Split Suppy Advice Please

[Yorkie]

I need a separate supply for the ring main in my garage (where the CU is located), because whenever something trips the RCCD in the CU I have to wind up the garge door by hand to get to the CU and reset, which is a pain. It takes ages to find the winder handle, usually in the dark and I'm not getting any younger.

I'm trying to spec the job properly before getting a sparky on the job and I'm keen to know exactly what's what. TBH, the last chap who replaced the CU 7 years ago goosed me quite badly and it ain't going to happen again.

I've already sourced a separate 2 position CU with a 63A RCD, plus a 32A MCB and a 6A MCB for the Christmas lights, all for free. I was expecting to be asking for the incoming 25mm tails to be split with a Henley Block and then feed the old and new CU separately. But when I cracked open the new CU to check it out for mounting requirements, it looks like the incoming terminals are 10mm max. Not even a chance of putting 16mm in, which is the size coming out of the meter to the 2 pole isolator. It's then 25mm through the wall to the CU.

So what are the options? I could ask for it to be fed from the main CU via a 50A RCD (for which there is space on the main breaker), but the problem is if a light bulb blows it can take out the main 100A breaker, so I'd be no better off than I am now. It's a split CU, with all the rings on an RCD, all the lighting circuits just on the 100A breaker covering the whole box. It's usually a light bulb going pop that causes the problem.

Are we allowed to have a 10mm supply coming out of the Henley Block to the new CU? I don't know what size the main fuse in the supply box is, but the carrier is 100A. There's no need to crack it open for isolation and I'd rather not bother with all the hassle of getting the YEB in to find out and re-seal it. Something tells me that in the worst case, 100A fizzing down a (short) 10mm cable isn't going to be allowed, but I need some advice please. Even if it's allowed, it would only be single insulated so how to we ensure that it is properly safe?

Wha'd'ya think?

 

[telectrix]

all standard RCDs will accept 25mm cables. several forum members are in your area and i'm sure would be glad to do the job without ripping you off.

 

[Flanders]

sounds a lot of messing about with the parts you have Your best bet is a 3 way split load CU with 2 main RCDs and an RCBO for you garage socket this should keep nuisance tripping down to a minimum

 

[Gazthesparky]

If the fuse board is a duel RCD then I would have the garage supply taken off the RCD side and just put onto a normal MCB as long as the circuit isn't buried in the wall tHen it doesn't need a RCD covering the circuit


failing that as you say you could have another CU fitted via henely blocks and supply the garage from that but that seems a lot of work if you could reconfigure the existing set up

 

[Yorkie]

all standard RCDs will accept 25mm cables. several forum members are in your area and i'm sure would be glad to do the job without ripping you off.

True. But the neutral rail won't go bigger than 10mm, hence my question.

 

[Marvo]

There's usually at least one hole in the neutral bar that should accept a 16 or 25mm wire.

 

[Yorkie]

If the fuse board is a duel RCD then I would have the garage supply taken off the RCD side and just put onto a normal MCB as long as the circuit isn't buried in the wall tHen it doesn't need a RCD covering the circuit
failing that as you say you could have another CU fitted via henely blocks and supply the garage from that but that seems a lot of work if you could reconfigure the existing set up

No, unfortunately, although it was a quite expensive Square D 16 way unit, there's only one RCD and all the lighting circuits MCBs come directly off the 100A 2 pole breaker. I've checked and there isn't enough room to split it to give two RCDs, even though this is obviously safer and now quite common.

As it is at present, if a tungsten bulb goes pop, it can (and does) take out both the 6A MCB and the main 100A switch. Granted, in ten years time when we've changed everything to low energy and LCDs this won't be a hassle, but I need an interim solution.

Not sure if I'm explaining myself too well, but does it make sense?

 

[Flanders]

Don't under stand how it can take out the 100 amp switch as this is not a breaker unless it is an RCD ?

 

[Yorkie]

There's usually at least one hole in the neutral bar that should accept a 16 or 25mm wire.

No, honest, I've just checked again. It's a Wickes jobbie that someone gave me, so maybe I got what I deserved, but it still leaves me with a problem to solve.

I don't want to get a sparky on site then find out it's going to be a start again job and chuck away what I already have.

 

[Marvo]

I'd be worried if a popping filament lamp takes out a 100A main breaker to be honest.

 

[Gazthesparky]

Can you post a picture of the consumer unit as stated it carnt take out a 100amp switch

 

[Marvo]

Can you upload some photos to Imageshack or Photobucket of the existing CU from various angles?

 

[Yorkie]

Don't under stand how it can take out the 100 amp switch as this is not a breaker unless it is an RCD ?

Dunno. But sometime it does if we get a really, really bright flash when a bub goes. I thought the 100A was some sort of rating, can they get a bit weedy with age?

 

[Wiresandthings]

Id say the bright flash when the lamp goes is even more of a worry than a lamp supposedly taking out a 100a breaker!! It must be a 100a rated rcd as main switch as 100A main switches don't trip

 

[Marvo]

Switches don't trip when there's a fault they're purely a device for manually isolating the supply. MCB's and RCBO's trip to a fault. I assumed your switch is an MCB (circuit breaker) when you said it tripped.

 

[Yorkie]

Thanks for the replies so far. My son-in-law is a sparky (but industrial, not Part P registered) so I'll get him to pull the lid off tomorrow and I'll take some photos.

Thinking back, when the CU was fitted the brand new 100A breaker was duff, wouldn't latch, and he had to fix something up for a day or two until he got a replacement (hence the reason for me getting goosed and over charged for labour). Maybe the replacement wasn't too smart either? I'll think about having it swapped while we're at it, just to be sure.

The consensus seems to be that if the switch is okay, just get the new CU fed from an RCD in the main CU. If so, what size should I ask for, is 50A big enough?

 

[Marvo]

I think photos would go a long way in getting better advice. Surely if your sons a sparky you could persuade him to install an RCBO for your garage circuit and take it off the RCD. Threaten to torture him with some childhood photos....it always worked for me when my parents did.

 

[Yorkie]

Update............... I'm glad I'm not doing the job myself.... .. 'cos I'm a berk.

Of course the new CU will take 25mm tails - they go into the RCD, not in the neutral rail. Agghh!. So all's well.

But of course, if all is well in the main CU, is it best practice to split the tails and feed each CU separately, or just to take it out of the main CU on a big ol' RCD (not under cover of the RCD that's in there)?

I don't mind spending a few quid extra so long as it's as good as it gets.

 

[Yorkie]

I think photos would go a long way in getting better advice. Surely if your sons a sparky you could persuade him to install an RCBO for your garage circuit and take it off the RCD. Threaten to torture him with some childhood photos....it always worked for me when my parents did.

Always an option, but I thought is was notifable work and he can't Part P register it?

I keep threatening to sponsor him to get accreditation, but the problem is that business is so good there's never any time. In any case, if the cables aren't as thick as your finger he's not always too interested!