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Conduit Fill Calculator

NEC conduit fill check and smallest-size recommendation.

Mode

Wires

What is conduit fill calculator?

The NEC conduit fill rule caps how much wire area can occupy a conduit’s cross-section. The limits depend on wire count: 53% for one conductor, 31% for two, 40% for three or more. Violate the rule and you risk overheating, insulation damage, and a failed inspection.

This calculator handles two modes:

  • Fill check — you specify the conduit size; it tells you whether your wires fit.
  • Find size — you specify only the wires; it returns the smallest trade size that passes.

Wire and conduit areas come from NEC Chapter 9 Tables 4 and 5, NFPA 70-2023. The supported subset covers most residential and light-commercial installs: five insulation types (THHN, THWN-2, XHHW, RHH/RHW, TW), fifteen wire sizes from 14 AWG up to 500 kcmil, three conduit types (EMT, RMC, PVC-40), and trade sizes 1/2 inch through 4 inch.

This is a fill-check tool only — it does not derate ampacity, calculate voltage drop, or substitute for an engineered design. Always confirm against your current code edition and local AHJ requirements.

When to use a conduit fill calculator

  • Residential branch-circuit fill check — Confirm a 1/2-inch or 3/4-inch EMT run with #12 or #10 THHN passes NEC fill limits.
  • Sub-panel feeder sizing — Find the smallest RMC or PVC-40 size that handles your feeder conductors plus an equipment grounding conductor.
  • Low-voltage trunk to an outbuilding — Plan a single trunk run from the main panel to a detached structure with mixed conductor sizes.

How to use the Conduit Fill Calculator

  1. Pick the conduit type and (in Fill Check mode) trade size — EMT for residential indoor, RMC for outdoor / heavy-duty, PVC-40 for underground or wet locations.
  2. Add wires — Each row is one (insulation, AWG, count) combination. Click <strong>+ Add wire</strong> to mix different gauges in the same run.
  3. Read the result — Fill Check shows the fill % and pass/fail. Find Size walks through trade sizes and returns the smallest one that satisfies the NEC fill rule.

Worked examples

3 × THHN 12 AWG in 1/2" EMT

Input:  Conduit=EMT 1/2", wires=3 × THHN 12
Output: Fill 13.1%, Max 40%, ✓ Passes

4 × THWN-2 4/0 in 1-1/2" RMC

Input:  Conduit=RMC 1-1/2", wires=4 × THWN-2 4/0
Output: Fill 62.5%, Max 40%, ✗ Fails — try a larger size

Same wires in Find Size mode

Input:  Conduit=RMC, wires=4 × THWN-2 4/0, mode=Find size
Output: Smallest passing conduit: 2"

Frequently asked questions

What's the NEC fill limit?
Per NEC Chapter 9, the maximum conduit fill is 53% for one conductor, 31% for two, and 40% for three or more. This calculator applies the rule automatically based on the total wire count you enter.
Where do the wire and conduit area numbers come from?
NFPA 70 (NEC), Chapter 9, Tables 4 (conduit cross-sectional areas) and 5 (approximate conductor areas). Spot-checks of THHN 12 AWG (0.0133 sq in), EMT 1/2" (0.304 sq in), and RMC 2" (3.408 sq in) are validated by the test suite.
Which insulation types are supported?
THHN, THWN-2, XHHW, RHH/RHW, and TW — the five insulations covering the vast majority of residential and light-commercial work. Broader coverage is a planned future enhancement.
Which conduit types are supported?
EMT (electrical metallic tubing), RMC (rigid metal conduit), and PVC-40 (Schedule 40 PVC). These cover most residential and commercial installations. IMC, FMC, LFMC, and others are planned future additions.
What's the difference between THHN and THWN-2?
THHN is the most common building wire — heat-resistant nylon-coated. THWN-2 has the same area (same lookup row in Table 5) but is rated for wet locations. Either will give the same fill result here.
Does this account for derating?
No — fill check and ampacity derating are separate calculations. NEC 310.15(C) requires ampacity derating when more than 3 current-carrying conductors share a raceway. This calculator covers the physical fill rule only; check ampacity separately.
What if no size up to 4" passes?
Find Size mode will tell you. When even 4" exceeds the fill rule, the answer is to split the bundle into multiple parallel conduit runs, each within the fill limit.
Is this a substitute for an engineered design?
No — this is a sanity check tool. Always confirm against current NEC + local amendments and follow your authority-having-jurisdiction's inspection requirements for any installed work.

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