De-rating a ring circuit

[KeenPensioner]

Hi, I'm not an electrician but have worked as a mate and I'm studying in my own time - I know my limitations. I watch a lot of videos (John Ward and others) and have come across an issue I don't understand. An 80A 30mA RCCD has 4 Type B 32A MCBs hanging off it and the sparky (

) talks about "de-rating" the rings to 20A to "free up space". I understand how maximum demand and selectivity works but can someone please explain what this experienced and competent sparky is trying to do? How can you de-rate a circuit without removing a load?

 

[Sintra]

His thinking is probably that the 4x32A mcb’s will total 128A if fully loaded. This would then be above the 80A rating of the RCD. I would be more concerned about the lack of additional protection on the shower room lighting.

 

[Deleted account]

I don’t understand how deratong a 32A to A 20A would free up space.
Maybe by derating he thinks he can use more amp age else where. This is not true.

Yes it would limit what load you can put on the circuit but in reality you would plug in exactly the same appliances with exactly the same load.

Ive written this post without looking at video. I’ll watch it now to see if it sheds any light.

 

[Wilko]

Hi - this is the virtue of using RCBO or RCD with the same current rating as the main fuse .

 

[Deleted account]

Just watched it.

Yes derating those Mcb’s will not magically mean less load.
A pointless exercise.

Well spotted keen pensioner.

What I will say Is having all those sockets on one residual current device is not good design. Saying that, it was acceptable when the board was originally installed but not now.
To then decide to put a shower circuit on the same residual current device, that is not good design to the current regs.

 

[FatAlan]

I have watched the video but can’t remember or be bothered to watch again. Did he de-rate the 4x32a and then add an additional MCB for the shower circuit?

 

[Sintra]

I have watched the video but can’t remember or be bothered to watch again. Did he de-rate the 4x32a and then add an additional MCB for the shower circuit?

Yes I think that was his plan.

 

[Ian1981]

Perhaps he’s making sure that the rccb has the required overload protection in relation to the main fuse and not relying solely on the final circuit diversity factors which is now a requirement.

 

[EricMark]

When working with discontinued products there is always a problem working out what to do, looking at TLC website both the MCB and the RCD are discontinued. One would need to contact Contactum to find if there is a replacement.

I know we all tend to simply look for some thing that will fit, however the reason it's called a consumer unit, not distribution unit is it's type tested, so we are not permitted to fit anything which was not type tested with the unit, I have seen it done many times, but would not make a video showing it being done.

So options are a consumer unit which is up to date, with a MCB/RCD/RBCO which is designed for the unit, be it on henley blocks as separate unit, or complete change, or combine some ring circuits and use the existing 32A MCB which means a shower that uses under 32 amp.

I know it's a pain to change the consumer unit, but that was really the only way forward, anything else is a bodge. Likely I would also bodge, however I would not make a video of the bodge.

It does not show the DNO fuse, if the DNO fuse is 80 amp or less, then there is nothing wrong with a 80 amp RCD being fitted, this house has a 60 amp DNO fuse, and twin fuse box/consumer unit so some thing like 20 ways. Power use has dropped with LED bulbs and induction hobs etc.

But it has never been correct to fit a 80A RCD with 4 x 32A MCB's with a supply over 80A, however what one does not know is was the supply fuse over 80A when the board was fitted? So you have no idea who was at fault.

 

[westward10]

Perhaps he’s making sure that the rccb has the required overload protection in relation to the main fuse and not relying solely on the final circuit diversity factors which is now a requirement.

At least you will be able to spot where he has been all the ring finals will be 20A.

 

[EricMark]

I have down rated a ring to 20A because there was a break in the ring somewhere, and also a radial which had 4 mm for most of the run, then some one seems to have thought it was a ring, and extended with 2.5 mm. But in both cases it was a temporary measure until fault could be corrected, and I did not make a video of doing it.

 

[Risteard]

Perhaps he’s making sure that the rccb has the required overload protection in relation to the main fuse and not relying solely on the final circuit diversity factors which is now a requirement.

It seems from comments he has written below the video that he believes that maximum demand including diversity needs to be assessed from the very basic guidance in the On-Site Guide. As correctly pointed out by many it will have absolutely no effect on the actual load connected.

Replacing the RCCB may have been more sensible depending on the rating of it, but I suspect they're all obsolete devices anyway which will be practically impossible (or hugely expensive) to source.

Probably another DB for the new work might have been more sensible.

 

[EricMark]

Replacing the RCCB may have been more sensible depending on the rating of it, but I suspect they're all obsolete devices anyway which will be practically impossible (or hugely expensive) to source.

Probably another DB for the new work might have been more sensible.

I hunted for a 100A rated RCD but could only find 80A, so would assume never made? I also looked for 40A MCB and yes they were made, but there was already 128 amp worth of MCB's on that RCD, I admit unlikely to draw 80 amp never mind 100 amp if that's the incoming fuse size, but however you juggle it, to get below 80 amp would mean 40 amp shared between the 4 rings, so 10A each which would be silly.

So only option would be to combine all 4 ring finals, this seems wrong to my mind, there must have been a reason to have 4 circuits to start with, so to combine seems wrong. Even two final ring circuits would be over the 100A when combined with the shower.

In real terms it is unlikely that there would be any problem leaving the 80A RCD in and simply adding a 40A MCB, it is unlikely you would exceed the 80A and even if you did likely it would take the 100A limit of incoming fuse.

However this is the major problem when making a video and sticking it on line. You have to follow the rules, be it the RCD size or route of cables from socket not being in safe zones, does not matter if unlikely to cause a problem, you don't publish videos showing the errors. Personally I would not clip the cables so if a screw or nail was put in, the cable was free to move out of the way, but that's not the right way either.

 

[tedjrr]

I'm certainly not an electrician but have always had a fear over 32A MCB protection on domestic ring mains. I'm in the process of replacing MCBs with RCBOs, as a general improvement. There are two ring mains in the house, one goes to all the domestic bits and covers most of the house, and the other just supports the kitchen where the washing and washing-up machines are. The first is all 2.5mm t&e, with all the sockets actually on the ring. The second is much shorter, has fewer sockets, and again, no spurs. So I'd propose to de-rate the first to a 20A RCBO, keeping the second rated at 32A. This makes sense to me as the second ring main has the larger loads and things like a fridge and freezer.

Am I missing some logic?

 

[DPG]

I'm certainly not an electrician but have always had a fear over 32A MCB protection on domestic ring mains. I'm in the process of replacing MCBs with RCBOs, as a general improvement. There are two ring mains in the house, one goes to all the domestic bits and covers most of the house, and the other just supports the kitchen where the washing and washing-up machines are. The first is all 2.5mm t&e, with all the sockets actually on the ring. The second is much shorter, has fewer sockets, and again, no spurs. So I'd propose to de-rate the first to a 20A RCBO, keeping the second rated at 32A. This makes sense to me as the second ring main has the larger loads and things like a fridge and freezer.

Am I missing some logic?

What is your reasoning for dropping the MCB size of the ring?