5.4 Raceways, Cables & Bus Ducts (0404)

Raceways, Cables & Bus Ducts (NEC Articles 300, 342–362, 320–340, 368)

Quick Answer: Article 300 is the general wiring methods article; Articles 342–362 cover individual raceways (RMC, IMC, EMT, RNC, FMC, LFMC, wireways); Articles 320–340 cover cables (AC, MC, NM, UF); and Article 368 covers busways. Raceway fill is calculated from Chapter 9 Tables 1, 4, and 5; for same-size conductors in a single raceway, Annex C gives a quick precomputed count.

Raceway Types & Support Spacing

Note the article numbers: Article 355 is reinforced thermosetting resin conduit (RTRC) — liquidtight flexible nonmetallic conduit is Article 356, and nonmetallic wireways are Article 378.

Cable Types

General Installation Rules — Article 300

• 300.4 — protection against physical damage: (A) cables and raceways through bored holes in wood members must be at least 1¼ in. from the nearest edge or protected by a 1/16 in. steel plate; (D) cables or raceways run parallel to framing members get the same 1¼ in. rule; and 300.4(G) requires insulated bushings or fittings where a raceway terminates and contains 4 AWG or larger insulated conductors, so the conductor insulation is not abraded at the raceway end.
• 300.11 — raceways and cables must be securely fastened in place and supported by the building structure; independent support wires used to support raceways or cables above a suspended ceiling must be distinguishable and secured at both ends. The ceiling grid itself is not a support.
• 300.17 — the number and size of conductors in a raceway must not be more than will permit dissipation of heat and ready installation or withdrawal; the Chapter 9 fill tables implement this.
• Number of bends — each raceway article states its own 360-degree limit: 358.26 for EMT, 344.26 for RMC, 342.26 for IMC, 352.26 for PVC. There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points.
• 358.24 — bends in EMT must be made so the tubing is not damaged and its internal diameter is not effectively reduced (this is the how made rule, not the how many rule).

Raceway Fill — Chapter 9, Table 1

The 60% column printed in Table 4 is not a fourth row of Table 1 — it comes from Note 4 to Chapter 9 and applies only to conduit or tubing nipples not exceeding 600 mm (24 in.) in length between boxes, cabinets, and similar enclosures. Adjustment factors for more than three current-carrying conductors do not apply to those nipples either.

The method: total the conductor areas from Table 5 (insulated conductors) or Table 8 (bare conductors), then find a raceway in Table 4 whose column for your conductor count is at least that large. Table 4 already prints the 53%, 31%, 40%, and 60% values, so in practice you compare your conductor total directly against the printed column instead of dividing.

Worked Conduit Fill Example — 3 #6 THHN in EMT

Step 1 — Conductor area. From Chapter 9, Table 5, one #6 THHN conductor occupies 0.0507 in². Three conductors: 3 × 0.0507 = 0.152 in².

Step 2 — Fill percentage. Three conductors fall in the "more than two" row of Table 1, so 40% of the raceway's internal cross-section may be filled.

Step 3 — Compare against the 40% column of Table 4. The EMT section of Chapter 9 Table 4 reads:

Step 4 — Select the raceway. We need at least 0.152 in². Trade size 1/2 EMT offers only 0.122 in² at 40% — too small. Trade size 3/4 EMT offers 0.213 in², which exceeds 0.152 in². The minimum acceptable EMT is therefore trade size 3/4.

Watch the columns. The most common arithmetic failure here is reading a 40% value out of the 100% column (or the reverse). A 1 in. EMT is 0.864 in² total and 0.346 in² at 40% — those two numbers are five times apart, and mixing them up will oversize a raceway by two trade sizes.

Annex C Shortcut

For runs containing only one conductor size and type, Annex C, Table C.1 lists the maximum number of each conductor size in each EMT size. The counts are simply the Table 4 column divided by the Table 5 conductor area and rounded down: for #6 THHN, 0.122 ÷ 0.0507 = 2 conductors in 1/2 in. EMT, and 0.213 ÷ 0.0507 = 4 conductors in 3/4 in. EMT — confirming that three #6 THHN fit trade size 3/4. Annex C is the faster check, but remember two limits: it is informative, not enforceable, and it only works when every conductor in the raceway is the same size and type.

Number of Conductors & Thermal Limits

Fill and ampacity are two separate tests, and both must pass. 300.17 requires the number and size of conductors to permit heat dissipation and ready installation or withdrawal. Independently, 310.15(C)(1) requires the ampacity adjustment factors (80% for 4–6 current-carrying conductors, 70% for 7–9, 50% for 10–20) whenever more than three current-carrying conductors share a raceway or cable — even when the raceway passes the 40% fill test with room to spare. Note that a neutral carrying only the unbalanced current of a balanced 3-wire or 4-wire circuit is not counted as a current-carrying conductor, but the neutral of a 4-wire wye circuit with a major portion of nonlinear load is.

Busways / Bus Duct — Article 368

Busways are prefabricated assemblies of busbars in an enclosure used to distribute large blocks of power. 368.30 requires busways to be securely supported at intervals not exceeding 5 ft (1.5 m) unless otherwise designed and marked. 368.10 restricts busway to locations where it is not subject to severe physical damage or corrosive vapors, and generally prohibits it outdoors or in wet or damp locations unless identified for such use. 368.17 covers overcurrent protection, including the reduction-in-size rules for busway taps, and 368.56 requires branches from a busway to be made with listed devices.

Inspector Focus

Verify: (1) the raceway or cable type is permitted in that location — EMT is not for severe physical damage, and NM cable is not permitted at all in most commercial construction; (2) supports meet the spacing in the governing article, including the within-3-ft-of-each-box rule for rigid raceways and the within-12-in. rule for flexible raceways and cables; (3) bends between pull points do not exceed 360 degrees; (4) conduit fill does not exceed 40% for more than two conductors, using the correct Table 4 column; (5) the 310.15(C)(1) adjustment factors have been applied where more than three current-carrying conductors share the raceway; and (6) busway is supported at 5 ft or less with listed tap fittings.