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Hi

My consumer unit is located in the garage, the cables are currently clipped to a solid wall above the unit. I am considering putting the cables in trunking to give them some more protection, the trunking would be just under a meter long.

I believe its common practice to use trunking in this situation but from my understanding of the regulations grouping factors need to be applied. For most cables this is fine as they have less than 30% load but I am concerned about the ring main supplying kitchen and utility.
The ring is 2.5mm protected by 32amp RCD in a split CU with RCD's. It's impractical to change the size of the cable and I am reluctant to use a 20A MCB as it could be quite heavily loaded for short periods; Kettle, washing machine, dishwasher, tumble dryer etc.
There will be other cables in the trunking that could have a reasonable load immersion heater on 2.5mm radial and 7.5 KW cooker on a 10mm radial circuit. Other ring just has TV's, lights and small items on. Lighting circuits are mainly LED now.

Please can it be confirmed that trunking is standard practice and how is it justified under the regulations?
 
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Welcome to the forum mate.

My consumer unit is located in the garage, the cables are currently clipped to a solid wall above the unit. I am considering putting the cables in trunking to give them some more protection, the trunking would be just under a meter long.

What type of damage do you think the cables may get?

I believe its common practice to use trunking in this situation but from my understanding of the regulations grouping factors need to be applied. For most cables this is fine as they have less than 30% load but I am concerned about the ring main supplying kitchen and utility.

This is true regarding the 30%, but if the cables are suitably spaces from one another then grouping factors do not apply.

The ring is 2.5mm protected by 32amp RCD in a split CU with RCD's. It's impractical to change the size of the cable and I am reluctant to use a 20A MCB as it could be quite heavily loaded for short periods; Kettle, washing machine, dishwasher, tumble dryer etc.

According to the regs, putting the 2.5mm in trunking will de-rate the cable to 20A, which is not suitable for a ring (FRC)

There will be other cables in the trunking that could have a reasonable load immersion heater on 2.5mm radial and 7.5 KW cooker on a 10mm radial circuit. Other ring just has TV's, lights and small items on. Lighting circuits are mainly LED now.

This will need looking at with regards to the difference in 'clipped direct' and 'in trunking' for the cable rating.

Can you not put a raised plate in front of the cables to protect them, then you still get the air flow, and they are still 'clipped direct'?
 
Welcome to the forum mate.



What type of damage do you think the cables may get?



This is true regarding the 30%, but if the cables are suitably spaces from one another then grouping factors do not apply.



According to the regs, putting the 2.5mm in trunking will de-rate the cable to 20A, which is not suitable for a ring (FRC)



This will need looking at with regards to the difference in 'clipped direct' and 'in trunking' for the cable rating.

Can you not put a raised plate in front of the cables to protect them, then you still get the air flow, and they are still 'clipped direct'?
Thanks for the welcome and reply.

Your statement about de-rating to 20A matches my understanding of the regs.

I am looking to protect them because the roof space above is being used for storage and the last 10cm of the cables could be knocked by things in storage. I do have some chipboard covering them at the moment but bit concerned its presumably quite flammable. Given the consumer unit is still plastic it could act as a chimney to take a fire from the consumer unit up through a wall and into the loft above the kitchen.

I can look at using a more fire resistant material but still very curious as to why using conduit appears to be common practice without taking in to account grouping factors, I have seen this done in houses and there are numerous you tube videos/forum posts from qualified electricians showing trunking combined with 2.5mm ring main and 32A MCB's.
 
Typo? 20A is suitable for a ring fella 433.1.204

No typo mate... just me being thick. Thanks for the correction.

Also, I believe this would be in ref. method B, so rated to 24A

I'm not going good today, I think i will go back to bed.. Totally mis-read the reg page.
 
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I am looking to protect them because the roof space above is being used for storage and the last 10cm of the cables could be knocked by things in storage. I do have some chipboard covering them at the moment but bit concerned its presumably quite flammable. Given the consumer unit is still plastic it could act as a chimney to take a fire from the consumer unit up through a wall and into the loft above the kitchen.

Personally I think there is nothing wrong with plastic consumer units. It depends on who has done the wiring to them that can cause fires.
If you are concerned about having a fire within your plastic CU then you can buy a fire suppression tube that fits into the top of the CU.
It's in one of the thread on here. I'll see if I can find it. Hope fully someone will beat me to it. My brain isn't working on all cylinders today. ?
 
Typo? 20A is suitable for a ring fella 433.1.204
Not my comment but for my situation at least the load in the ring at times could easily exceed 20A, I haven't done any analysis but given a 3KW kettle draws 13amps on it's own with 3 other fairly heavy loads (dishwasher, washing machine, tumble dryer) I am sure it will be well over 20A for short periods.
 
Not my comment but for my situation at least the load in the ring at times could easily exceed 20A, I haven't done any analysis but given a 3KW kettle draws 13amps on it's own with 3 other fairly heavy loads (dishwasher, washing machine, tumble dryer) I am sure it will be well over 20A for short periods.
We are at cross purposes fella. 20A is the minimum rating for the cables of a ring, if they are to be protected by a 32A breaker.
 
eachlegof the ring will happily withstand 20A. that gives you 40A at the mid-point. somewhat less closer to the ends if the ring. OCPD is 32A. no problem.
 
We are at cross purposes fella. 20A is the minimum rating for the cables of a ring, if they are to be protected by a 32A breaker.
Thanks this is a very useful clarification.

Also just realised there may be a more elegant approach to this, I could use perforated PVC cable tray with a cover which protects the cables, gives a more elegant mounting and also an improved rating for the cable.
 
Trunking from consumer unit appears to be standard practice with 2.5mm 32Amp ring mains. Given cable grouping rating factors how is this covered in the regulations?

In some cases, it probably doesn't comply. However, in typical domestic usage there is a generous margin on cable ratings so it doesn't cause any practical issues and is routinely ignored. The rating is based on a maximum working temperature for the conductor, often 70°C. Exceeding this temperature up to a certain limit doesn't immediately destroy the cable, it shortens its life by accelerating the reactions that degrade the insulation. But it is rare for cables in domestic situations to run near their Iz for more than a small fraction of their useful life.

Cable heating is proportional to the square of the load current. Take a 2.5mm T+E rated at 23A ref. method B without grouping. Running at 16A - half the ring MCB rating - that would only experience (16/23)² = 48% of the heat dissipation, and hence about half the temperature rise. We could tape a pair of them tightly together both carrying 16A and still be cooler than one running at 23A, due to having a larger surface area.

Take instead six of these cables (Cg = 0.57) grouped together which will have an Iz of 23 * 0.57 = 13A. Run them at 16A (three rings fully loaded to 32A) and the temperature rise might be as great as (16/13)² = 1.5 times higher than rated, i.e. the cable is significantly overheated and will have a shortened life. This involves a load current of 6 * 16 = 96A which might be more than your total supply, but if by good fortune you have a 100A cutout fuse, then you will clock up an electriciy bill of 96A * 230V * £0.15/kWh * 24 * 365 / 1000 = £29,000 per annum. If you use a mere £23,000 per annum with a continuous load of 18kW spread over three rings grouped together, the cables won't be overloaded and will live out their full design life.

If the same current is shared by a larger number of circuits grouped together, the temperature rise will be reduced because the parallel resistance of the cables is lower. The worst case is always the smallest number of cables that can be fully loaded with the available total current. For those who would point out the fusing factor of an MCB that allows those cables to run at 1.45 * In, remember that the cable rating Iz already took that into account. A cable with an Iz of 20A can be protected by a 60898 of 20A so the cable is actually rated to take 29A under abnormal conditions, but the design should avoid that occurring.

Tl, DR; Cables usually have an easy life in domestic installations.
 
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No typo mate... just me being thick. Thanks for the correction.



I'm not going good today, I think i will go back to bed.. Totally mis-read the reg page.

Even though I knew you were wrong you still gave me a massive brown trousers moment then for a second.?

Just installed 13 Rings in dado trunking..

Then thought, hang on this can't be true, surely??
 
Lucien Thanks for all the great explanation and calculations, which make things far clearer.
I have a much better understanding now and I feel far more comfortable that I can make a good decision now.
I have an electronics engineering/communications background and always feel uncomfortable doing things a certain just because they are standard practice without feeling sure it's also good practice.
 
@Alan Beard ... Found it.
That looks cool! Though I don't want the pointy end of 2 screws sticking out the front of my consumer unit! They could have used machine screws and nuts...
 

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