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Conduit Size Calculator

Enter your conductor bundle and get the minimum trade size across EMT, RMC, IMC, PVC Schedule 40, and PVC Schedule 80 side by side. NEC 2020 Chapter 9 Tables 1, 4, and 5. No need to run the calculation five times — one entry, one table.

Conductors
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How to size conduit for a conductor bundle

  1. Enter each conductor's insulation type (THHN/THWN-2 or XHHW-2), size, and quantity. Add a row for each distinct conductor spec. Don't forget the EGC — it counts toward fill.
  2. Check "Nipple" if the run is 24 inches or shorter — the 60% fill exception per NEC Table 1 Note 4 applies, reducing the required trade size.
  3. Read the minimum trade size for each conduit type from the comparison table. The fill percentage column shows how tightly packed the conductors are.
  4. Choose the conduit type appropriate for the installation: EMT for exposed interior, PVC for underground, RMC for exposed exterior or physical protection.

NEC reference

Conduit sizing uses NEC 2020 Chapter 9. Table 1 sets fill limits by conductor count. Table 4 gives the internal cross-sectional area for each conduit type and trade size (the values differ for EMT, RMC, IMC, PVC Sch 40, and PVC Sch 80 at the same trade size). Table 5 gives approximate conductor areas including insulation. The minimum trade size is the smallest whose allowable fill area (total area × Table 1 limit) is at or above the total conductor area.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

Why conduit type changes the minimum size

All conduit types use the same Table 1 fill percentages, but their internal areas differ for the same nominal trade size. A larger internal area means you can fit the same conductor bundle into a smaller trade size. IMC typically has the largest internal area, followed by RMC, then EMT, then PVC Sch 40, then PVC Sch 80. The gap between types is largest at small trade sizes and narrows at larger sizes.

Practical example: five #12 THHN conductors (0.0665 in² total, 40% fill limit). Minimum sizes:

EMT → ¾ in. (allowable 0.213 in², fill 31.2%)

RMC → ¾ in. (allowable 0.220 in², fill 30.2%)

IMC → ¾ in. (allowable 0.234 in², fill 28.4%)

PVC Sch40 → ¾ in. (allowable 0.203 in², fill 32.7%)

PVC Sch80 → 1 in. (allowable 0.275 in², fill 24.2%)

Four of the five conduit types fit the bundle in ¾ in.; PVC Sch 80 requires 1 in. because its ¾ in. bore is too small (0.409 in² × 40% = 0.164 in², which is below the 0.0665 in² bundle). The comparison table shows this immediately.

Selecting a conduit type

NEC Article 300 and individual conduit articles (358 EMT, 344 RMC, 342 IMC, 352 PVC) govern where each type is permitted. Common guidance:

  • EMT — exposed interior work, dry and damp locations, commercial and industrial, not permitted in concrete or where subject to severe physical damage
  • RMC — all locations, including outdoor, underground with PVC sleeve, and where physical protection is required
  • IMC — similar to RMC with lighter weight, permitted in most locations RMC is approved
  • PVC Sch 40 — underground, concrete-encased, wet locations, not where exposed to physical damage (unless listed for that use)
  • PVC Sch 80 — exposed locations where additional physical protection is needed, above grade applications

This calculator shows the minimum trade size for all types simultaneously so you can confirm sizing for the type that suits the installation before committing.

Frequently asked questions

What is the minimum conduit size allowed by the NEC?

NEC 358.20 (EMT), 344.20 (RMC), 342.20 (IMC), and 352.20 (PVC) set ½ in. as the minimum trade size for all metallic conduit types. The fill requirement from Chapter 9 then determines whether a conductor bundle actually fits within the fill limit for that trade size.

Do I use the same fill percentage for all conduit types?

Yes. NEC Chapter 9 Table 1 fill percentages (53% / 31% / 40% by conductor count, 60% for nipples) apply to all raceway types. Only the internal areas in Table 4 differ by conduit type and trade size.

How do I count conductors for the fill limit?

Count every conductor in the raceway: phase conductors, neutral, and EGC. A 3-wire circuit (A, B, C) plus a neutral and a bare EGC is 5 conductors — fill limit is 40%. A single-phase 2-wire circuit (hot + neutral, no EGC) is 2 conductors — fill limit is 31%.

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