EMT conduit fill table
EMT (Electrical Metallic Tubing) conduit fill establishes the maximum number of conductors that can be placed in a thin-wall metal tube, ensuring safety, thermal dissipation and ease of installation. The calculation basis is Chapter 9 of the NEC (NFPA 70), which defines cross-sectional areas, fill factors and ampacity adjustments. The typical fill table for THHN/THWN conductors, with a fill factor of 40 % (more than two conductors), is the most commonly used practical reference on site.
EMT conduit dimensions
Section titled “EMT conduit dimensions”The dimensions of electrical metallic tubing (EMT) define the internal capacity to accommodate conductors. Standard commercial diameters in inches and their metric equivalents are used.
| Trade size (in / metric) | Outside diameter (mm / in) | Inside diameter (mm / in) | Internal cross-sectional area (mm² / in²) |
|---|---|---|---|
| 1/2“ (16 mm) | 17,9 / 0.706 | 15,8 / 0.622 | 196 / 0.304 |
| 3/4“ (21 mm) | 23,4 / 0.922 | 20,9 / 0.824 | 343 / 0.532 |
| 1“ (27 mm) | 29,5 / 1.163 | 26,6 / 1.05 | 555 / 0.860 |
| 1-1/4“ (35 mm) | 38,4 / 1.510 | 35,1 / 1.38 | 967 / 1.50 |
| 1-1/2“ (41 mm) | 44,2 / 1.740 | 40,9 / 1.61 | 1313 / 2.03 |
| 2“ (53 mm) | 55,8 / 2.197 | 52,5 / 2.07 | 2165 / 3.36 |
| 2-1/2“ (63 mm) | 73,0 / 2.875 | 69,3 / 2.73 | 3771 / 5.85 |
| 3“ (78 mm) | 88,9 / 3.500 | 85,2 / 3.36 | 5701 / 8.84 |
| 3-1/2“ (91 mm) | 101,6 / 4.000 | 97,4 / 3.83 | 7451 / 11.55 |
| 4“ (103 mm) | 114,3 / 4.500 | 110,0 / 4.33 | 9503 / 14.73 |
Conductor fill table (EMT)
Section titled “Conductor fill table (EMT)”Maximum number of copper conductors with THHN/THWN insulation allowed in an EMT conduit, applying a fill factor of 40 % (more than two conductors). Reference values per NEC, Chapter 9, Table C.1.
| Trade size (in / mm) | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG | 1/0 AWG | 2/0 AWG | 4/0 AWG | 250 kcmil | 350 kcmil | 500 kcmil |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1/2“ (16 mm) | 11 | 9 | 5 | 2 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 3/4“ (21 mm) | 21 | 16 | 10 | 5 | 3 | 2 | 1 | 1 | 0 | 0 | 0 | 0 | 0 |
| 1“ (27 mm) | 34 | 26 | 16 | 9 | 6 | 4 | 2 | 1 | 1 | 0 | 0 | 0 | 0 |
| 1-1/4“ (35 mm) | 54 | 42 | 26 | 14 | 10 | 7 | 4 | 3 | 2 | 1 | 1 | 0 | 0 |
| 1-1/2“ (41 mm) | 73 | 57 | 36 | 19 | 14 | 9 | 5 | 4 | 3 | 2 | 1 | 1 | 0 |
| 2“ (53 mm) | 112 | 88 | 55 | 30 | 22 | 14 | 8 | 7 | 5 | 3 | 2 | 1 | 1 |
| 2-1/2“ (63 mm) | 179 | 140 | 88 | 48 | 35 | 23 | 13 | 11 | 8 | 5 | 4 | 3 | 2 |
| 3“ (78 mm) | 273 | 213 | 134 | 73 | 53 | 35 | 20 | 17 | 13 | 8 | 6 | 5 | 3 |
| 3-1/2“ (91 mm) | 357 | 279 | 175 | 96 | 70 | 46 | 26 | 22 | 17 | 10 | 8 | 6 | 4 |
| 4“ (103 mm) | 472 | 369 | 232 | 127 | 92 | 61 | 35 | 29 | 22 | 14 | 11 | 8 | 6 |
Cross-sectional area formula
Section titled “Cross-sectional area formula”The fill calculation is based on the sum of the conductor areas compared to the usable cross-sectional area of the conduit, multiplied by the corresponding fill factor.
A_total = Σ (nᵢ × aᵢ) ≤ A_allowable = FR × A_interior
| Variable | Description | Unit |
|---|---|---|
| A_total | Total area occupied by conductors | mm² |
| nᵢ | Number of conductors of size i | – |
| aᵢ | Cross-sectional area of one conductor (including insulation) of size i | mm² |
| A_allowable | Maximum area allowed for the set of conductors | mm² |
| FR | Fill factor (0.53; 0.31 or 0.40 depending on number of conductors) | – |
| A_interior | Internal cross-sectional area of EMT conduit | mm² |
Fill factors per NEC
Section titled “Fill factors per NEC”The NEC defines three fill factors based on the number of conductors sharing the conduit.
| Number of conductors | Fill factor (FR) | Percentage of interior area |
|---|---|---|
| 1 | 0.53 | 53 % |
| 2 | 0.31 | 31 % |
| 3 or more | 0.40 | 40 % |
Ampacity adjustment factors
Section titled “Ampacity adjustment factors”When multiple current-carrying conductors are installed in the same conduit, the current-carrying capacity of each conductor must be reduced to avoid overheating, per NEC Table 310.15(C)(1).
| Number of current-carrying conductors | Adjustment factor (ampacity multiplier) |
|---|---|
| 1 – 3 | 1.00 |
| 4 – 6 | 0.80 |
| 7 – 9 | 0.70 |
| 10 – 20 | 0.50 |
| 21 – 30 | 0.45 |
| 31 – 40 | 0.40 |
| 41 and more | 0.35 |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum number of 12 AWG THHN conductors allowed in a 1/2 inch EMT?
Section titled “What is the maximum number of 12 AWG THHN conductors allowed in a 1/2 inch EMT?”The maximum is 9 conductors, applying the 40% fill factor.
What percentage of the interior area is used for three or more conductors in the same EMT conduit?
Section titled “What percentage of the interior area is used for three or more conductors in the same EMT conduit?”A maximum occupancy of 40% of the interior area is allowed, equivalent to a factor of 0.40.
How many 10 AWG THHN conductors can be installed in a 2 inch EMT?
Section titled “How many 10 AWG THHN conductors can be installed in a 2 inch EMT?”Up to 55 10 AWG conductors can be installed, according to the fill table with the 40% factor.
What is the inside diameter of a 1 inch EMT conduit?
Section titled “What is the inside diameter of a 1 inch EMT conduit?”The inside diameter is 1.05 inches / 26.6 mm, which provides an interior area of 0.860 in² / 555 mm².
How does having 9 current-carrying conductors affect ampacity?
Section titled “How does having 9 current-carrying conductors affect ampacity?”With 9 current-carrying conductors, the ampacity of each conductor is multiplied by 0.70, meaning it is reduced by 30% from the nominal value.
How many 4/0 AWG conductors fit maximum in a 3 inch EMT?
Section titled “How many 4/0 AWG conductors fit maximum in a 3 inch EMT?”The maximum allowed is 8 4/0 AWG conductors, considering 40% fill and THHN/THWN insulation.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/conduit-size-d_1738.html
- southwire.com: https://www.southwire.com/calculator-conduit