UK Impact on cable rating if trunking used to consumer unit

[Alan Beard]

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?

 

[Spoon]

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'?

 

[Alan Beard]

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.

 

[Pretty Mouth]

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)

Typo? 20A is suitable for a ring fella 433.1.204

 

[Pretty Mouth]

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

 

[Spoon]

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.

 

[Spoon]

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. ?

 

[Pretty Mouth]

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.

You and me both mate, it's actually 23A ?

 

[Alan Beard]

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.

 

[Pretty Mouth]

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.