Conduit and Tubing Fill: Chapter 9 Tables 1, 4, 5 and 8 and the Nipple Rule

Conduit fill is the calculation the Wiring Methods area is built around: two calculation items on the Journeyman and Master exams and a regular Residential Wireman question. The exam is open book, so the skill is knowing which table, which column, and which rounding rule — fast. Written from the 2026 text of Chapter 9.

1. The rule — Table 1

Table 1 — maximum percent of the raceway's cross-sectional area that conductors and cables may occupy:

Why 31% for two? Two round conductors side by side waste the space between them — the code is deliberately conservative there.

Notes to Table 1 that get tested:

1. Annex C tables give the maximum number of same-size conductors per trade size — use them when every conductor is the same type and size.
2. Table 1 applies to complete conduit or tubing systems, not to short sleeves used only to protect exposed wiring or cable from damage.
3. Equipment grounding and bonding conductors count in the fill, using their actual dimensions (insulated or bare).
4. Nipples: a conduit or tubing nipple not over 24 in. long between boxes, cabinets or similar enclosures may be filled to 60 percent, and the 310.15(C)(1) adjustment factors do not apply.
5. Conductors not in Chapter 9 (multiconductor cables, optical fiber cables) — use actual dimensions.
6. Mixed sizes — use actual dimensions or Table 5 / 5A for the conductors and Table 4 for the raceway.
7. Rounding, same-size conductors: if the calculated count has a decimal of 0.8 or more, round UP to the next whole number. Below 0.8, round down. (Sizing a raceway for a single conductor or cable: a result of 0.8 or more permits one.)
8. Bare conductors — Table 8 dimensions.
9. A multiconductor cable, optical fiber cable or flexible cord of two or more conductors is treated as a single conductor; an elliptical cable uses its major diameter as the diameter of a circle. Assemblies of single insulated conductors without an overall covering are counted as individual