Conduit Fill Calculator

Compute conduit fill for up to three conductor groups. Enter counts and **outer diameters** (inches), pick a conduit type & trade size or provide a custom internal diameter. The calculator applies the common NEC fill limits: 1 conductor → 53%, 2 conductors → 31%, 3+ conductors → 40%.

Pick a dataset or choose Custom to enter your own internal diameter (ID).
Trade size list changes with conduit type. Shows common sizes.
Only used when type is Custom. Otherwise, ID comes from the table.

Conductors

Total number of identical conductors in group A.
Approx ODs: 14 THHN ≈ 0.111″, 12 ≈ 0.130″, 10 ≈ 0.164″, 8 ≈ 0.213″.
Optional: second conductor group.
Outer diameter in inches.
Optional: third conductor group.
Outer diameter in inches.
Click an example to auto-fill and see results immediately.
Shows: per-group areas, conduit area, allowed fill %, pass/fail check, and suggested minimum trade size for the chosen type.

Result

Total Conductors

—
Sum of all groups.

Actual Fill

—
= (Total conductor area / Conduit area) × 100%.

Allowed Fill

—
53% for 1, 31% for 2, 40% for 3+ conductors.

Area Summary (in²)

—
Conductor total vs. conduit area (and remaining to limit).

Status & Suggestion

—
Shows pass/fail and the smallest trade size (same type) that meets fill.
Helping notes:
  • Fill checks use cross-sectional **areas**. For round wires: A = π·d²/4. For conduit: use internal diameter.
  • Common NEC limits: 1 conductor → 53%, 2 conductors → 31%, 3+ conductors → 40%. Verify any special-case rules that may apply.
  • Wire ODs vary by insulation and manufacturer. Use your spec sheet values for best accuracy.
  • Dataset diameters for EMT & PVC-40 are approximate and for learning. Always confirm with current tables.

What is a Conduit Fill Calculator?

A Conduit Fill Calculator helps electricians, engineers, and DIYers determine how many conductors of given sizes can safely fit inside a conduit. It applies National Electrical Code (NEC) fill limits and area math to prevent overcrowding, reduce heat buildup, and maintain pullability. By converting wire outside diameters (or published cross-sectional areas) and conduit internal area into a single percentage, the tool quickly flags compliance and suggests alternate sizes if needed. Use it during design, estimation, and field checks to avoid rework and inspection failures.

About the Conduit Fill Calculator

The calculator accepts conductor outside diameter \(d_i\) (or area \(A_i\)) for each type/size and the conduit’s internal diameter \(D\) (or area). It sums conductor areas and compares them with NEC Chapter 9, Table 1 limits: 53% (one conductor), 31% (two conductors), and 40% (more than two).

$$A_{\text{wire},i}=\pi\left(\frac{d_i}{2}\right)^2 \quad\text{(if diameter given)}$$ $$A_{\text{cond}}=\sum_{i=1}^{k} n_i\,A_{\text{wire},i},\qquad A_{\text{conduit}}=\pi\left(\frac{D}{2}\right)^2$$ $$\%\text{Fill}=100\cdot\frac{A_{\text{cond}}}{A_{\text{conduit}}}$$ $$A_{\text{allow}}=\alpha\cdot A_{\text{conduit}},\ \ \alpha= \begin{cases} 0.53,& k=1\\ 0.31,& k=2\\ 0.40,& k\ge 3 \end{cases}$$ $$n_{\max}=\left\lfloor \frac{A_{\text{allow}}}{A_{\text{wire}}}\right\rfloor \quad\text{(same size conductors)}$$

When many current-carrying conductors share a raceway, ampacity adjustments may apply. The calculator can display the count for derating reference while you consult NEC 310.15(C)(1). Render these formulas responsively with MathJax; use math.js for the same numeric evaluations shown by the equations.

How to Use this Conduit Fill Calculator

  1. Select conduit type and size (to get internal diameter/area) or enter \(D\) directly.
  2. Enter each conductor’s outside diameter \(d_i\) (or published area \(A_i\)) and quantity \(n_i\).
  3. The tool computes \(A_{\text{cond}}\), \(\%\text{Fill}\), and compares to the correct NEC limit \(\alpha\) based on conductor count.
  4. If overfilled, increase conduit size or reduce conductor size/count; optionally view \(n_{\max}\) using the formula above.

Examples (using the same formulas)

Example 1 — Three same wires in 1/2″ conduit:
Assume \(D=0.824\ \text{in}\Rightarrow A_{\text{conduit}}=\pi(0.412)^2\approx 0.533\ \text{in}^2.\) Three THHN wires with \(d=0.130\ \text{in}\Rightarrow A_{\text{wire}}=\pi(0.065)^2\approx 0.0133.\) $$A_{\text{cond}}=3\times 0.0133=0.0399,\quad \%\text{Fill}=100\cdot \frac{0.0399}{0.533}\approx 7.5\% \ (<40\%)\ \checkmark.$$

Example 2 — Mixed sizes, more than two:
Two conductors \(d=0.162\Rightarrow A=0.0206\) and four conductors \(d=0.130\Rightarrow A=0.0133.\) $$A_{\text{cond}}=2(0.0206)+4(0.0133)=0.0944.$$ With \(A_{\text{conduit}}=0.533\), \(\%\text{Fill}=17.7\%<40\%\) ⇒ acceptable.

Example 3 — Single large cable in small conduit:
One cable \(d=0.70\Rightarrow A_{\text{cond}}=\pi(0.35)^2=0.3848.\) With \(A_{\text{conduit}}=0.533\) and \(\alpha=0.53\): $$\%\text{Fill}=100\cdot \frac{0.3848}{0.533}=72.2\%\ (>53\%)\ \Rightarrow\ \text{not permitted}.$$

7 FAQs

Q1: Which NEC limits does this follow?
NEC Chapter 9, Table 1: 53% (one), 31% (two), 40% (three or more). Always confirm the latest code edition.

Q2: Do I use wire gauge or outside diameter?
Use outside diameter or published area for the specific insulation (e.g., THHN). Different insulations change the OD.

Q3: How do bends affect fill?
Fill limits are independent of bends, but excessive bends increase pulling tension. Follow bend count/radius rules elsewhere in the NEC.

Q4: Does neutral count for derating?
It depends. Certain neutrals are current-carrying for ampacity adjustment; check NEC 310.15 notes for your circuit type.

Q5: Can I mix wire types in one conduit?
Yes if rated for the same temperature/voltage and suitable for the raceway. Use each type’s actual OD/area in calculations.

Q6: What if my conduit type isn’t listed?
Enter the internal diameter/area manually from the manufacturer’s datasheet; the formulas remain the same.

Q7: How do I find maximum number of identical conductors?
Compute \(n_{\max}=\left\lfloor \alpha A_{\text{conduit}}/A_{\text{wire}}\right\rfloor\) with \(\alpha\) chosen from 53%, 31%, or 40% as applicable.

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