be used in the
following:
(9) Where subject to physical damage
ARTICLE 320 ARMORED CABLE: TYPE AC
320.12 Uses Not Permitted. Type AC cable shall not be
used as follows:
(1) Where subject to physical damage
ARTICLE 330 METAL-CLAD CABLE: TYPE MC
330.12 Uses Not Permitted. Type MC cable shall not be
used under either of the following conditions:
(1) Where subject to physical damage
I think that has it covered.
312.5 Cabinets, Cutout Boxes, and Meter Socket Enclosures. Conductors entering enclosures within the scope of this article shall be
protected from abrasion and shall comply with 312.5(A) through (C).
(C) Cables. Where cable is used, each cable shall be secured to the
cabinet, cutout box, or meter socket enclosure. Exception: Cables with
entirely nonmetallic sheaths shall be permitted to enter the top of a
surface-mounted enclosure through one or more nonflexible raceways not
less than 450 mm (18 in.) and not more than 3.0 m (10 ft) in length,
provided all of the following conditions are met:
(a) Each cable is fastened within 300 mm (12 in.), measured along the
sheath, of the outer end of the raceway
(b) The raceway extends directly above the enclosure and does not
penetrate a structural ceiling.
(c) A fitting is provided on each end of the raceway to protect the
cable(s) from abrasion and the fittings remain accessible after
installation.
(d) The raceway is sealed or plugged at the outer end using approved
means so as to prevent access to the enclosure through the raceway.
(e) The cable sheath is continuous through the raceway and extends
into the enclosure beyond the fitting not less than 6 mm (1⁄4 in.).
(f) The raceway is fastened at its outer end and at other points in
accordance with the applicable article.
(g) Where installed as conduit or tubing, the cable fill does not
exceed the amount that would be permitted for complete conduit or
tubing systems by Table 1 of Chapter 9 of this Code and all applicable
notes thereto. Informational Note: See Table 1 in Chapter 9, including
Note 9, for allowable cable fill in circular raceways. See
310.15(B)(3)(a) for required ampacity reductions for multiple cables installed in a common raceway.
Best Answer
You mean 800.133. And this very concept of mixing comms and power is highly improbable. I know you're casting around for a way to do this, but this sort of "hopeful reading" is very highly prone to confirmation bias.
NM-B is always useless in conduit
Using NM-B in the conduit wiring method is not illegal, but buys you nothing but headaches. It's very difficult to wrestle, and takes up way too much space (counts as a round wire of the wide dimension). It's not going to bring you closer to running comms and power in the same conduit.
As a rule, NM cable in conduit doesn't buy you any advantages at all. This is one warning that you may be barking up the wrong tree.
800.133 only affirms a case that's already allowed.
Elsewhere in NEC there is a rule that you can run low voltage or comms wiring in Class 1 methods, and commingle with power lines, if and only if the entire comms circuit, every inch soup to nuts, is wired in Class 1 wiring methods.
It's not even really an exception. You see, Class 1 wiring has no minimum voltage. If you want to install to mains spec and then run 6V on it, be my guest. Nothing says you can't use class 1 wiring to convey signal instead of power, and that's exactly what happens between some smart switches, for instance. The only exception being made is use of smaller than 14AWG wire. Here's an example:
Presumably, you want your ethernet cables to exit class 1 wiring methods at a faceplate, then proceed via the decidedly not-class-1 ethernet cable to your most definitely not class 1 Ethernet card or dongle. So like my thermostat, your entire comms circuit is not in class 1 wiring. That is exactly what you can't do.
Also I believe Ethernet cable is not insulated well enough and not a large enough wire diameter.