Raceways, Boxes, and Fittings
Raceway types and where they belong
Article 300 gives the general wiring-method rules. Then know the individual articles: 342 IMC, 344 RMC, 348 FMC, 350 LFMC, 352 PVC, 358 EMT. Rigid metal and IMC suit hazardous or corrosive areas; PVC is limited by temperature and by support spacing; EMT is the everyday choice for exposed interior work; flexible connections are used at motors and vibrating equipment.
Fill, bends, and support
Conduit fill comes from Chapter 9. Table 1 gives 53 percent for one conductor, 31 percent for two, and 40 percent for three or more. Annex C gives ready-made combinations. Total bends between pull points may not exceed 360 degrees. EMT must be supported within 3 ft of a box and every 10 ft thereafter. A 24 in nipple is the classic exception that allows 60 percent fill.
Box fill and pull boxes
Box fill under 314.16 counts each conductor by its volume from Table 314.16(B). Each device counts as two conductors of the largest size, each cable clamp as one, and equipment grounding conductors together as one quarter of the largest conductor's volume. Pull-box sizing under 314.28 uses 8 times the trade size for straight pulls and 6 times for angle or U pulls.
Worked example
A box of 20.3 cubic inches with 12 AWG (2.25 cubic inches each): four conductors = 9.0; one ground = 2.25; one device = 4.5; clamps = 2.25. Total 18.0, which fits.
Rule of thumb
40 percent fill for three or more conductors, 360 degrees maximum between pull points, 6 in free conductor at each box.