Home / Networks & telecom

Conduit and cable tray fill

Enter the cables by outside diameter to find the smallest conduit that holds them within the fill limit and whether the chosen cable tray is large enough, without forcing the pull or overheating the cables.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Check
Cables

Outside diameter from the cable data sheet. Examples: Cat 6 UTP about 0.25 in; Cat 6A 0.28 to 0.30 in; THHN 10 AWG about 0.16 in. A multiconductor cable counts as one cable.

More than 4 cable types? Paste the list from Excel

One row per type: description (optional), outside diameter and quantity. Diameter in page units (mm in metric, inches in imperial).

See what was read
Pathway

Keep pull points within manufacturer and code limits: NEC allows no more than 360° of bends between pull points. Power and communications cables should not share a conduit.

—Conduit that complies
—

Fill in each conduit size

Same cables in each trade size of the chosen type. Green: within the limit; red: above it.

Tap the chart to see the values.

Tray fill versus width

Standard widths at the usable depth entered (at most 6 in counts).

Tap the chart to see the values.

Next step

Take this page’s results to another calculator with the fields already filled in.

Derate power conductors grouped in this conduit →

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Cable area A = Σ n_i · π · d_i² ÷ 4; fill = A ÷ (π · ID² ÷ 4)
NEC Chapter 9 Table 1 limit: 53% (1 cable), 31% (2), 40% (3 or more); ITC-BT-21 (Spain): internal area ≥ 2.5, 3 or 4 × A
Cable tray: fill = A ÷ (width × min(usable depth; 6 in)) ≤ 50% ladder/ventilated, 40% solid bottom
Jam ratio: 2.8 ≤ 1.05 · ID ÷ d ≤ 3.2 with three equal cables

Assumptions and limits

  • Inside diameters for EMT, RMC and PVC Schedule 40 from NEC Chapter 9, Table 4; Brazilian rigid PVC and EN 61386 metric tubes use typical catalog values (check the manufacturer).
  • A multiconductor cable counts as a single cable (NEC Chapter 9, Note 9). Conduit nipples up to 24 in may be filled to 60% (Note 4), which is not used here.
  • Cable tray uses NEC 392.22(A)(3) and (4) for control, signal and communications cables. Power cables follow 392.22(A)(1) and (2) and ampacity rules instead.

Standards and technical documents

  • NFPA 70, National Electrical Code (NEC) 2023: Chapter 9, Tables 1, 4 and 5, and Article 392 (Cable trays)
  • ANSI/TIA-569-E: Telecommunications Pathways and Spaces
  • IEC 61386-1: Conduit systems for cable management
  • IEC 61537: Cable management — Cable tray systems and cable ladder systems
  • NEMA VE 2: Cable Tray Installation Guidelines

U.S. regulations

  • NFPA 70 (NEC) Chapter 9 and Article 392, as adopted by state and local jurisdictions
  • NEC Articles 358 (EMT), 344 (RMC) and 352 (PVC conduit)
  • OSHA 29 CFR 1910 Subpart S: electrical

Frequently asked questions

What is the maximum conduit fill?

NEC Chapter 9, Table 1 allows 53% for one cable, 31% for two and 40% for three or more conductors or cables.

How many Cat 6 cables fit in a 1 inch conduit?

A 1 in EMT has 0.864 in² inside. At 40% fill (0.346 in²), it holds 7 Cat 6 cables of 0.25 in OD (0.049 in² each). With 0.30 in Cat 6A, it holds 4.

Why is the limit only 31% for two cables?

Two cables side by side span almost the whole diameter and tend to jam during the pull, especially in bends. Three or more settle together more easily.

What is the cable tray fill limit for data cables?

For control, signal and communications cables only, NEC 392.22(A)(3) and (4) allow 50% of the cross-section in ladder or ventilated trough tray and 40% in solid bottom tray, using at most 6 in of depth.