How many #12 THHN can I legally pull in 3/4-inch EMT, and how do you figure that if it's a mix of sizes?

Answer:

For 3/4-inch EMT (actual ID ≈ 0.740"), the maximum number of #12 THHN conductors is determined by Table 1, Chapter 9 of the NEC, using the 40% fill rule for more than two conductors.

Step 1: Find Conduit Fill Capacity

• 3/4" EMT allows 40% fill (per Table 1, Chapter 9).
• From Table 4, 3/4" EMT has a total area of 0.598 in².
• 40% of 0.598 = 0.239 in² available for conductor fill.

Step 2: Find Area of #12 THHN

• From Table 5, #12 THHN has an area of 0.0133 in².

Step 3: Calculate Number of Conductors

• 0.239 ÷ 0.0133 ≈ 17.97
• Maximum = 17 #12 THHN conductors

For Mixed Sizes:

Use the same method:

1. Find the area of each conductor size from Table 5.
2. Multiply each by the number of conductors of that size.
3. Add all areas together.
4. Total must not exceed 0.239 in² (for 3/4" EMT at 40% fill).

Example: (10 × 0.0133) + (2 × 0.0211) = 0.133 + 0.0422 = 0.175 in² → OK

Note: If more than 3 current-carrying conductors are installed, derating per Table 310.15(C)(1) may apply. Also, a grounding conductor (if required) counts toward fill but is not current-carrying.

Let me know if you need help with derating or specific wire combinations.

Straight #12 THHN in 3/4" EMT: 16 conductors maximum.

The Calculation (NEC Chapter 9, Tables 4, 5, & 1):

1. Conduit Area (Table 4): 3/4" EMT = 0.213 in² (100% area).
2. Fill Limit (Table 1): >2 conductors = 40% fill. Allowable Area: 0.213 × 0.40 = 0.0852 in²*.
3. Wire Area (Table 5): #12 THHN = 0.0133 in² per conductor.
4. Math: 0.0852 ÷ 0.0133 = 6.4 → 16 conductors (NEC rounds down; Annex C Table C.1 confirms 16).

Mixed Sizes? Calculate Total Cross-Sectional Area:

1. Look up each conductor’s area in Table 5 (THHN/THWN) or Table 5A (compact).
2. Sum the areas of all conductors (including grounds/neutrals).
3. Compare total to the 40% fill limit for that conduit size (Table 4).

Example: (4) #10 + (6) #12 THHN in 3/4" EMT

• #10 THHN: 0.0211 in² × 4 = 0.0844 in²
• #12 THHN: 0.0133 in² × 6 = 0.0798 in²
• Total: 0.1642 in² > 0.0852 in² (Limit) → NO GOOD.

Critical Notes:

• Equipment Grounding Conductors (EGCs) count toward fill (250.118, 300.17).
• Derating (310.15(C)(1)): 10–20 current-carrying conductors = 50% ampacity. 16 #12s (all current-carrying) = 20A × 0.50 = 10A max.
• Nipples ≤ 24" (300.17(B)): 60% fill allowed (24 #12s), but derating still applies.
• Always verify Annex C tables for standard same-size pulls; calculate manually for mixes.