Industrial electrical conduits: Standards & selection guide

A common mistake is using residential electrical conduit (typically specified for building construction) in factory installations. They may look identical — but technically, they are two entirely different products.

2.2 Rigid conduit vs. flexible conduit

In practice, both types are used together in industrial facilities: rigid conduit for long trunk runs (along machines or cable trays), and flexible conduit at equipment connection points (panel entries, sensor mounting points, robot joints).

2.3 Material — The decisive factor for service life and long-term cost

Each material has its own range of applications—there is no single material that is best in every situation. A common mistake is choosing PVC simply because it is familiar and inexpensive, and then using it in environments it is not designed to withstand. The table below provides a quick comparison of the five most common materials:

💡Learn more about the characteristics, properties, and applications of each type of corrugated pipe by material

2.4 Cable fill capacity — A frequently overlooked criterion

This criterion directly affects safety but is often neglected when selecting conduit size. According to the National Electrical Code (NEC) and equivalent standards, the cable cross-sectional fill ratio relative to the conduit bore must remain within permitted limits:

• 1 cable: maximum 53% of conduit interior area
• 2 cables: maximum 31%
• 3 or more cables: maximum 40%

Exceeding these limits causes two problems: heat accumulates inside the conduit and cannot dissipate (leading to insulation damage), and pulling cables through during installation or maintenance becomes very difficult — risking jacket abrasion.

Additionally, conduit material affects the actual internal bore size: metal conduit has thinner walls, providing a larger internal diameter than plastic conduit of the same nominal size. This must be accounted for when sizing conduits for multi-cable applications.

3.2 TCVN 7417 — Vietnamese standard equivalent to IEC 61386

In Vietnam, the TCVN 7417 series is published by the Directorate for Standards, Metrology and Quality, and is equivalent to IEC 61386. It serves as the mandatory reference standard when designing electrical systems in construction projects:

• TCVN 7417-1:2010 — General requirements (equivalent to IEC 61386-1)
• TCVN 7417-25:2015 — Conduit installed in the ground (equivalent to IEC 61386-25)

3.3 IP rating (IEC 60529)

The IP (Ingress Protection) rating per IEC 60529 indicates the degree of protection a conduit provides against dust and water ingress. This is the most important rating for outdoor or wet environment installations:

Choosing the wrong conduit size is a very common error — too small and cables cannot be pulled through and heat builds up; too large and the installation is wasteful and unwieldy. The first step is to look up the available size range for the product line, then apply fill ratio principles to select the appropriate size.

5.1 Size ranges by HELU product line

Each HELU conduit line is available in multiple sizes within a defined outer diameter range. The table below is compiled from the product catalog to help engineers quickly narrow down their options:

Once the right product line for the environment has been identified, the next step is to select the correct size within that range. This is where the detailed datasheet for each product line is needed to obtain the exact inner diameter for each size — e.g., Ø12, Ø16, Ø20, etc. Datasheets can be downloaded directly from the HELU product page, or contact the HELU Vietnam technical team for sizing assistance.

5.2 Sizing principle — Fill ratio

Precisely calculating conduit size using formulas requires the actual outer diameter of each cable from the manufacturer's datasheet, and can become complex when multiple cable types share the same conduit. However, a fundamental guideline applies:

The total cross-sectional area of all cables inside the conduit must not exceed a defined percentage of the conduit's internal bore area. Per the National Electrical Code (NEC) and equivalent standards:

• 1 cable: maximum 53% of conduit interior area
• 2 cables: maximum 31%
• 3 or more cables: maximum 40%

This principle ensures two things: heat generated by the cables can dissipate out of the conduit, and cables can be pulled in or replaced later without jamming or jacket abrasion.

In practice, to ensure accuracy and avoid errors, provide the full cable list (quantity, cross-section, and outer diameter of each cable) to the manufacturer or distributor for professional sizing advice — especially for applications involving multiple cables or cables with non-uniform outer diameters.