Conduit sizing table.

The conduits accommodate the cables through the walls of an installation and are an important part of the electrical infrastructure, that is, the part of the installation that does not use electricity to function but that is part of the electrical installation. All the components that support, fix and protect cables and other electrical components are part of this infrastructure, such as boxes, panels, fasteners, etc.

They constitute a relatively inexpensive part of the electrical installation, being the first items of the electrical installation to be installed and commonly the first on the shopping list requested by the electrician. Perhaps because the conduits are relatively low in price compared to other components of the electrical installation, their importance is underestimated.

The NBR 5410 makes it very clear that if necessary, the use of conduits, these must be standardized, there are several specific NBRs for each type of conduit, both for flush and flush conduits. There are basically three important standards for conduits:

  • NBR 15465 Plastic conduit systems for low voltage electrical installations – Performance requirements
  • NBR 5597 Carbon steel conduit and accessories, with protective coating and NPT thread – Requirements
  • NBR 5598 Carbon steel conduit and accessories, with protective coating and BSP thread – Requirements

In residential electrical installations the most common conduit is the flexible corrugated type. Normally manufactured in PVC or similar material they have a low cost and a good malleability and are found in the following gauges:

Standardized sections for corrugated conduit.

Within the NBR5444-Graphical symbols for building electrical installations, a table is stipulated for the standardized conversion from inches to millimeter based on NBR5626-Building installation for cold water.

Equivalence table between inches and millimeters.

It is extremely important to use this table for conversion because the vast majority of the conduits are dimensioned in millimeters and then their equivalence in inches must be found because it is in this unit of measurement that the manufacturers make the conduits available.

There is a mathematical method taking into account a number of factors for the correct sizing of an conduit. Thus, it is important to have the data from the manufacturers of the conductors for a correct dimensioning of the conduits, which is confusing due to the large number of variations in cable manufacturing.
In the case of simpler installations, the table below can be used in order to reference and simplify the dimensioning of the conduits, this table is not absolute but its consultation is simplified due to the ease of interpretation and little margin of error.

Conduit sizing table.

This table takes into account two criteria, the number of cables in a conduit and the section of these conductors. For your use, just select the column containing the number of cables inside the conduit in question, then cross with the line referring to the cable section inside this conduit, the intersection between column and line results in the inch value of the appropriate conduit to accommodate these cables.

See the example:

Two circuits of general-purpose sockets passing in the same conductor, the selected column will be that of 6 cables, two phases, two neutrals and two earths, one phase, one neutral, and one earth for each circuit. The cables used for these circuits according to the project are 2.5mm².

Example of using the conduit sizing table.

Observing the intersection between column and line, it is noted that the appropriate conduit to support the two circuits in the example is a 1 ”conduit.

This table already takes into account an adequate occupancy rate for the conduit, and this rate is important to ensure the proper temperature inside the conduit as well as ease of cable routing and future maintenance of these circuits inside the conduit.