Calculate conduit size (electrical conduit sizing).

In this article, we explain how to use NOM-001-SEDE-2012 to calculate conduit size dimensions based on the number of wires, their gauge, and the type of insulation they use.

To calculate conduit size dimensions according to NOM-001-SEDE-2012 in Mexico, you first need to understand some basic concepts related to conduit capacity for housing electrical conductors according to insulation type, gauge, and number of conductors.

Here is a step-by-step guide to performing the calculation:

Step 1: Identify the Quantity and Type of Conductors

Before selecting the conduit, it’s important to know:

Number of conductors: How many wires need to be installed in the conduit.
Conductor gauge: The size of the wires, measured in AWG (American Wire Gauge).
Type of conductor insulation: Such as THHN, XHHW, etc. The insulation type affects the necessary space in the conduit.

Step 2: Consult the Conduit Table in NOM-001-SEDE-2012

NOM-001-SEDE-2012 provides specific tables (Table 9 of the NOM-001) indicating the maximum allowable fill in conduits for different conductor types and sizes. Relevant sections include:

Conduit fill percentage table: According to the standard, the maximum allowed fill for a conduit depends on the number of conductors:
- 1 conductor: 53% conduit fill.
- 2 conductors: 31% conduit fill.
- 3 or more conductors: 40% conduit fill.
Conduit capacity by diameter table: This helps determine the conduit size based on the conductors to be housed.

Step 3: Calculate the Conductor Occupancy Area

To calculate the conductors’ occupancy area, you should:

Look up the area in square millimeters (mm²) for each conductor type and gauge in the NOM-001 table.
Multiply the area of one conductor by the total number of conductors of that type.
Add the areas of all conductors to be installed in the conduit.

Step 4: Determine the Required Conduit Area

Using the total occupancy value calculated in the previous step, select a conduit whose diameter allows for the appropriate fill percentage:

We now have the minimum conduit area you need from Step 3.
Use the conduit dimension table to select a conduit with an internal area equal to or greater than your calculated requirement, typically using the fill percentage for three or more conductors (40%).

Practical Example: Calculate conduit size

Suppose you want to install three THHN conductors of 12 AWG in a conduit. We’ll calculate the necessary conduit size:

Conductor area: According to the table, a 12 AWG THHN conductor has an approximate area of 8.6 mm².
Total conductor area: 3 conductors x 8.6 mm² = 25.8 mm².
Minimum conduit area: 25.8 mm². Simply refer to the conduit table for three or more conductors and select a size larger than the minimum required.
Select the Conduit Diameter. Check the conduit dimensions table in NOM-001-SEDE-2012 and choose a conduit with an internal diameter that provides at least 24.83 mm² of available space to house the conductors.

Table for THHN cable area, from the example above.

Final Considerations calculate conduit size.

Environmental factors: If the conductors will be in high-temperature areas, they may need greater spacing, potentially affecting conduit size.
Additional conductors: If you plan to add more conductors in the future, consider a conduit size larger than the minimum needed.

By following these steps and consulting the tables in NOM-001-SEDE-2012, you can select the appropriate conduit size and ensure compliance with safety and efficiency standards in electrical installation.

We recommend an app that performs this calculation and more:

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