David Watts is the mastermind behind YouTube channel SparkyNinja, which offers unbiased help with electrical training, UK regulation and design, among many other topics. Here he discusses cable strapping.
Have a look at these completed electrical installations:
These photos were taken to show examples of good workmanship, which as we know is one of the fundamental requirements from BS 7671:2018 (134.1.1) and something we are often striving to find good examples of.
Good workmanship by one or more skilled or instructed persons and proper materials shall be used in the erection of the electrical installation.
One of the things I always observe fairly quickly when I see these new installations is the way all the line conductors and neutral conductors are cable tied together – most often to achieve that professional and good workmanship result.
But was this grouping together considered within the installations electrical design?
With electrical installations, sometimes the designer is also the person carrying out the construction and initial verification. Sometimes there is a separate designer, constructor and inspector within the business. Other times, these roles may all be carried out by different organisations altogether.
So how can we determine if this was considered by the designer or if it’s just the installers attempt at good workmanship actually being its undoing?
Well trying to keep this brief, let’s explain a couple of things.
Firstly, we need to understand that these limits of current-carrying-capacity are to allow the conductor to reach this limiting temperature but to then ensure that it does not go beyond that value.
This is because the conductor needs to be able to give-off the thermal energy created by the nature of the load. The more load, the more thermal energy created.
If we don’t limit a conductor’s load then we cannot limit the thermal energy, or temperature of which the conductor is allowed to reach. If it exceeds this limit then the insulation will become damaged and degraded.
For example, let’s assume a circuit has been designed for a 5kW 230V fixed load. The designer has calculated a 2.5mm² cable to be used through the normal means which I will summarise here:
Design current = Ib
Ib = P/V, so Ib = 5000/230 = 21.74 Amps
The wiring system is singles in conduit, cable choice is 70⁰ thermosetting (off the shelf stuff).
Referring to Table 4D1A of BS 7671 we can see that this cable type has a current-carrying-capacity of 24 Amps if we select 2.5mm² (don’t forget it’s a single phase circuit).
Now on inspecting this completed installation we notice that the distribution board has been installed like this:
This installation contains 20 final circuits and as you can see they have all been cable-tied together in a bunch to make the installation look a lot neater.
The negative effect of this is that all these conductors are now impacting each other’s limit of current-carrying-capacity. This is because they are imposing each other’s ability to release the thermal energy as they all reach their limiting temperature under load.
This illustration shows how the cables in the group greatly impact on each other’s ability to dissipate thermal energy
It should be noted that if any of these conductors are either off-peak, lower than 30% of the grouped rating or similar then they may not need to be considered as they are not impacting our circuit.
To approach this issue we refer to BS 7671:2018 Regulation 523.5, which further references Table 4C1 in Appendix 4 of the Standard.
Referencing this table we identify a rating factor of 0.38 needs to be applied to our design for cable selection
Applying this factor to one of the equations in Appendix 4 we adjust the required cable size.
It = Ib/Cg, so It = 21.74/0.38 = 57.2 Amps.
So with this in mind, referring back to table 4D1A, these cables should be increased to 16mm² to accommodate this grouping factor, as 16mm² has a current-carrying-capacity of 76 A, the very first value higher than 57.2 Amp!
Now obviously this would result in a very bad way to design this system and a great waste in resource and money to change the circuit from a 2.5mm² to a 16mm²…
So to conclude: STOP STRAPPING YOUR CABLES TOGETHER!!
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