Conduit fill, and the smallest size that fits
List the wires going in, ground included. See the fill percentage for any trade size and the smallest EMT, rigid or PVC conduit that stays within the NEC limit.
Count every conductor, including grounding and bonding conductors (Note 3). Areas are Table 5 values for those insulation types; for bare conductors, compact-stranded wire or cables, use the actual dimensions. A cable counts as one conductor at its overall area (Note 9).
| Trade size | Total area | Allowed | Fill |
|---|
Fill = conductor area ÷ the raceway's total internal area (Table 4). For conductors all of one size and type, Note 7 lets a count ending in .8 or more round up, and this calculator applies it; Informative Annex C also lists those same-size maximums by raceway, so check it (and whether your area adopted it) for counts right at the limit. Mixed sizes use the exact areas. An estimate: local amendments and the inspector (AHJ) govern.
Add up the cross-sectional areas of every conductor from NEC Chapter 9 Table 5, including the ground, and divide by the conduit's internal area from Table 4. More than two wires may fill 40%, two wires 31% and one wire 53%. Four 12 AWG THHN in 1/2 in. EMT are 0.0532 in² of 0.304 in², or 17.5%.
How to use the conduit fill calculator
Pick the raceway
EMT, rigid metal conduit, or PVC Schedule 40 or 80, and the trade size you plan to use.
List the conductors
Up to four rows of count, insulation type and size, including grounding conductors.
Read the fill
Fill % in your size, the allowed area, the smallest size that fits and the most of one wire it holds.
What the conduit fill calculator does
Chapter 9 Table 1 limits
53% for one conductor, 31% for two, 40% for more than two, and 60% for nipples of 24 in. or less.
Table 4 areas
Total internal area of EMT, RMC and PVC Schedule 40 and 80, trade sizes 1/2 to 6 (EMT to 4).
Table 5 wire areas
THHN, THWN and THWN-2, and XHHW and XHHW-2, 14 AWG to 1000 kcmil.
Mixed sizes
Up to four kinds of wire at once, added up by area.
Note 7 rounding
For one size and type of wire, a count ending in .8 or more rounds up, as the code allows.
- Fill limits
- NEC 2023 Chapter 9 Table 1: 1 wire 53%, 2 wires 31%, over 2 wires 40%; nipples 60% (Note 4)
- Raceway areas
- Chapter 9 Table 4 total area (100%) for EMT, RMC, PVC Schedule 40 and Schedule 80
- Conductor areas
- Chapter 9 Table 5, THHN/THWN/THWN-2 and XHHW/XHHW-2
- Same-size shortcut
- Informative Annex C lists maximum counts of one conductor type and size
- Not included
- IMC, flexible and liquidtight conduit, bare and compact conductors, cables, jam ratio
Conduit Fill Calculator FAQ
How do you calculate conduit fill?
Look up each conductor's area in NEC Chapter 9 Table 5, multiply by how many there are, and add them. Divide by the conduit's total area in Table 4. The result must not exceed 40% for three or more conductors, 31% for two or 53% for one.
Does the ground wire count in conduit fill?
Yes. Note 3 to the Chapter 9 tables says equipment grounding and bonding conductors, insulated or bare, are included, using their actual dimensions.
How many 12 AWG THHN fit in 1/2 inch EMT?
Nine. 40% of 1/2 in. EMT is 0.1216 in², and each 12 AWG THHN is 0.0133 in², which allows 9.1 wires. In 3/4 in. EMT the same wire fits 16.
What is Annex C?
Informative Annex C in the NEC lists the maximum number of conductors of one type and size for each trade size of each raceway. It's a shortcut for same-size wire; mixed sizes need the area calculation. Annexes are informative unless your jurisdiction adopts them.
Why does Schedule 80 PVC hold fewer wires?
Its walls are thicker, so the inside is smaller. 1 in. Schedule 80 has 0.688 in² of total area against 0.832 in² for Schedule 40, so the same wires can need the next trade size up.
Does conduit fill change the wire's ampacity?
Fill doesn't, but the number of current-carrying conductors does: more than three in a raceway means derating under Table 310.15(C)(1). The wire size calculator applies that.
Where the numbers come from
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