Conduit Fill and Box Fill Calculator
NEC Chapter 9 conduit fill and 314.16 box fill, including Note 7 rounding and the separate ampacity derate that fill alone never warns you about.
Conductors in the raceway
Fill and derating are two different rules
Passing one says nothing about the other, and this is where people come unstuck. Chapter 9 limits you to 40% fill so the wire can be physically pulled without stripping its insulation on the way in. NEC 310.15(C)(1) separately cuts ampacity once more than three current-carrying conductors share a raceway, because they cannot shed heat into each other. A conduit stuffed to exactly 40% with ten current-carrying conductors is entirely legal on fill and needs its ampacity halved — and nothing in the fill calculation warns you.
- The fill percentages are not monotonic, on purpose. One conductor gets 53%, two get 31%, three or more get 40%. Two circles inside a circle is the geometrically awkward packing case, so it gets the tightest limit.
- Chapter 9 Note 7 lets you round up at 0.8. Where every conductor is the same size and the calculation lands on a decimal of 0.8 or larger, the next whole number is permitted. That is why Annex C gives 22 × 14 AWG in ¾″ EMT when the plain division gives 21.98.
- Grounds count for fill but not for derating. Neutrals count for both on a three-phase wye circuit feeding non-linear loads, and not at all on a balanced three-wire circuit — the distinction that catches people on shared-neutral runs.
- A nipple of 24 inches or less gets 60% and no derate at all. Over 24 inches neither concession applies.
- Fill is only half of whether a pull will go. NEC 358.26 caps you at 360° of bend between pull points, and four 90° bends will defeat a conduit sitting well inside its fill limit.
A planning aid, not a design. Local amendments and the inspector govern, and box volume should always be read from the marking on the box rather than a chart.
How to use
- Pick the conduit type and trade size, then add each conductor size and count.
- Untick "carries current" for equipment grounds — they fill but do not derate.
- Check the fill percentage AND the ampacity derate; they are separate rules.
- Switch to the box tab for 314.16 volume, where each device counts as two.
Frequently asked questions
Why is the limit 40 percent?
Because fill limits are about pulling, not heat. Forty percent is what lets the wire go in without stripping its insulation against the conduit wall or the other conductors on the way. That is also why the numbers are not monotonic: one conductor gets 53 percent, two get 31 percent, and three or more get 40 percent, because two circles inside a circle is the geometrically awkward packing case and gets the tightest limit of the three.
I passed the fill check, so is my conduit fine?
Not necessarily, and this is where most people come unstuck. NEC 310.15(C)(1) separately cuts ampacity once more than three current-carrying conductors share a raceway, because they cannot shed heat into each other. A conduit at exactly 40 percent fill with ten current-carrying conductors is entirely legal on fill and needs its ampacity halved. Fill and derating are different rules and satisfying one says nothing whatever about the other.
Why does my count differ by one from the chart?
Almost certainly Chapter 9 Note 7. Where every conductor is the same size and the calculation lands on a decimal of 0.8 or larger, the next whole number is permitted. That is why Annex C gives 22 fourteen-gauge THHN in three-quarter inch EMT when the plain division gives 21.98. A calculator that omits Note 7 comes up one short and disagrees with the table your inspector reads from.
Do grounds count toward fill?
Yes for fill, no for derating. Every conductor in the raceway takes up physical room, including equipment grounding conductors, so they all count in the area calculation. Only current-carrying conductors count for the 310.15(C)(1) derate, and a ground never carries current in normal operation. Neutrals are the subtle case: they count for both on a three-phase wye circuit feeding non-linear loads, and not at all on a balanced three-wire circuit.
What is a nipple and why does it get more room?
A raceway 24 inches or less between two enclosures. Chapter 9 Note 4 permits 60 percent fill in one and exempts it from bundling derate entirely, because conductors passing through such a short length have enclosures either side to shed heat into and there is very little pull distance to damage insulation over. Go past 24 inches and neither concession applies.
How do I count box fill?
Each conductor entering and terminating counts as one, and so does one passing through unbroken. All the equipment grounds together count as one, and so do all the cable clamps. Each device on a yoke counts as TWO of the largest conductor in the box, which is the rule that fills most boxes. Pigtails that both start and end inside the box count as nothing at all.
Can I just look up my box volume?
Use the marking on the box instead. Plastic boxes are stamped inside with their cubic inch capacity, and metal boxes of the same nominal size genuinely vary between manufacturers. Ganged boxes, extension rings and plaster rings all add their own marked volume. Overfilling is a real hazard rather than a paperwork issue: it damages insulation as the device is pushed home, and that damage is inside a wall where nobody will see it.
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