Use this free conduit fill calculator to find the smallest conduit that can legally hold your conductors under the National Electrical Code (NEC). Add one or more wire sizes, choose the raceway type, and the calculator applies the fill limits of NEC Chapter 9, Table 1 using the conductor areas in Table 5 and conduit areas in Table 4.
How conduit fill is calculated
- Add up the conductor areas. NEC Chapter 9, Table 5 lists the cross-sectional area of each insulated conductor, for example 0.0133 in² for a 12 AWG THHN wire.
- Count every conductor, including equipment grounding conductors (Chapter 9, Note 3).
- Apply the fill limit from Table 1: 53% for one conductor, 31% for two, and 40% for three or more.
- Compare with the conduit area in Table 4 and pick the smallest size whose allowable area is at least the total wire area.
| Number of conductors | Maximum fill (NEC Chapter 9, Table 1) |
|---|---|
| 1 | 53 % |
| 2 | 31 % |
| 3 or more | 40 % |
| Nipple 24 in. or shorter | 60 % (Note 4) |
Worked examples
Example 1: six 12 AWG THHN plus one 12 AWG ground in EMT
Total area = 7 × 0.0133 = 0.0931 in². The 40% allowable area of 1/2″ EMT is 0.304 × 0.40 = 0.122 in², so 1/2″ EMT is enough (76% of the allowable fill).
Example 2: 225 A feeder, four 4/0 AWG THHN plus one 4 AWG THHN ground
Total area = 4 × 0.3237 + 0.0824 = 1.377 in². 2″ EMT allows 1.342 in² (too small), while 2-1/2″ EMT allows 2.343 in², so 2-1/2″ is the minimum.
Maximum number of THHN wires in EMT
When all conductors are the same size, NEC Annex C gives the maximum count directly. These match the calculator:
| EMT size | 14 AWG THHN | 12 AWG THHN | 10 AWG THHN |
|---|---|---|---|
| 1/2″ | 12 | 9 | 5 |
| 3/4″ | 22 | 16 | 10 |
| 1″ | 35 | 26 | 16 |
Remember that conduit fill is only one check. More than three current-carrying conductors in a raceway also require ampacity adjustment under NEC 310.15(C)(1), so size your wires with our wire size calculator as well.
Frequently asked questions
How many 12 AWG wires can go in a 3/4 inch EMT?
Up to 16 conductors of 12 AWG THHN/THWN-2 fit in 3/4-inch EMT at the 40% fill limit (NEC Annex C, Table C.1). Ground wires count toward the total.
What is the 40% conduit fill rule?
NEC Chapter 9, Table 1 limits the total cross-sectional area of three or more conductors to 40% of the conduit internal area. This leaves space to pull wires without damaging insulation and allows heat to dissipate.
Do ground wires count in conduit fill?
Yes. Equipment grounding and bonding conductors must be included in the conduit fill calculation (NEC Chapter 9, Note 3).
Is PVC conduit fill different from EMT?
Yes. The fill percentage is the same, but PVC has thicker walls and a smaller internal area than EMT of the same trade size, so it holds fewer wires. Schedule 80 PVC holds fewer still.
What if my wires are XHHW instead of THHN?
XHHW insulation is slightly thicker in small sizes, so each wire takes more space. Choose XHHW/XHHW-2 in the calculator to use the correct areas from NEC Chapter 9, Table 5.
Related calculators and guides
These calculators give engineering estimates for planning and learning. Final designs must follow your local code (NEC, IEC/BS 7671, PEC or the authority having jurisdiction) and be checked by a qualified electrical engineer or licensed electrician.
