How many cables can fit in an conduit?

When it comes to electrical projects, it is very common to have questions such as, how many circuits can pass in the same conduit? How many cables can fit in a conduit? Do you have a maximum number of circuits? In this article, we will show how many cables can be passed through a conduit.

Conduits

The conduits are responsible for storing the electrical conductors through the walls, floor and ceiling of the civil construction, protecting the cables against external damage. Conduits are an important part of the infrastructure of the electrical installation, even if they do not need electricity to be useful, it represents a relatively inexpensive part of the electrical installation.

In electrical projects, the conduits can be represented with different symbols, such as conduits that pass through the floor, ceiling and walls, if the conduits are going up or down a wall for example. Informing even the diameter of the conduits both in millimeters and inches. Observe the symbologies for the conduit in the image below.

Various types of conduit symbologies.

There are different types of conduits and for each type of conduit, there is a specific NBR. But there are basically three important rules about conduit:

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

In residential electrical installations, the most used conduit is the corrugated flexible type because it has an excellent malleability, usually made of PVC or similar material.

Conductor

Another fundamental component for the infrastructure of the electrical installation that is protected by conduits is the conductors. Electrical conductors, which are better known as cable or wire, are responsible for conducting electrical current and connecting electrical components and devices. The cables can have different applications in the electrical installation and there is the appropriate symbology for all types of conductors, as we can see in the image below.

Symbols of conductors used in electrical design.

Occupancy rate Conduits

A very common question among electricians about conduit and cables is the number of cables that can pass through a conduit. The more cables and circuits that pass through the same conduit, the more heat will be generated inside.

Heat is a very important factor in electrical installations, as the heat increases inside the conduit, the electrical current capacity of the cables decreases, that is, if the number of cables and circuits inside the conduit increases, the cable section must also increase to compensate for the loss of electrical current conduction capacity.

The standard NBR 5410 has a table that shows the correction factor for the cables according to the number of circuits that are passing in the same conduit, it is table 42. In the table there is no limit of circuits or multipolar cables per conduit.

However, the conduits have space limits, because when placing thinner cables in the conduit, the number of circuits and cables may be larger, but this depends on the dimensioning of the cables, because if the cables are thicker the number of circuits and cables decreases. But, in the NBR 5410 standard in item 6.2.11.1.6 and in letter (a), it speaks of the occupation rate of the conduits .

In case of maintenance, the cables in an conduit must be able to be easily removed. Also new cables must be able to be installed easily. For this reason, the NBR 5410 standard fixes the maximum space of the conduit that can be occupied:

  • 53% in the case of a driver
  • 31% in the case of two drivers
  • 40% in the case of three or more drivers

When an installation is being made, it is also important to think about the future need for increased circuits, and the standard also talks about this in item 4.2.5.4.

The expansions must reflect not only in the power supply, but also in the occupation rate of the ducts and the distribution boards. There are specific calculations to size the conduits according to the number of cables, but there are tables on the number of cables inside the conduit.