National Electrical Code Top Ten Tips: Article 312 and Article 314 -- Enclosures and boxes

Based on the 2023 NEC

by Mark Lamendola

Please note, we do quote from copyrighted material. While the NFPA does allow such quotes, it does so only for the purposes of education regarding the National Electrical Code. This article is not a substitute for the NEC.

These are the ten Article 312 and Article 314 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

1. You must repair any non-combustible surfaces that are broken or incomplete so there's a maximum 1/4 inch gap at the edge of a cabinet or cutout box employing a flush-type cover [312.4].
    
2. Use the proper fittings at enclosure openings, to protect and secure the cables and wiring [312.5]. This rule is commonly violated, so be looking for it when reviewing or inspecting an installation.
    
3. Table 312.6(A) provides the minimum bending space at terminals and the minimum width of wiring gutters, based on wire size and the number of wires per terminal. Note that this applies to power cables. Signal cables may require much more room, so check the specs on each type of signal cable you are using.
    
4. Table 312.6(B)(2) provides the minimum wire-bending space at terminals, based on wire size and the number of wires per terminal. How does this table differ from 312.6(A)? If you have wiring gutters, use Table 312.6(A). If you don't have wiring gutters, use Table 312.6(B)(2). You will notice that the minimum wire bending space is the same for size 8 AWG but as the conductor gets larger you see a larger minimum wire bending radius required where you don't have wiring gutters. Sidenote: The new table naming convention with the 2023 revision should have had Tables 312.6(A)(1) and 312.6(A)(2) or 312.6(A)(1) and 312.6(B)(1) rather than how it came out.
    
5. 312.8 provides various clearances within an enclosure or cabinet.
    
6. 314.2 You cannot use round boxes where raceway or connectors requiring locknuts or bushings are to be connected to the side of the box [314.2]. This is because the locking mechanism will have only a fraction of the contact with the box that the design needs for adequate locking. You'll be lucky to get two of the "points" to actually hit the box, whereas with a flat surface all of those points engage with the surface you are locking to.
    
7. 314.3 places certain restrictions on nonmetallic boxes. Generally, you go all non-metallic or all metallic with raceways and boxes. No mix and match.
    
8. 314.4 requires all metal boxes to be "grounded and bonded", but it means they should be bonded if on the load side.
    
9. 314.16 provides the details of box volume calculations, box fill, conductor fill, clamp fill, support fittings fill, device fill, equipment fill, equipment grounding conductor fill, and other factors. It includes NEC Table 314.6(A), which the minimum volume of that qualifies an enclosure to be a certain box trade size. It also shows the maximum number of conductors a given box trade size can legally hold. Some guy named Phil came up with these rules. Just kidding. This is really the heart of the entire article. Take the time to work through a couple of sample projects so you can understand the rules and how to apply them.
   
10. 314.21 provides the same requirement as 312.4: You must repair any non-combustible surfaces that are broken or incomplete so there's a maximum 1/4 inch gap at the edge of a cabinet or cutout box employing a flush-type cover.

The other NEC Articles from this point forward to the end of NEC Chapter 3 are short, and each is specific to a type of cable or wireway.