Conduit fill calculator
Add the conductors going into the raceway to get the fill percentage, the limit that applies, and the smallest trade size that fits.
Inputs
Result
Maximum 12 AWG THHN conductors by trade size
For the conduit type selected and the first wire size in your list, all conductors the same size.
| Trade size | Max conductors |
|---|
How it works
Conduit fill compares the total cross-section of the conductors with the inside area of the raceway. The limits come from NEC Chapter 9, Table 1:
| Number of conductors | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| Over 2 | 40% |
Conductor areas come from Chapter 9, Table 5, for THHN, THWN and THWN-2 insulation. Raceway areas come from Table 4 for the type and trade size you choose. When every conductor is the same size, Chapter 9, Note 7 allows the count to round up to the next whole number if the calculation lands at 0.8 or more, which is how the tables in Annex C are built.
What this calculator does not cover
- Other insulation types, such as XHHW, RHW and compact conductors, have different areas.
- Nipples of 24 in. or less may be filled to 60%, under Chapter 9, Note 4.
- Cables, bare conductors and multiconductor assemblies are calculated differently.
- Trade sizes 2-1/2 and 3-1/2 are listed for EMT only, until their published areas for rigid and PVC are confirmed.
Questions
How many 12 AWG wires fit in 1/2 inch EMT?
Nine 12 AWG THHN conductors fit in 1/2 in. EMT at 40% fill. In 3/4 in. EMT the limit is 16, and in 1 in. EMT it is 26.
Do ground wires count toward conduit fill?
Yes. Equipment grounding and bonding conductors count toward fill and are included in the conductor count.
Why is the limit 31% for two conductors?
Two round conductors can jam against each other inside a raceway more easily than one or three, so Table 1 sets a lower limit of 31% for exactly two.
How many 14 AWG wires fit in 3/4 EMT?
Twenty-two 14 AWG THHN conductors. The calculation gives 21.98, and Chapter 9, Note 7 allows rounding up when the decimal is 0.8 or more.
Can I mix wire sizes?
Yes. Add a row for each size. With mixed sizes the total area must stay within the limit, and the Note 7 round-up does not apply.
More Electrical calculators
Voltage drop calculator
Drop in volts and percent for copper or aluminum, single-phase, three-phase or DC.
Electrical load calculator
Dwelling service size by the NEC 220.82 optional method, with every demand factor shown.
Wire size & ampacity calculator
Conductor size and breaker from the load, with ambient correction and bundling adjustment applied.