Brief Introduction
To professional engineers, the concept of cable ladders, cable trays should not be something new that need my lenghty and worthless explanation.To fresh graduate engineer, I quote wikipedia:
- In the electrical wiring of buildings, a cable tray system is used to support insulated electric cables used for power distribution and communication. Cable trays are used as an alternative to open wiring or electrical conduitsystems, and are commonly used for cable management in commercial and industrial construction. They are especially useful in situations where changes to a wiring system are anticipated, since new cables can be installed by laying them in the tray, instead of pulling them through a pipe
Considerations for sizing of cable tray / ladder
For most projects, you need to route and sizing the cable ladder system to satisfy the followings:
- It can physically house all the cables and deliver it to required location. Cables weight, cables Outer Diameter (OD) must be taken into consideration
- It have space for future expansion: one of the common practice in industrial setting is, instead of replacing, we lay new cables on top of old cables, thus, we need some space to carter for that. Normally, the project requirement is 20% spare space.
- Restriction of number of layers of cables laid: Some company restrict the number of layers of cable can be stacked, but it is often not applicable for instrument cable.
- Segregation between type of cables: This is more applicable when we do routing of ladder system:
- Minimum distance between Electrical ladder - Instrument/Telecom ladder should be 300mm
- Some companies request Intrinsically Safe to be segregated from Non Intrinsically Safe cables. Thus, a separate routing is required
- Some companies request A/B routing for safety system, following Norsok standard. This should be taken into consideration when we do routing.
Into sizing of cable tray / ladder
While there are many consideration for instrument cable tray system, the method to size it is actually pretty simple.
Cable tray/ ladders are categorized by width and height. Most project specified the height of the ladder/ tray, so that the entire system will fit with each other, while width can be adjusted by reducers / enlargers.
Common height:
50mm (tray only) / 100mm (tray and ladder) / 150mm (ladder). (some vendors go by inch, so it would be 2"/4"/6").
Common width:
Tray: 50-100-150-300-450-600-900 mm (2/4/6/12/18/24/36 inch)
Ladder 150-300-450-600-900 mm .(6/12/18/24/36 inch)
I have attached a sizing program from a vendor for this calculation. It's very comprehensive, so if you happen to need to size the ladder quickly, this should help tremendously. But when your client want to know how you size your cable tray/ladder and do the cable fill check, you need to do the previous step by yourself.
- Get a list of all the cable, and cable OD
- Get the cross section area of cable by square the cable OD
- Sum all the cable cross section area together
- Double the result.
- Add in the spare requirement.
- Pick the next largest cross section area for cable tray.
- Get the largest cable OD, and multiple it with 10 times minimum. That's the minimum bending radius you should pick.
- It's good habit to put cable weight in to consideration. However, most cable tray/ladder system are designed to take the heaviest load possible, therefore we can reasonable neglect it (don't blame me for it if it fails :)
Cable tray/ ladders are categorized by width and height. Most project specified the height of the ladder/ tray, so that the entire system will fit with each other, while width can be adjusted by reducers / enlargers.
Common height:
50mm (tray only) / 100mm (tray and ladder) / 150mm (ladder). (some vendors go by inch, so it would be 2"/4"/6").
Common width:
Tray: 50-100-150-300-450-600-900 mm (2/4/6/12/18/24/36 inch)
Ladder 150-300-450-600-900 mm .(6/12/18/24/36 inch)
I have attached a sizing program from a vendor for this calculation. It's very comprehensive, so if you happen to need to size the ladder quickly, this should help tremendously. But when your client want to know how you size your cable tray/ladder and do the cable fill check, you need to do the previous step by yourself.
cable-tray-sizing-program.xls |