Does this look ok?

[M15TRY]

Hi, new to the forum and no electrical qualifications or regular experience doing domestic electrical work. I recently purchased a newish build which is a few years old. It looks like the original house builder installed the main consumer unit and a secondary consumer unit in the garage as part of the original construction. Subsequently the old owner added a third consumer unit into a lean-to shed which sits on the back of the garage (although no part p cert in paperwork for this).

I’ve had a bit of a nosy around and I’m now contemplating whether I should call an electrician out to evaluate the circuits and if required do some kind of corrective work if it’s not sound. Before I waste both my time and that of fellow tradesmen I thought I would map out my set up and seek any advice (or simply put my worries to rest). Please see attached. My main concerns is loading to garage and the adequacy of MCB / RCD ratings.

Many thanks.

 

[David Prosser]

What do the electrical certificates say ?

 

[Dartlec]

There are calculations that can be done to determine the 'actual' loading, but it's late so I won't look them up now.

Couple of things that jump out though.

Are the RCDs all 30mA? If so you appear to have double/triple stacked RCDs of the same type, which is not a good idea as you can't guarantee which one will trip for any particular fault. If it's armoured cable from the house then it doesn't need to be RCD protected at the house end.

Why is there a gas cooker/oven in the garage? Or is it run from there because of a short run to the kitchen? How often is it used?

40A does seem a bit low for the feed to the garage with the car charger, appliances and oven, but in practise it may not be an issue.

The Shed seems fine - apart from the RCD issue.

It would help to know the approximate lengths of the feed cable, as that may affect any calculations.

I think there is enough there to be worth getting an electrician round to have a look.

 

[M15TRY]

What do the electrical certificates say ?

Not sure what I'm meant to be looking for over the several pages.... But I'm glad you asked this question as on inspection the garage installation isn't even on there.... Despite both house and garage CU being the same Eaton model and fitted by Clarkson Evens Ltd. Guess this is what happens when you buy a Bovis home. :-(

 

[pc1966]

Is the garage CU fedc oven really 13kW? That implies 56A!

Otherwise there is nothing that really stands out as dangerous, just that the feed to the garage CU could not support many of loads together, and you wont have selectivity between CU protection. I.e. in the event of a hard fault on, say, the shad 3kW heater it could conceivably trip house 40A => garage 20A => shed 16A all in one go, putting off the lights in both shed and garage.

This is a fundamental issue with MCB, as if you get enough current to trip the "instant" magnetic trip then quite often by time the lowest breaker has opened to clear the fault the higher up ones will already be past the point of no return and trip as well. Fuses are more forgiving if you have at least a 1.6:1 ratio between them they will normally be selective in isolating the down-stream fault.

 

[pc1966]

Of course, none of us can see the quality of workmanship, but if that is sound and has been tested then the selectivity and total load / possible diversity limits (above) are the main things I can see.

 

[M15TRY]

There are calculations that can be done to determine the 'actual' loading, but it's late so I won't look them up now.

Couple of things that jump out though.

Are the RCDs all 30mA? If so you appear to have double/triple stacked RCDs of the same type, which is not a good idea as you can't guarantee which one will trip for any particular fault. If it's armoured cable from the house then it doesn't need to be RCD protected at the house end.

Why is there a gas cooker/oven in the garage? Or is it run from there because of a short run to the kitchen? How often is it used?

40A does seem a bit low for the feed to the garage with the car charger, appliances and oven, but in practise it may not be an issue.

The Shed seems fine - apart from the RCD issue.

It would help to know the approximate lengths of the feed cable, as that may affect any calculations.

I think there is enough there to be worth getting an electrician round to have a look.

Thanks.

All three RCDs are 30mA. The armoured cable runs to the outside of the garage at floor level, then there is a small junction box outside the garage, out of which comes a grey un amoured cable running up the inside wall to the consumer unit which is above head height. I'm guessing the rcd in the house is there to protect that little bit of unprotected cable.

The old owner had a second kitchen installation done in the garage hence the cooker and appliances. Guessing they used to do some kind of catering.

As for feeds... Probably 20m of amoured cable from main CU to garage, plus 2.5m of unarmoured able as mentioned above. Then another 10m from garage CU to shed CU (all internal going through the loft of the garage and straight into the shed which is attached to the garage.

 

[jakew009]

2.5mm t&e is an interesting choice for feeding the 7kw car charger on a 32a breaker.
Are you sure it’s 2.5mm and not 4mm? Have you got a certificate for when that was fitted?

 

[pc1966]

2.5mm t&e is an interesting choice for feeding the 7kw car charger on a 32a breaker.
Are you sure it’s 2.5mm and not 4mm? Have you got a certificate for when that was fitted?

Well spotted! Yes, 32A is OK for 2.5mm ring but not a radial of that sort of load.

 

[Dartlec]

Thanks.

All three RCDs are 30mA. The armoured cable runs to the outside of the garage at floor level, then there is a small junction box outside the garage, out of which comes a grey un amoured cable running up the inside wall to the consumer unit which is above head height. I'm guessing the rcd in the house is there to protect that little bit of unprotected cable.

The old owner had a second kitchen installation done in the garage hence the cooker and appliances. Guessing they used to do some kind of catering.

As for feeds... Probably 20m of amoured cable from main CU to garage, plus 2.5m of unarmoured able as mentioned above. Then another 10m from garage CU to shed CU (all internal going through the loft of the garage and straight into the shed which is attached to the garage.

From some quick number crunching, the limitation on load may be the unarmoured bit of cable in the garage. If it's 10mm Twin and Earth that would take 64 Amps, while 10mm Armoured cable will take 85 Amps (making some assumptions about installation conditions).

Having said that, the chances of any overload being a genuine risk to the cable are slim, though it may not do the 40A MCB much good long term if it is consistently overloaded, but not enough to trip it.

In the real world, the practical loads can probably be considered quite a bit smaller. It is unlikely that the oven will be on overnight, when the car charger is likely to be, for example. And some of the other loads are likely to only be at full load intermittently.

The selectivity issue between RCDs may well be the biggest potential issue - where a fault in either the shed or the garage might trip any or all of the 3 RCDs, tripping out your house downstairs socket and lights. There is also the issue with MCBs tripping as mentioned by @pc1966

If you aren't using the garage oven at the same time as other loads, then the only potentially dangerous issue may be the car charger, if it really is 2.5mm on a 32A breaker. That could easily be solved by uprating the cable to 4mm.

If the concern of nuisance tripping bothers you, then the armoured cable could likely be removed from the RCD at the house end and protected with with a suitable MCB (63A perhaps), or even a suitable fused unit.

Does the certificate you have say what the size of the main fuse is? Likely to be 100A, but if all this is running off a 60A supply then someone was a bit optimistic (though in reality you'll almost never pop a suppliers main fuse with an overload)

For a more recent new build I'd have rather seen RCBOs used, but electrical installation in new builds rarely reflects the selling price in my experience.

It may be worth approaching Clarkson Evans direct for the certificate for the garage, for your records in future. Perhaps the house buyer paid them separately for the garage "catering" installation as they were on site, which is why it didn't come through the house builders pack.

 

[jakew009]

I’m guessing the car charger was either a DIY addition, or perhaps started life as a 3kw model and has been ‘upgraded’ to a 7kw one by swapping the breaker

 

[Dartlec]

Slightly off topic, but which (flowchart?) program did you use for the diagram @M15TRY? It's come out quite neatly