Conduit Fill Calculation: Using NEC Chapter 9 Tables 1, 4 and 5

With three or more wires, conduit fill can't exceed 40% of the conduit's inside area. One wire can fill 53%, and two wires 31%. For a conduit fill calculation, add wire areas from NEC Chapter 9 Table 5 and compare them to the allowed area in Table 4. If every wire is the same size and type, the Annex C charts give you the count without the math.

Conduit fill calculator

Pick the raceway, then enter each wire size and type, including the ground. The calculator applies the right fill limit and finds the smallest trade size that works.

Chapter 9 Table 1: the notes that change the answer

The fill percentage depends only on how many conductors or cables are in the raceway. For mixed sizes, use Table 5 (or 5A) for the wires and Table 4 for the conduit (Note 6). Other notes that change the result:

• Note 2: Table 1 doesn't apply to a section of conduit used only to protect exposed wiring or cable from damage.
• Note 3: equipment grounding and bonding conductors count, at their actual size, insulated or bare.
• Note 4: a nipple 24 inches or shorter between boxes or cabinets can be filled to 60%. Ampacity adjustment for more than three current-carrying wires doesn't apply in it.
• Note 7: when all wires are the same size, a decimal of 0.8 or more rounds up to the next whole wire. For a single wire, 0.8 or more means one wire is allowed.
• Note 9: a multiconductor cable counts as one conductor. An oval cable is figured as a circle using its widest dimension.

Conduit fill calculation, worked example

The job: four 6 AWG THHN conductors and one 10 AWG THHN equipment ground in EMT. What's the smallest EMT that works?

1. Wire areas (Table 5): 6 AWG THHN is 0.0507 in². 10 AWG THHN is 0.0211 in².
2. Add them: 4 × 0.0507 = 0.2028, plus the ground: 0.2028 + 0.0211 = 0.2239 in².
3. Fill column: five wires is over two, so 40%.
4. Table 4, EMT, 40% column: ¾-inch allows 0.213 in², too small. 1-inch allows 0.346 in², which works.

Answer: 1-inch EMT. Without the ground, 0.2028 in² would fit in ¾-inch. Even a bare stranded 10 AWG ground (0.011 in² from Table 8) makes it 0.2138 in², just over 0.213.

Conduit fill chart: same-size wires with Annex C

When every wire is the same size and type, the tables in Informative Annex C list the maximum count for each raceway and trade size. Enter one wire row in the calculator above, and it runs the same math.

Two checks with Tables 4 and 5:

• 12 AWG THHN in ¾-inch EMT: 0.213 ÷ 0.0133 = 16.02, so 16.
• 12 AWG THHN in 1¼-inch EMT: 0.598 ÷ 0.0133 = 44.96. The decimal is over 0.8, so Note 7 rounds up to 45.

Mixed sizes always need the full calculation. Compact-stranded wire has its own tables, such as Table C.1(A).

Common conduit fill exam traps

• Leaving out the ground, or using 40% for two wires (they get 31%).
• Using the 100% column of Table 4. That's the full area, not the allowed fill.
• Using the wrong raceway. EMT and Schedule 80 PVC of the same trade size have different areas.
• Forgetting ampacity. Fill can pass while more than three current-carrying wires still need derating. See JCR's ampacity adjustment guide.

The pull box at the end of the run has its own rule, 314.28. JCR's angle pull box question walks through it.

Conduit fill questions

How many 12 AWG THHN wires can go in ¾ EMT?

Sixteen. ¾-inch EMT allows 0.213 in² at 40%, each 12 AWG THHN is 0.0133 in², and Table C.1 lists 16. That count includes any 12 AWG THHN ground in the raceway.

Does the ground wire count in conduit fill?

Yes. Note 3 to Chapter 9 Table 1 includes equipment grounding and bonding conductors. Use the ground's actual size, from Table 5 if insulated or Table 8 if bare.

What is the 40 percent rule for conduit?

It's the Chapter 9 Table 1 limit for three or more conductors or cables. Their combined area can't exceed 40% of the conduit's inside area, from the 40% column of Table 4. One wire gets 53% and two get 31%.

What part of the NEC covers conduit fill for same-size wires?

Informative Annex C. Its tables list the maximum number of conductors of one size and type for each raceway type and trade size.

Do I have to use Annex C for conduit fill?

No. Annex C is informational, built from Chapter 9 Tables 1, 4 and 5, so both methods give the same answer. It's faster when every wire is the same size and type.

What fill percentage applies to one multiconductor cable?

53%. Note 9 to Chapter 9 Table 1 treats a multiconductor cable as a single conductor, so one cable gets the one-wire limit.

Related guides: Ampacity derating step by step · Pull box sizing for an angle pull (314.28) · 200 amp service wire size (Table 310.12)

More worked examples: voltage drop calculation, box fill calculation (314.16) and all NEC Answers.