1What this calculator tells you
This tool checks whether your conductors fit inside a raceway within the limits of the National Electrical Code (NFPA 70). Pick the conduit type and trade size, the conductor insulation and size, how many conductors you are running, and whether it is a standard run or a short nipple.
You get the fill percentage, the maximum the NEC allows for that conductor count, the usable area in square inches, and a clear pass or fail.
2The NEC fill limits
The allowable fill depends on how many conductors share the raceway, not on what they carry. These percentages are from NEC Chapter 9, Table 1.
3How many wires fit — quick reference
A frequent question is simply “how many of this wire fit in that conduit?” The table below shows the fill percentage for some common THHN combinations, all comfortably within the 40% limit. Read it as a sanity check, then use the calculator for your exact run.
Notice how quickly small conduits fill: nine 12 AWG wires already reach 39% in half-inch EMT, so the next wire would push it over.
Fill is only one of several checks a real installation has to pass. Treat a “pass” here as necessary, not sufficient.
- Ampacity derating is separate. Once a raceway carries more than three current-carrying conductors, NEC 310.15 requires you to derate their ampacity — that can force a larger wire (and then a larger conduit) even when fill looks fine.
- Pulling and jam ratio. Long runs with several 90° bends may need a conduit 25–50% larger than the fill minimum so the wire can actually be pulled without damage.
- Mixed wire sizes. This tool assumes one conductor size and a count. For a mix, look up each size’s area in Table 5, add them up, and divide by the conduit area.
- Other raceways. IMC, FMC, ENT and LFNC have their own Table 4 areas; this MVP covers the four most common types.
- Grounds count. Equipment grounding and bonding conductors count toward fill — include them in your number.
- Select the conduit type and trade size you plan to install.
- Pick the conductor insulation (THHN/THWN-2 or XHHW) and the wire size.
- Enter the total number of conductors, including equipment grounds.
- Choose Standard run or Nipple, then press Calculate.
The readout shows your fill percentage against the allowable limit and the usable area left. If it fails, step up one trade size and recalculate. Want your own number? Enter your values in the calculator and hit Calculate.
1The conduit fill formula
Fill percentage is the total conductor area divided by the internal area of the conduit. Both areas come straight from NEC Chapter 9 tables, so no geometry is needed.
Where:
- Σ conductor area the sum of every conductor’s cross-sectional area (area per wire × count).
- conduit area the total internal cross-sectional area of the raceway.
- fill limit the NEC maximum for your conductor count (53% / 31% / 40% / 60%).
2Worked example
Take three 12 AWG THHN conductors in 1/2-inch EMT, run as a standard length. Work it one line at a time:
Make the same run a nipple (24 inches or shorter) and the limit jumps to 60%, because the short length keeps heat and pulling stress low — the same wires would then fill only about 13% of a much higher allowance.