Conduit Fill Calculator (NEC)
Check NEC conduit fill for up to six groups of insulated wires. Select the conduit type and trade size, then enter each wire group’s insulation, size, and quantity.
Check Conduit Fill for Your Wire Schedule
Conduit Fill Formula
Where:
- \(A_{\mathrm{w}}\) — combined area of all wires (in²)
- \(n_i\) — quantity of wires in row \(i\)
- \(a_i\) — area of one wire in row \(i\), including insulation (in²)
- \(A_{\mathrm{c}}\) — internal area of the selected conduit (in²)
- \(F\) — percentage of conduit area occupied by wires
The calculator sums the areas of the entered insulated conductors, then divides by the conduit’s internal area. It compares the result with the NEC Chapter 9, Table 1 fill limit for the total number of wires: 53% for one, 31% for two, or 40% for three or more.
Conduit Fill – Calculation Example
Example: the default entries are three 12 AWG THHN/THWN-2 wires in 1/2 in EMT. Each wire occupies 0.0133 in², and the EMT internal area is 0.304 in².
\[A_{\mathrm{w}}=3(0.0133)=0.0399\ \mathrm{in}^2\]
\[F=\frac{0.0399}{0.304}\times100\%=13.13\%\]
Result: 13.13% fill is below the 40% limit, so the selected conduit passes. The smallest passing EMT trade size in the calculator is 1/2 in; its maximum capacity for identical 12 AWG THHN/THWN-2 wires is nine.
How to use the calculator
- Choose Conduit type—EMT, RMC, PVC Schedule 40, PVC Schedule 80, or FMC—and select the proposed Trade size.
- Under Wire row 1, choose the wire’s Insulation and Wire size, then enter its Quantity as a whole number.
- Use Wire row 2 through Wire row 6 for other insulation-and-size combinations. Leave an unused row’s Quantity at zero. Enter at least one wire overall.
- Read Total Wire Area, Conduit Internal Area, and Conduit Fill. Compare the fill with Maximum Allowed Fill and check Fill Check for PASS or FAIL.
- If needed, review Smallest Passing Trade Size for the same conduit type. Read Maximum Identical Wires and Calculation Details to see how the separate identical-wire capacity was determined.
What conduit fill tells you
Conduit fill is the portion of a raceway’s internal cross-sectional area occupied by wires. The area used for an insulated conductor includes its insulation, not just its metal core. For a mixed schedule, each row contributes its per-wire area multiplied by its quantity. The calculator adds those contributions before determining the fill percentage.
Under the ordinary NEC Chapter 9, Table 1 limits used here, the permitted percentage depends on the total number of wires, not on the number of distinct wire sizes or rows. For example, three wires entered together in one row and three wires entered across separate rows both use the 40% limit. A PASS means the entered wire area does not exceed that limit for the selected conduit area; it is not an approval of the entire electrical installation.
Smallest Passing Trade Size checks the same wire schedule against the listed sizes of the selected conduit type, from 1/2 in through 4 in. If none passes, the result says “None through 4 in.” Maximum Identical Wires answers a different question: how many copies of the first wire row with a nonzero quantity fit in the selected conduit. It checks the one-, two-, and three-or-more-wire limits separately. It is not additional capacity available alongside a mixed schedule.
Assumptions and practical checks
- The calculation uses built-in conduit internal areas and insulated-wire areas for the listed selections. Confirm the applicable values against the NEC edition adopted in your jurisdiction and the actual products specified for the job.
- It assumes individual insulated conductors in an ordinary raceway run. It does not evaluate multiconductor cable assemblies, bare conductors, or special fill provisions such as those that may apply to short nipples.
- Count every conductor that occupies the raceway, including insulated equipment grounding conductors. A zero-quantity row contributes no area and does not affect the wire count.
- Fill is only one design check. Conductor ampacity adjustment, temperature correction, raceway installation requirements, and pulling conditions may require separate review. If you are selecting conductors, use the wire gauge calculator as a related reference; check the circuit with the circuit breaker size calculator and assess run length with the voltage drop calculator.
Frequently Asked Questions
How do I check NEC conduit fill by hand?
Look up the internal area for the exact raceway type and trade size in NEC Chapter 9, Table 4. Find each insulated conductor’s area in the applicable conductor table, such as Table 5 or 5A, and multiply it by the number of those conductors. Add the areas and compare the total with the permitted area from Table 1. Use the unrounded areas for the comparison.
Is 40% conduit fill a good target or just a maximum?
For three or more conductors, 40% is the ordinary NEC maximum, not a recommended design target. A lower fill can make pulling easier, especially on long runs or runs with bends. A raceway that passes the area check may still need review for pulling difficulty, conductor ampacity adjustment, and other installation requirements.
Does a 3/4-inch conduit have a 3/4-inch inside diameter?
No. Trade size is a nominal designation, not a measurement of the conduit’s inside diameter. The usable cross-sectional area also varies by raceway type; for example, PVC Schedule 80 has less internal area than PVC Schedule 40 of the same trade size. Select the specified raceway type rather than calculating fill from the trade-size label.
How do I convert conductor or conduit area between square inches and square millimeters?
Multiply square inches by 645.16 to get square millimeters; divide square millimeters by 645.16 to get square inches. Convert both the conductor areas and conduit area to the same unit before calculating a fill percentage. For a code check, use the applicable published or listed dimensions rather than estimating insulation thickness from the wire’s AWG size.
Do ground and neutral wires count toward conduit fill?
Yes. An insulated equipment grounding conductor and a neutral each occupy physical space, so include them in the fill count and area, along with any other conductors installed in the raceway. Do not confuse this with ampacity adjustment: the rules for counting current-carrying conductors are a separate check and can treat grounding conductors and some neutrals differently.
Why can adding a second wire make a conduit fail even if one wire fits?
The permitted percentage drops from 53% for one conductor to 31% for two. Adding a second wire increases occupied area while reducing the allowable percentage, so a previously passing selection can fail. For three or more conductors, the ordinary limit changes to 40%; each added wire still contributes area, so check the complete group rather than assuming the higher percentage guarantees a pass.
Can I use the same fill area for THHN, XHHW-2, and THW wires of the same AWG size?
Not necessarily. AWG identifies the conductor size, while insulation type affects its overall dimensions and the area used for fill. Some size-and-insulation combinations happen to share a tabulated area, but others do not. Check the applicable table for each specified wire rather than substituting an area based only on AWG, especially when mixing insulation types.