5.6 Conduit Fill: Chapter 9 Tables and Annex C

Why Fill Is Limited

Raceway fill limits exist for two reasons: heat dissipation and pulling tension. A raceway packed with conductors runs hotter and requires more force to pull, damaging insulation. The percentages in Chapter 9 Table 1 are derived from an assumption of a jam ratio and pulling geometry, not from any thermal calculation — the thermal side is handled separately by the 310.15(C)(1) adjustment factors.

Chapter 9, Table 1 — Percent of Cross Section

The two-conductor case being the tightest looks counterintuitive but comes from the geometry of two circles inside a circle — the wasted space is greatest with two.

The Notes to Table 1

The notes are as enforceable as the table:

• Note 1. See Informative Annex C for the maximum number of conductors and fixture wires, all of the same size and insulation type, permitted in trade sizes of the applicable conduit or tubing.
• Note 2. Table 1 applies only to complete conduit or tubing systems and is not intended to apply to sections used to protect exposed wiring from physical damage.
• Note 4. Where conduit or tubing nipples having a maximum length not to exceed 600 mm (24 in.) are installed between boxes, cabinets, and similar enclosures, the nipples shall be permitted to be filled to 60 percent of their total cross-sectional area, and 310.15(C)(1) adjustment factors need not apply to this condition.
• Note 6. For conductors not included in Chapter 9, such as multiconductor cables and optical fiber cables, the actual dimensions shall be used.
• Note 7. When calculating the maximum number of conductors permitted in a conduit or tubing, all of the same size (total cross-sectional area including insulation), the next higher whole number shall be used to determine the number of conductors permitted when the calculation results in a decimal of 0.8 or larger.
• Note 8. Where bare conductors are permitted, the dimensions for bare conductors in Table 8 shall be used.
• Note 9. A multiconductor cable or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.

Note 9 is a favorite exam item: a single 12/3 MC cable pulled through a conduit is one conductor for fill, using the major diameter of the ellipse.

The Two-Table Method

For conductors of mixed sizes or types, work in three steps:

1. From Chapter 9 TABLE 5, look up the approximate area (sq in)
   of each conductor by insulation type and size. Multiply by quantity.
2. Sum all conductor areas, including any equipment grounding conductor.
3. From Chapter 9 TABLE 4, find the raceway type and read down the
   column for the applicable fill percentage (usually the Over 2 Wires 40%
   column) until you find a trade size whose area equals or exceeds your sum.

Table 5 Values to Recognize

Approximate area in square inches for THHN/THWN/THWN-2:

Note that XHHW and THW have larger areas than THHN at the same AWG because the insulation is thicker. Always match the insulation type in the question to the correct Table 5 column — substituting THHN areas into an XHHW problem is the most common error.

Table 4 Values to Recognize — EMT, 40 Percent Column

Each raceway type has its own Table 4 section. EMT, RMC, IMC, PVC Schedule 40, and PVC Schedule 80 all have different interior areas at the same trade size — Schedule 80 is noticeably smaller because of its thicker wall.

Worked Example 1 — Mixed Sizes

How large an EMT is needed for three 2 AWG THHN copper conductors, one 4 AWG THHN neutral, and one 8 AWG THHN equipment grounding conductor?

3 x 2 AWG THHN  @ 0.1158 = 0.3474 sq in
1 x 4 AWG THHN  @ 0.0824 = 0.0824 sq in
1 x 8 AWG THHN  @ 0.0366 = 0.0366 sq in
                           ------------
Total conductor area       0.4664 sq in

Five conductors means "over 2" and therefore the 40 percent column. From Table 4, EMT:

• 1-1/4 in at 0.598 sq in — 0.4664 fits.
• 1 in at 0.346 sq in — too small.

Answer: trade size 1-1/4 EMT. Note that the equipment grounding conductor is counted in the fill calculation even though it is not counted as a current-carrying conductor for ampacity adjustment. Those two counts are different and both appear on exams.

Worked Example 2 — Same Size, Use Annex C

How many 12 AWG THHN conductors are permitted in trade size 3/4 EMT?

All conductors are the same size and type, so Informative Annex C, Table C.1 applies directly: 16 conductors.

Verify by the long method:

Table 4, 3/4 EMT, 40 percent = 0.213 sq in
Table 5, 12 AWG THHN         = 0.0133 sq in
0.213 / 0.0133 = 16.02 conductors

16.02 truncates to 16. Note that Note 7's round-up applies only at 0.8 or larger — 16.02 does not round to 17.

A case where Note 7 does apply: if the calculation had produced 16.85, the answer would be 17 conductors.

Worked Example 3 — Nipple

Eight 6 AWG THHN conductors pass through an 18-inch RMC nipple between two enclosures. What trade size is required?

8 x 6 AWG THHN @ 0.0507 = 0.4056 sq in
Note 4 permits 60 percent fill for a nipple 24 in or less.
Table 4, RMC, 60 percent column:
   trade size 1     = 0.531 sq in   -> 0.4056 fits
   trade size 3/4   = 0.307 sq in   -> too small

Trade size 1 RMC. And under Note 4, the eight conductors need no ampacity adjustment even though eight exceeds three.

Multiconductor Cables in Raceway

Under Note 9, treat each cable as a single conductor, using its overall diameter. For an elliptical cable, use the major diameter as the circle diameter and compute the area as pi times radius squared. Because you are computing the area of a circle enclosing a flat cable, the number is conservative — that is deliberate.

With one cable in the raceway you may use the 53 percent column; with two, the 31 percent column; with three or more, 40 percent.

What Annex C Cannot Do

Annex C is informative under 90.5(D), and its tables are precomputed for the case where all conductors are the same size and insulation type. It cannot be used where:

• conductor sizes are mixed;
• insulation types are mixed;
• multiconductor cables are being pulled;
• the raceway is a nipple being filled to 60 percent;
• bare conductors using Table 8 dimensions are present in the count.

In all of those cases, return to Tables 1, 4, 5, and 8.

Existing Raceways and Physical Damage Sleeves

Table 1, Note 2 exempts short sections of raceway used solely to protect exposed wiring from physical damage from the fill limits. A sleeve over NM cable where it passes through a garage wall is not a complete conduit system and is not subject to the percentages — though the sleeve must still be sized so the cable is not damaged during installation.