ConduitCheck
NEC 2017, 2020 and 2023
Stop counting cubic inches on the back of a scrap of Romex. Box fill, conduit fill, and ampacity derating worked out against the NEC, with the article and table cited on every answer.
What it calculates
- Box fill in cubic inches from conductor, device, clamp, and fitting counts
- Minimum box size for a given set of conductors
- Conduit and raceway fill percentage for mixed conductor sizes
- Minimum raceway size for a given conductor bundle
- Conductor ampacity with ambient temperature correction
- Ampacity adjustment for more than three current-carrying conductors
- Equipment grounding conductor sizing from overcurrent device rating
- Voltage drop over a run length
Code sections it cites
ConduitCheck names the section behind every result so you can look it up, defend it to an inspector, or check it against the edition your town adopted. It describes what each section requires in plain language — it does not reproduce the code text itself. Why that matters.
NEC 314.16
Box fill — the number of conductors permitted in outlet, device, and junction boxes.
NEC Table 314.16(A)
The volumes of standard metal boxes and the conductor counts they allow.
NEC 314.16(B)
How each item in a box — conductors, devices, clamps, fittings, and grounds — counts toward the fill volume.
NEC Chapter 9, Table 1
The maximum permitted fill percentage of a raceway based on how many conductors are in it.
NEC Chapter 9, Table 4
The internal cross-sectional areas of each raceway type and trade size.
NEC Chapter 9, Table 5
The cross-sectional areas of insulated conductors by size and insulation type.
NEC Table 310.16
Allowable ampacities for insulated conductors in a raceway, cable, or earth.
NEC 310.15
Ambient temperature correction and the adjustment factors applied when conductors are bundled.
NEC 250.122
Sizing the equipment grounding conductor from the rating of the overcurrent device.
Screenshots
Coming once the interface settles.
What it costs
Free
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- Every calculator: box fill, conduit fill, ampacity with ambient and bundling adjustment, EGC sizing, voltage drop
- Switch between NEC 2017, 2020 and 2023 to match your jurisdiction
- Article and table citation on every result
- Works fully offline
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