12.5 Conduit & Tubing Fill Calculations (Chapter 9 & Annex C)

What DLI expects

DLI's exam guide includes a full section on conduit fill. It explains that Annex C has tables for each type of conduit and tubing, listing the maximum number of conductors of one size and insulation type, for raceways longer than 24 in. For mixed sizes or insulation types, it walks through a three-step calculation using Chapter 9, Tables 1, 4, and 5.

The Chapter 9 tables

Key notes to Chapter 9 tables

• Equipment grounding and bonding conductors are counted in fill, using their actual dimensions (Table 5 for insulated, Table 8 for bare).
• Nipples not longer than 24 in. between boxes, cabinets, and similar enclosures may be filled to 60%, and the ampacity adjustment factors of Table 310.15(C)(1) do not apply to them.
• A multiconductor cable or flexible cord counts as one conductor, using its overall area; for an elliptical cable, use a circle based on the major diameter.
• When calculating the number of same-size conductors, use the next higher whole number only if the result has a decimal of 0.8 or more. DLI's guide calls this provision Note 8 and says it must be applied if you calculate instead of using Annex C.
• The fill limits apply to complete raceway systems, not to short sections used only to protect cables from physical damage.

Method 1: same size and type (Annex C)

DLI example: how many 10 AWG THWN conductors may be installed in trade size 1½ Schedule 80 PVC?

1. Use Table C.10 (Schedule 80 PVC).
2. Find the THWN section and the 10 AWG row.
3. Read the trade size 1½ column: 32 conductors.

If you calculate instead, divide the 40% area of the raceway (Table 4) by the area of one conductor (Table 5) and apply the 0.8 rounding rule.

Method 2: mixed conductors (Tables 1, 4, and 5)

DLI example: what minimum trade size of rigid metal conduit is needed for six 3 AWG THW, six 6 AWG THWN, one 8 AWG THWN, and twelve 14 AWG XHHW conductors?

1. Percent fill: more than two conductors, so 40% (Table 1).
2. Conductor areas (Table 5):

1. Raceway (Table 4, RMC, 40% column): trade size 1½ allows 0.829 in² (too small), and trade size 2 allows 1.363 in². The answer is trade size 2.

Common Table 5 areas (in²)

Common Table 4 values, 40% fill (in²)

Always use the table for the actual raceway type: Schedule 80 PVC has a thicker wall and much less area than EMT of the same trade size.

Worked example: EMT with an equipment grounding conductor

Size EMT for three 4 AWG THHN conductors and one 8 AWG insulated THHN EGC.

1. Four conductors, so 40%.
2. Areas: $3 \times 0.0824 = 0.2472$ and $1 \times 0.0366 = 0.0366$; total 0.2838 in².
3. EMT 40% column: ¾ = 0.213 (too small); 1 = 0.346 (enough). Use trade size 1 EMT.