This is a blog post by Steven McFadyen, which first appeared on myElectrical.com, July 25th, 2011
A recurring theme I encounter is cable sizing. Now many installations are unique and require special consideration. However, a lot of the time cable selection is a repeated activity. When looking at low voltage power cables I generally always start with the same basic strategy.
- Default to using XLPE - why bother with other insulations (PVC, rubber, etc.). XLPE is well established, cost competitive and doesn't have any of the degradation or fire related issues of other insulations. You will also end up with a smaller cross sectional area. Only in special circumstances would you need to look at other installation types.
- Use armoured - buried cable mechanical protection is essential. For indoor cables, the use of armouring is not necessary. However, even indoors you have the benefit of using the armouring for the CPC. On indoor cables, choose armoured or not dependant on local practice.
- Use LSZH (low smoke zero halogen) sheath - smoke and toxic fumes in a fire situation are not good. Why not just avoid the issue.
- Calculate the current rating using an acceptable method. I tend to use the method given in BS 7671 as this is applicable where I work. Calculate the rating taking into account both the design current and protective device rating and apply the necessary derating factors.
- Calculate the voltage drop - again you can use BS 7671 and check it complies with local regulations. The voltage drop needs to be the sum of all cables in a circuit (from source to end load).
- Ensure the cable can take the fault level - for most larger cables this tends not to be a problem, but for smaller cables, it can be an issue.
- Use myCableEnginering to carry out the calculations - it makes life easier.
- Be practical - make sure your cable size is reasonable. If you end up with a 50 mm2 cable on a 2 A load due to meeting voltage drops or fault levels, start to look a the system design concept itself.
while you cannot say "once you have selected one cable you have selected all cables', you may be able to get away with saying "once you have selected a few cables you have selected most cables"
Finally, we need a disclaimer here. While the above is suitable for most situations (low voltage power), it does not cover every case. There are situations which are different, unique or require some special consideration. To evaluate these situations, one of the best things is to understand fully the characteristics of the load the cable will be supplying, the environment it is being installed in and be aware of other overriding issues. If you can do this, any necessary adjustments to the eight point plan often become apparent.