What the Electrical Conduit Fill Calculator does
This calculator checks how full a conduit is. Enter the cables by overall diameter and count, choose IEC or NEC rules, and it gives the total cable area, the permitted fill percentage for that number of cables, the smallest conduit that complies, the fill in the size you pick and the spare capacity left for future circuits.
The critical input is the overall diameter of each cable, including its insulation and sheath - not the conductor cross-section. A 2.5 mm² conductor is about 1.8 mm across, but the insulated cable is nearer 3.6 mm and occupies four times the area. Indicative diameters are filled in for standard sizes, and you should type over them with the figures from your cable data sheet whenever the answer matters.
How to use it
- Pick IEC or NEC. NEC works to Chapter 9 Table 1 - 53% for one conductor, 31% for two, 40% for more than two - while IEC-derived practice uses a 40% space factor for conduit and 45% for trunking.
- Add a row for each cable type, choose the standard size to pre-fill an indicative diameter, then correct it from the data sheet.
- Enter how many of each. The total count decides the NEC fill percentage, so adding a second conductor to a single one actually tightens the limit from 53% to 31%.
- Leave the conduit size on "find the smallest that complies" for a recommendation, or pick a size to check a run that already exists.
- Read the spare capacity to see how many more of the first cable type would still fit - useful when someone asks whether one more circuit can go down an existing route.
Reading the results
The permitted fill percentage is about pulling, not heat. The limits exist so cables can be drawn in without damaging their insulation, and so they can be drawn out again later.
The area needed is the total cable area divided by the fill percentage. It is the internal area the conduit must have, and it is the figure to compare against a manufacturer's data.
Fill percentage in the selected conduit is total cable area over internal conduit area. Under the limit is a pass on fill alone.
Spare capacity is expressed both as an area and as a count of the first cable type, because that is how the question is usually asked on site.
Worked example: six 12 AWG THHN conductors in EMT
THHN 12 AWG has an approximate overall diameter of 3.30 mm, so each occupies π x 3.30² / 4 = 8.553 mm². Six of them come to 51.32 mm².
More than two conductors means a 40% fill limit under NEC Chapter 9 Table 1, so the conduit needs at least 51.32 / 0.40 = 128.3 mm² of internal area.
Half-inch EMT has about 196 mm² internally, which is enough: the fill comes out at 51.32 / 196 = 26.2%. There is 78.4 - 51.32 = 27.1 mm² of permitted space left, which is another three 12 AWG conductors before the 40% limit is reached.
Note what happens if only one of those conductors were present: the limit would be 53% rather than 40%, because a single cable can always be pulled through the middle. Two conductors get the harshest limit of all, 31%, because two circles jam against each other and the conduit wall.
None of that says the nine conductors would be a good idea. Nine current-carrying conductors in a raceway attract a 70% NEC ampacity adjustment factor, so the circuits they form would each need much larger conductors - which would then not fit. Fill and derating pull in opposite directions, and both have to be satisfied.
Formulas and scoring rules
- Area of one cable
A = pi x d^2 / 4d is the overall diameter over the outer sheath, in millimetres.- Total cable area
A_total = sum over types of (count x area each)- Internal area needed
A_conduit >= A_total / (fill% / 100)- Fill percentage
fill% = A_total / A_conduit x 100- NEC fill limits
1 conductor 53%, 2 conductors 31%, over 2 conductors 40%NEC Chapter 9, Table 1. The 31% figure for exactly two is not a typing error.- IEC-derived space factor
40% for conduit, 45% for trunkingA long-standing convention; the standards themselves work in cable factors and conduit factors rather than a single percentage.
Why two conductors get a tighter limit than three
It looks like a mistake in the table until you draw it. One cable in a conduit can lie anywhere and will pull straight through the middle, so a generous 53% is allowed. Three or more round cables settle into a roughly triangular bundle that behaves like a single larger circle, and 40% works.
Two cables are the awkward case: they lie side by side, spanning the full diameter of the conduit, and jam against each other and both walls as the bundle twists during a pull. The 31% limit reflects that geometry, and it is the reason a two-conductor run sometimes needs a bigger conduit than a three-conductor one.
Fill is not the only limit
A conduit run also has a bend allowance - the NEC permits no more than 360 degrees of bends between pull points - and a practical draw-in length beyond which the pulling tension damages insulation. Cable manufacturers publish maximum pulling tensions and minimum bending radii, and a lubricated pull with a winch on a long run is a calculation of its own.
Then there is heat. Cables bunched in a conduit cannot reject heat to the air, so a grouping or adjustment factor applies to every circuit in it. A conduit that passes on fill but fails on derating has simply moved the problem from the containment to the cable, and the cable size calculator is where you will find it.
Limitations: what the result does not prove
- The conduit internal areas here are for common EMT and rigid PVC sizes and vary between manufacturers and conduit types. Check the maker's data sheet for anything that matters.
- The cable diameters are indicative for standard single-core cables. Armoured, multi-core, flexible and fire-rated cables are all larger, sometimes considerably.
- Fill is only one check. Bends, draw-in length, pulling tension, bending radius and the grouping derating for the circuits inside all apply separately.
- NEC Chapter 9 also has a specific rule for conduit nipples up to 600 mm, which may be filled to 60%. That case is not modelled here.
- Nothing here is a compliance statement. A qualified engineer must verify the installation against the applicable standard and local regulations.
Privacy: where your data goes
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Standards and sources
- NFPA 70 - National Electrical Code - checked 19 Sep 2026
- IEC 60364-5-52 - Selection and erection of wiring systems - checked 19 Sep 2026
- IEC 61386-1 - Conduit systems for cable management: general requirements
Frequently asked questions
What is the 40% conduit fill rule?
It is the NEC Chapter 9 Table 1 limit for more than two conductors in a raceway: the total cross-sectional area of the cables may not exceed 40% of the conduit's internal area. One conductor may fill 53% and two may fill only 31%, so the rule is really three numbers rather than one.
Do I measure the conductor or the whole cable?
The whole cable, over its outermost covering. A 2.5 mm² conductor is only about 1.8 mm across, but with insulation it is around 3.6 mm - and area scales with the square of diameter, so using the conductor size understates the fill by a factor of four.
Does the earth conductor count towards fill?
Yes. Fill is about physical space, so every conductor in the conduit counts, including the protective earth and any separate neutral. That is different from the ampacity adjustment factor, which counts only current-carrying conductors and excludes the earth.
Can I fill a conduit to 40% and forget about derating?
No, and this is the trap. Fill and derating are separate rules pulling in opposite directions. Nine current-carrying conductors might fit on fill while attracting a 70% ampacity adjustment under NEC 310.15(C)(1) or a 0.50 grouping factor under IEC practice, which forces every circuit in the conduit to a larger size - which then may not fit.
What about trunking rather than conduit?
IEC-derived practice conventionally allows a 45% space factor for trunking against 40% for conduit, because a trunking lid comes off and the cables are laid rather than pulled. Enter the trunking's internal area as a custom conduit if it is not in the list, and remember that the grouping derating still applies to whatever is in it.
How many cables can I add to an existing conduit?
The spare capacity figure answers that for the first cable type in the list: it is the permitted area minus the area already used, divided by one cable's area. Before adding them, check the grouping derating for the circuits already in the conduit - more cables may mean the existing ones are now undersized.
Does a conduit nipple have a different limit?
Yes. NEC Chapter 9, Note 4 permits a nipple no longer than 600 mm (24 inches) to be filled to 60%, and no ampacity adjustment applies over that short length. This calculator uses the ordinary limits, so a nipple will look more crowded here than the code requires.
Are the diameters in this tool accurate enough to order materials?
They are good enough for a first check and for teaching the method, but not for ordering. Real diameters vary by manufacturer, insulation type and voltage rating, and armoured or fire-rated cables are much larger. Take the figures from the data sheet of the cable you are actually buying.
Last reviewed by the A2Z.Tools team against the sources listed above.
