Introduction
A cable pathway or raceway is a protective channel or enclosure made of materials like metal or plastic, used to manage and safeguard electrical cables and wires.
It serves to organize and shield cables from physical damage, environmental elements, and interference.
There are various types, including surface-mounted, flush-mounted, and underground pathways, each suited to different installation needs.
In this blog post, we will discuss two types of cable pathways: conduits and cable ladders. As a bonus, we will also cover cable pits.
Discover the essential role of cable pathways in electrical installations! 🌐🔌 Learn how they organize and shield cables from damage and interference. #ElectricalEngineering #CableManagement #Conduits #Cableladder #SparkyCalc
Power Cable Conduits
Various types of conduits are available, including metal, non-metallic, flexible, and liquid-tight options, each suited to specific applications.
In Australia, conduits housing power cables are colored orange, while those for telecommunications are white.
In this section, we will explore items such as standard conduit markings, standard conduit sizes, and conduit installation methods
Conduits Material of Construction
Electrical conduits can be made from various materials, including PVC, polyethylene, metal (such as steel, aluminum, or galvanized steel), fiberglass, and flexible options (like non-metallic, liquid-tight, or flexible metallic).
The choice depends on factors like environment, type of wiring, and budget, but here is a quick guide on the selection process.
- PVC is affordable, lightweight, and resistant to moisture and corrosion, making it suitable for indoor use. However, it might not be as durable in high-temperature areas.
- Metal conduits are sturdy, making them ideal for outdoor use or areas prone to physical stress. They’re fire-resistant and handle high temperatures, though they can be pricier and more challenging to install.
- Fiberglass conduits are excellent in corrosive environments, resistant to chemicals and moisture. They’re lightweight and easy to install, but not as durable as metal.
- Flexible conduits are suitable for bending or flexing situations. Non-metallic is lightweight but less durable, liquid-tight is for high-moisture areas, and flexible metallic is more durable but may be harder to install
Conduit Markings
Conduits normally have markings on their body at intervals of between 1m to 3m, indicating the manufacturer’s name, the standard to which they comply (e.g., AS/NZS61386.21), the material of construction (e.g., PVC), and the size
Conduit diameters
Conduit sizes can be somewhat confusing to understand. Nominal conduit sizes 16mm to 63mm refer to outside diameters, whereas sizes 65mm and above refer to approximate internal diameters.
This can be seen in the table below. Nominal sizes 80 (NZ) and 100 (NZ) are used in New Zealand, while sizes 80 (AU) and 100 (AU) are used in Australia.
Conduit lengths
Unless otherwise specified by the supplier or through a special order, conduit sizes are typically 4m in length for diameters up to 63mm, and 6m in length for diameters greater than 63mm.
Maximum number of cables in a conduit
The maximum number of cables (N), that can be installed in a conduit in given by the formula;
- For one cable in a conduit = 0.5
- For two cables in a conduit = 0.33
- For three or more cables in conduit = 0.4
The value of the space factor provided here is general. Remember, the space factor is used to restrict the number of cables that can be packed into a conduit.
This is done primarily to control the temperature increase within the conduit, as the ampacity of a cable is dependent on temperature.
It also ensures that cables can be pulled in and out without requiring excessive force.
Therefore, the space factor vary depending on the length of the run and the number of bends involved.
Conduits in underground trenchs
Care should be taken when installing conduits underground. Conduits should be installed in smooth trenches without sharp edges that can damage the conduit.
Backfill material that contains large rocks, paving material, cinders, or sharply angled substances such as rocks or site debris shall not be used.
As a minimum, conduits containing LV cables should be buried to a depth of 500mm. A typical trench where a Power conduit shares a trench with a Telecommunications conduit is as shown below.
Cable ladders
The next cable pathway we will discuss is cable ladders. Cable ladders are a type of cable raceway used in electrical installations.
As the name suggests, they are essentially open structures that resemble ladders, designed to support and organize electrical cables in a safe and secure manner.
These ladders are typically made of metal (such as steel or aluminum) and consist of horizontal rungs connected by vertical side rails.
Standard ladder widths are 150mm, 300mm, 450mm, 600mm and 900mm, representing the inside dimension between side-rails and serving as the maximum width available for carrying cables. Straight lengths measure 6 meters
Cable Ladders or Cable Trays?
Cable trays and cable ladders are both support systems used in industrial and construction settings to manage electrical and data cables.
Cable trays are flat, open-bottomed structures resembling trays or baskets, while cable ladders consist of side rails with rungs in between, resembling a ladder.
Cable trays are easier to install and provide easier access to individual cables for maintenance.
They have a lower load-bearing capacity and offer less protection against environmental factors.
Cable ladders, on the other hand, can support heavier loads, provide better protection against dust and moisture, but are slightly more complex to install.
In this article we will be referring to cable ladders but most pf the things discussed here are equally applicable to cable trays.
Cable ladder Load Capacity
Because the cable density remains fairly constant in a total installation, the widest ladders carry the most load, and each smaller width carries proportionately less load.
However, the load carrying capacity of any class of ladder is independent of the width.
Calculation Example
Let’s look at a quick example of sizing a cable ladder for load-bearing;
A NEMA 1 cable ladder, available in standard sizes ranging from 150mm to 900mm, provides load-bearing information as shown in the top image.
Additionally, we intend to install Prysmian single core firestop FS110 0.6/1kV cables, with cable data as displayed in the second image, on this tray.
We aim to check if its feasible to installing ten 50m x 70mm cables in the 3m span type NEMA 1 cable tray. Please calculate this and send me an email if you believe you have the correct solution.
Cable ladder spans
Cable ladders will need to be supported at regular intervals as they traverse a building or any structure.
It’s common for the building’s structure to determine the span for supporting the cable ladders, but there’s still some room for choice.
If the available supports are spaced farther apart, the cable ladder might need to be slightly loaded to stay within its deflection limits, a topic we’ll discuss later.
Additionally, it’s possible to use two 300mm wide ladders side by side instead of one 600mm wide ladder.
This allows for the selection of a lighter ladder category for the entire project. However, it’s often inconvenient to use more than one ladder category in the same installation
Cable Laying Depth
Different makes and models of cable ladders may have varying cable laying depths. As a general rule, the shallower the ladder, the lower the cost per meter, but it may require more frequent support.
Keep this in mind for your next design or installation. Choosing a cheaper cable ladder might ultimately become more expensive due to the need for additional support and the extra labor hours required to install those supports.
Material of Construction Selection
Cable ladder material of construction is also an important step as part of selecting the right cable ladder for the job.
Cable ladders are made from different materials each with its different applicable environments and costs.
Here is a quick rundown that will help you select the next cable ladder for your project;
Galvanised Steel
- Cost-effective and suitable for most outdoor conditions.
- Expected life of about ten years in industrial settings. However, sensitivity to certain chemicals and weather conditions may influence lifespan.
Aluminium
- Although more expensive than galvanised steel, it excels in marine environments with salt exposure.
- Offers a long maintenance-free life and is lighter and easier to handle, leading to quicker installation.
- Typical lifespan exceeds twenty years in industrial or marine applications.
Stainless Steel
- Considered for environments demanding extremely high corrosion resistance, reliability, and where servicing after installation is challenging. An example includes offshore oil drilling platforms.
Cable Ladders Equipontential Bonding and Protective earthing.
According to the AS/NZS3000, conductive conduit, tube, pipe, trunking, and similar wiring enclosures, conductive sheaths, armours, and screens of cables, conductive framework used for mounting electrical equipment, and catenary wires for the support of cables are all considered part of protective earthing.
Also, joins and the fixed and hinged components of the framework are not considered good earthing joints unless connected by means of a flexible protective earthing conductor.
This means all joins in cable ladders should be connected by appropriately sized flexible protective earthing conductors.
Cable Pits
Cable pits are placed at regular intervals between 50 and 100 meters on cable routes to allow ease of pulling cables through.
They are also placed at points where there is a change of direction or elevation in the cable route, intersection points with other cable routes, or planned future projects.
When sizing and selecting cable pits, it is important to consider the size and type of cables that will be installed, as well as the need for future expansion.
Cable pits should also be large enough to allow for easy access to cables and other ground services.
In Australia, there is currently no standard specifically addressing pit performance.
Pits are selected based on their intended use, which in turn determines their size and location. Pit covers must be capable of withstanding traffic loads.
In most cases, the standard AS3996, which covers loading classes, is utilized to ensure the appropriate selection of pit covers. Here is a bit of information to help you select your cable pit cover.
Light Duty Pit Cover (L)
These are for use on footways where it is possible for light vehicles and animals to mount the footway e.g. tractors and livestock.
Medium Duty Pit Cover (M)
These are for use in pedestrian areas and occasionally places open to vehicles not exceeding 3.7 tonnes loaded.
Heavy Duty Pit Cover (H)
These are for use in carriageways open to vehicles with loads not exceeding 7 tonnes.
Rounding Up
That’s all we have for you today on this topic. If you’ve learnt something new from this post, you might also be interested in other articles we offer on related subjects available on our blog’s homepage.
While you’re here, take a moment to explore our collection of electrical engineering calculators. Cheers!