Conduit Fill Calculator
Size your raceway right the first time. Built on NEC Chapter 9, Tables 1, 4, and 5—no overfilled conduits, no re-pulls.
Switch to CECHow We Calculate Conduit Fill
This calculator uses conductor areas from NEC Chapter 9, Table 5 and conduit internal areas from Table 4. Fill percentages come from Table 1.
The calculation is straightforward:
- Sum conductor areas — Look up each wire size/type in Table 5, multiply by quantity
- Determine fill percentage — Based on total conductor count (Table 1)
- Find minimum conduit — Select smallest conduit where conductor area ≤ allowable fill area
Conduit Fill: What Every Electrician Should Know
Why are the fill percentages different for 1, 2, and 3+ conductors?
The percentages account for heat dissipation and physical pulling considerations. With one large conductor, you need room to bend it. With two conductors, they can lay side by side and jam easily. With three or more, they can triangle and pull more smoothly.
The 31% limit for two conductors is actually the most restrictive—this catches situations like pulling two large feeders where jamming is a real risk.
What's a nipple and why does it allow 60% fill?
A nipple is a short section of conduit, typically less than 24 inches, used to connect two enclosures. Since you're not pulling wire through it (just laying it in), the physical pulling concerns don't apply.
NEC Chapter 9, Note 4 to Tables permits 60% fill for nipples—useful for those tight panel-to-panel connections.
Do equipment grounding conductors count toward fill?
Yes. Per NEC 250.122, equipment grounding conductors are included in conduit fill calculations. Use the bare conductor area from Table 5 for bare grounds, or the insulated area if you're running insulated EGCs.
What about derating with high conductor counts?
Conduit fill and ampacity derating are separate issues, but they often hit at the same time. Per NEC 310.15(C)(1), when you have more than 3 current-carrying conductors in a raceway, you need to derate:
- 4-6 conductors: 80%
- 7-9 conductors: 70%
- 10-20 conductors: 50%
So even if the conduit fits, your conductors might need to be upsized for ampacity.
Which wire type should I use for calculations?
Use the actual insulation type you're installing. THHN is the most common for building wire—it has thinner insulation than XHHW or THW, so you can fit more in the same conduit.
If you're not sure, THHN is a safe assumption for typical commercial work. For wet locations or direct burial, you might be using XHHW-2 or similar—check the actual wire markings.
Can I mix different wire sizes in the same conduit?
Absolutely. The calculator handles mixed sizes—just add each wire type and size as a separate row. The fill calculation sums all conductor areas regardless of size.
This is common in practice: running a few #12s for 20A circuits alongside #10s for 30A circuits, plus a ground.