Garage supply

[JayBowden]

I was wondering if someone could help me. Im looking to install a db in my garage and i have 16mm 3 core sitting here, I understand its a bit ott. My issue is my DNO fuse is rated at 60A and i have a dual split RCD consumer unit which are both 63A. I take it I cant put a 63A mcb in that board and use the 16mm 3 core?

 

[sparkwright]

You would be more likely to use a smaller MCB, but can still use the larger cable.

What kind of things are you intending to run in the garage?

It would be a better design to fit a standalone switch fuse unit in the house, done by splitting the tails with a Henley or ISCO block, assuming the cable you are running to the garage is SWA.

And fit RCD protection in the garage consumer unit.

That way, a garage fault won't affect the house.

Alternatively, fitting a whole new consumer unit with RCBOs in the house may also solve the RCD problem.

 

[JayBowden]

Its only going to be power tools, washing machine and tumble dryer. There is a spare 40A mcb in the board that is rcd protected, could that be a choice?

 

[ebee]

Well it could be.
However you need to consider that a trip of that RCD for another reason will also trip out the garage (including lighting) and vice versa. How would that cause inconvenience/safety issues?
Putting it on its own RCBO from the consumer unit or a standalone switchfuse is an improvement.
Atually putting it on a non RCD supply from the consumer unit or on its own switchfuse is an improvement too in that respect, then make every circuit in the garage be on different RCBOs.

If you are in England or Wales then it comes under Part P so you must notify Local Building Control prior starting which includes further expense and possibly some difficulties too.
Or you can use the services of a registered competent person who might or might not allow you to assist in the works under their direct guidance and may or may not allow you to supply your own materials.
Or you can use a third party certifier (usually a competent person scheme member who has additional insurance and should have your job under regular surveillance, not just at the end of it).

So doing it yourself might give disadvantages in time and money.

 

[ericmark]

63 amp MCB? OK the old LoadMaster had 70 amp MCB's but most makes you will be lucky to find over a 45 amp. As to why you want so much power not sure? We tend to have 6, 16, 20, 32, 40 amp MCB's there are other sizes, but a supply to sockets is rarely over 32 amp, so supply to garage at 40 amp would allow a 32 amp ring final and lighting without risk that 40 amp will trip.

But in the main one only uses one power tool at a time, so a 16 amp supply is normally ample. But for some reason fitting a consumer unit needs notifying, and a FCU does not, so easy and cheap is 13 amp fuse in a FCU in house, and a 3 amp fuse in a switched FCU in garage for lights, and direct for a couple of sockets. Over that and one adds over £100 to cost, due to need to notify.

 

[JohnW2]

63 amp MCB? OK the old LoadMaster had 70 amp MCB's but most makes you will be lucky to find over a 45 amp.

A quick look at TLC indicates that they offer 50A MCBs for all the brands they deal with, and 63A ones for Contactum, Hager and MK - so not that rare, but, as you say, whether or not the OP needs one is a different matter.

 

[ebee]

So long as 1/ Overload, 2/ Short circuit & 3/ Earth fault are dealt with in an appropriate manner for the cables as installed we might just decide on the most readily available/cheapest Over current protective device.

 

[JohnW2]

So long as 1/ Overload, 2/ Short circuit & 3/ Earth fault are dealt with in an appropriate manner for the cables as installed we might just decide on the most readily available/cheapest Over current protective device.

Indeed - provided, of course, the OPD has an adequate rating for the anticipated loads.

I think your (2) is probably redundant. Since no cable has a neutral with a CSA less than that of the CPC, if the circuit's 'maximum Zs' is satisfied [i.e. your (3) ], then (2) would also inevitably be satisfied.

Kind Regards, John

 

[trojanhawrs]

Indeed - provided, of course, the OPD has an adequate rating for the anticipated loads.

I think your (2) is probably redundant. Since no cable has a neutral with a CSA less than that of the CPC, if the circuit's 'maximum Zs' is satisfied [i.e. your (3) ], then (2) would also inevitably be satisfied.

Kind Regards, John

An SWA where the armour is used as CPC would in some scenarios.

I was wondering if someone could help me. Im looking to install a db in my garage and i have 16mm 3 core sitting here, I understand its a bit ott. My issue is my DNO fuse is rated at 60A and i have a dual split RCD consumer unit which are both 63A. I take it I cant put a 63A mcb in that board and use the 16mm 3 core?

I would feed the garage with a 40A mcb from your board without RCD protection, and put an RCD in the garage board to protect the circuits (or individual RCBOs if you prefer). The SWA should be properly glanded and earthed at both ends preferably. If you already have the cable then fair enough, 16mm is a pig to work with though and compromises may have to be made in order to terminate at either end e.g. junction box before consumer unit or surface run cable etc etc. which may negate the cost saving of a smaller cable that may be suitable. The other loads in your house would have to be considered but there's a high chance the electrician would be able to make accommodation with diversity calcs.

 

[trojanhawrs]

Another consideration is the construction of your garage and whether it is detached. If it is and has a concrete floor or is made of metal it's important to know what type of supply you have to determine whether it is safe to export the earth.

 

[JohnW2]

An SWA where the armour is used as CPC would in some scenarios.

That's true, although I'm not sure that uis of any precatical mportance ...

Unless I've missed it, I don't think that the regs impose any particularly requirements in relation to short-circuit protection, other than that the breaking capacity of the the protective device should be adequate to cope with the PSCC.

In a situation such as you postulate, in which PSCC may be less that PEFC, if the device is capable of breaking the PEFC [per eBee's consideration (3) ], then it would certainly be able to handle the PSCC. I therefore think that what I said iis probably still true - that if fault protection is adequate in all respects (including device breaking capacity), then short-circuit protection would also be adequate.

Am I wrong?

 

[trojanhawrs]

That's true, although I'm not sure that uis of any precatical mportance ...

Unless I've missed it, I don't think that the regs impose any particularly requirements in relation to short-circuit protection, other than that the breaking capacity of the the protective device should be adequate to cope with the PSCC.

In a situation such as you postulate, in which PSCC may be less that PEFC, if the device is capable of breaking the PEFC [per eBee's consideration (3) ], then it would certainly be able to handle the PSCC. I therefore think that what I said iis probably still true - that if fault protection is adequate in all respects (including device breaking capacity), then short-circuit protection would also be adequate.

Am I wrong?

You're right about that. The PSCC or the PEFC may be higher than the breaker is capable of handling though so both would need to be measured/calculated and a suitable breaker selected.

 

[JohnW2]

You're right about that. The PSCC or the PEFC may be higher than the breaker is capable of handling though so both would need to be measured/calculated and a suitable breaker selected.

Sure, the breaker has to be able to handle the PFC, which is the higher of the PEFR and PSCC. Under most circumstances, the PSCC will be the higher but, as you've said, if its SWA with armour used as CPC, it's possible that the PEFR could be the higher.

However, I think that this is all pretty academic. This is a DIY forum, hence relates almost exclusively to domestic installations, in which PFCs are pretty modest. It may perhaps be different in other parts of the country, but I cannot recall having personally seen a domestic installation PFC appreciably greater than 1 kA, yet I don't think you'll find breakers that cannot handle 6 kA.

 

[trojanhawrs]

Sure, the breaker has to be able to handle the PFC, which is the higher of the PEFR and PSCC. Under most circumstances, the PSCC will be the higher but, as you've said, if its SWA with armour used as CPC, it's possible that the PEFR could be the higher.

However, I think that this is all pretty academic. This is a DIY forum, hence relates almost exclusively to domestic installations, in which PFCs are pretty modest. It may perhaps be different in other parts of the country, but I cannot recall having personally seen a domestic installation PFC appreciably greater than 1 kA, yet I don't think you'll find breakers that cannot handle 6 kA.

Good chance the Ze will be high enough to cover you but assuming fed direct from source 10m of 16mm SWA would have a 10kA short circuit fault current, and slightly more EFC with the 3rd core used as CPC and the armour earthed which is standard practice, going from this table

 

[JohnW2]

Good chance the Ze will be high enough to cover you but assuming fed direct from source 10m of 16mm SWA would have a 10kA short circuit fault current ...

Sure - but, as you imply, it's the Ze that is the saving grace.

Even if the SWA were of negligible length, 10 kA would imply a Ze of 0.023Ω - which is far far lower than I've personally ever seen in a domestic installation. I think that around 0.1Ω is the lowest I've ever seen (and that very rarely) and that (with negligible SWA length) would only result in a PEFC of 2.3 kA - and, in that situation, with a TN-C-S supply the PSCC would obviously be the same.