A conduit fill calculator for the NEC, with mixed wire sizes
This conduit fill calculator adds up the cross-sectional area of every wire you plan to pull, compares it with the inside area of the conduit, and tells you whether the run meets the fill limits of the US National Electrical Code (NEC) Chapter 9. Mix any number of sizes and insulation types: THHN, XHHW, THW or cables you measure yourself. It also lists every trade size of the chosen conduit and picks the smallest one that works.
It covers EMT, IMC, rigid metal conduit and PVC Schedule 40 and 80, using the internal areas of NEC Chapter 9 Table 4 and the conductor areas of Table 5 (2023 edition). For any other raceway, enter its inside diameter from the manufacturer’s data sheet. Everything runs in your browser.
Conduit fill is a code minimum, not the whole design: ampacity derating for bundled conductors, pulling tension and jamming are separate checks. Check the code edition your jurisdiction has adopted and ask your inspector when in doubt.
How to use the conduit fill calculator
- Choose the conduit type and trade size, or choose Other raceway and enter its inside diameter.
- For each group of identical wires, pick the insulation type and size and enter the quantity. Press Add wire for more groups and include the equipment grounding conductor.
- For a multiconductor cable or a wire not listed, choose Cable or other wire and enter its outside diameter or area from the data sheet.
- Tick Nipple 24 in or shorter for a short nipple between enclosures, which may be filled to 60%.
- Read the percent fill, the limit that applies and the trade-size table; Copy result puts it in your notes.
Conduit Fill Calculator features
Mixed wire sizes
Up to 12 rows of different sizes and insulation types, summed exactly as the NEC requires.
Right limit automatically
53% for one conductor, 31% for two, 40% for three or more, or 60% for a short nipple.
Smallest conduit that fits
Every trade size of the chosen conduit with its fill, so you can see the next size up at a glance.
Same-size maximum
How many of the first wire alone would fit, with the 0.8 round-up allowed for identical conductors.
Inches and millimetres
Areas in in² with mm² alongside, metric designators (16, 21, 27 …) and custom sizes in mm.
How conduit fill is calculated
Add the area of each conductor from Chapter 9 Table 5 (insulation included), then divide by the total inside area of the raceway from Table 4. The result must not exceed the Table 1 percentage for the number of conductors. Two conductors get the lowest limit, 31%, because two round wires can wedge across the conduit during a pull.
Example: three 8 AWG THHN (0.0366 in² each) and one 10 AWG THHN ground (0.0211 in²) total 0.1309 in². A 3/4 in EMT has 0.533 in², so the fill is 24.6%, under the 40% limit of 0.2132 in². A 1/2 in EMT allows only 0.1216 in², so 3/4 in is the smallest EMT that works.
| Conductors in the raceway | Maximum fill | Typical use |
|---|---|---|
| 1 | 53% | A single large feeder or one cable |
| 2 | 31% | Two conductors or two cables |
| 3 or more | 40% | Most branch circuits and feeders |
| Nipple ≤ 24 in | 60% | Short nipple between enclosures |
Rules that catch people out
- Equipment grounding and bonding conductors count toward fill, both in the number of conductors and in the area.
- A multiconductor cable counts as one conductor, with its area taken from the outside diameter.
- The round-up rule (use the next whole number when a count ends in .8 or more) applies only when all conductors are the same size, as in the NEC Annex C tables.
- Insulation matters: 12 AWG THHN is 0.0133 in² but 12 AWG THW is 0.0181 in², so the same conduit holds fewer THW wires.
- Compact-stranded conductors (Table 5A) and bare conductors are smaller; enter their area from the data sheet as an other wire.
Related calculators
Pick the conductor size first with the wire gauge calculator (ampacity and voltage drop), total the circuit with the electrical load calculator, or convert watts to amps with the watt calculator.
Conduit Fill Calculator FAQ
How do I use a conduit fill calculator for multiple wire sizes?
Enter one row per group of identical wires with its type, size and quantity. The calculator adds all the areas, counts every conductor to pick the 53, 31 or 40% limit, and compares the total with the conduit’s inside area.
What is the NEC conduit fill limit?
NEC Chapter 9 Table 1 allows 53% of the conduit’s inside area for one conductor, 31% for two and 40% for more than two. A nipple 24 in or shorter between enclosures may be filled to 60%.
How many 12 AWG THHN wires fit in 1/2 in EMT?
Nine. 40% of 0.304 in² is 0.1216 in²; divided by 0.0133 in² that is 9.14, which rounds down because the decimal is below 0.8.
Does the ground wire count in conduit fill?
Yes. Equipment grounding conductors count in the number of conductors and their area is added to the total, insulated or bare.
Which NEC edition does this use?
The fill rules and dimensions of NEC (NFPA 70) 2023 Chapter 9, cross-checked against an ICC reference and an engineering course. Jurisdictions adopt editions at different times, so check yours.
Can I use this for IEC or BS 7671 installations?
Not directly. UK and IEC practice uses cable and conduit factors from the On-Site Guide rather than the NEC percentages. You can still enter cable diameters and a conduit inside diameter to see the percentage of the bore that is filled.