Conduit fill calculator
You supply the conduit and conductor areas; the tool only compares their ratio to the percentage limit. It is not the code book, the local amendments or a qualified person — verify the areas and the applicable fill rule against your own book, and remember pulling tension and bend count can matter as much as the percentage.
Enter the conduit's internal area and the area of each conductor going in, set how many conductors there are, and the tool reports the fill percentage and whether it clears the limit for that number of conductors. Mixed sizes? Total their areas and enter that as one figure with a count of one.
Exemple détaillé
Three conductors at 22 mm² each fill 66 mm² of a raceway whose internal area is 200 mm². That is 66 ÷ 200 = 33% fill. Enter the limit your code sets for three conductors and the tool tells you whether 33% clears it; add a fourth conductor and the fill rises to 44%. Both numbers are arithmetic on the areas in this sentence — the allowed figure is the one you type in, off your own code book, and this page deliberately picks example areas that land nowhere near a published limit.
The same arithmetic in square inches: three conductors at 0.05 in² each fill 0.15 in² of a raceway with 0.5 in² inside, 0.15 ÷ 0.5 = 30% fill. (The 22 mm² conductors above are 0.034 in² each, and the 200 mm² raceway is 0.31 in² — the percentage does not care which unit the two areas are in, as long as it is the same one.)
Sur le chantier
The datasheet gives a diameter, not an area. A conductor listed at 4.5 mm overall diameter (insulation included) has an area of π ÷ 4 × 4.5² = 15.9 mm². Square the OD, times 0.785. Do the same for the conduit's internal diameter if the spec gives ID instead of area — and it is the internal figure, never the trade size. In inches it is the same line: a 0.18 in OD is 0.785 × 0.18² = 0.0254 in².
Mixed sizes add. Three at 20 mm² and two at 12 mm² in a 200 mm² raceway: 60 + 24 = 84 mm², which is 42% fill. Put the total of the small ones in as extra area on one of the big ones, or run the sum by hand and type it as one conductor of 84 mm² — the ratio is the same either way.
When it is over. If 42% is above the limit your book gives for five conductors, the next raceway up in the spec — say 300 mm² internal — brings the same conductors to 28%. That is the whole calculation: the limit comes from your book, the areas come from the datasheets, and this page divides. It never supplies the percentage.
Exercez-vous
Quick referencefor fill
- Remplissage
- Σ conductor areas ÷ raceway internal area × 100
- Area from a diameter
- 0.785 × d² — conductor OD or conduit ID, from the datasheet
- Mixed sizes
- add each conductor's area — count × area for each size, then sum
- The limit
- is your book's, for your conductor count — this page never supplies it
- Area from a diameter in inches
- 0.785 × d² in² — a 0.18 in OD is 0.0254 in²
- Unités
- 1 in² = 645 mm² · 1 mm² = 0,00155 in²
Mistakes to catchbefore you trust the number
- Using the conduit's nominal trade size as an area: a 1 in conduit does not have a 1 in² bore — the internal area comes from the conduit's own table, and you type it.
- Mixing units between the fields: a conductor area in mm² against a conduit area in in² gives a fill percentage that is nonsense. Set the toggle once and enter both in the same unit.
- Counting the ground as a free conductor; it takes area like any other, so it goes in the count.
What an inspector looks forthe electrical inspector
On this part of the job
- The number and size of conductors in each raceway against the fill allowed for that raceway. The allowance is a table in your own code book — type it into the tool and it does the division; the tool does not hold the table.
- Raceway support and spacing, and bushings or connectors where conductors enter a box or a cabinet.
- Bends between pull points, counted for the whole run rather than per length.
- Box and cabinet fill, which is a separate calculation from raceway fill and is the one more often missed.
- Whether the conductors were pulled in without damage — a scraped or stretched insulation on a tight pull is a visible fail.
Have this ready
- The conductor schedule: size, insulation type and count per raceway.
- Your code book open at the fill and dimension tables.
- Pull points left accessible.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La formule
fill % = (Σ conductor areas ÷ conduit internal area) × 100
Fill is simply the fraction of the pipe's cross-section the wires occupy. The ceiling — a percentage that depends on how many conductors share the raceway, because more conductors need more room to pull without damage — is set by your code book; enter it above and the tool checks your fill against it.
Questions
- How do I calculate conduit fill?
- Add up the cross-sectional area of every conductor going into the raceway, divide by the conduit’s internal area, and turn it into a percentage. This tool does that division once you enter the conduit’s internal area and the conductors’ areas — both of which you take from your own tables or datasheets.
- What is the fill limit?
- Your code book sets it: a maximum percentage of the conduit’s internal area that depends on how many conductors share the raceway. Look it up for your situation, type it into the limit field, and the tool tells you whether your fill passes. It ships no percentage of its own.
- Where do I get the conductor and conduit areas?
- From your conductor datasheets and conduit specifications, or by computing them — a round conductor’s area is π times its radius squared. The tool holds no dimension tables; it works on the areas you enter, which keeps it valid across insulation types and code editions.
- Does the tool tell me which conduit size to use?
- No — it tells you whether the fill you describe passes the percentage limit. Choosing a trade size is your call against your own tables. Enter a candidate conduit’s internal area and see if your conductors fit under the limit; if not, try a larger internal area.
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