Garage wiring - CU with greater capacity than submain?

[ChrisZx]

Now that I've got permission from the council to start my garage wiring, there are a few questions I'm not sure about.

Background: For the CU would be simple for me buy one with one 32 and one 6A mcb and a 40A rcd already fitted. There is an existing submain going to an outhouse that I'm wondering if I can reuse (outhouse is being disconnected) as it will save me time, effort and money. It's on a 32A mcb with 4mm2 SWA. Was thinking of an internal junction box so I can extend this to reach the garage, 35m total. Online calculators tell me that voltage drop should be fine for a directly clipped installation and 32A. Nothing out of the ordinary being used in the garage so 32A supply is fine.

Questions: Can I install a CU with 40A rcd with 32A+6A mcb's on a 32A submain? Will the voltage drop actually be ok for this? How do I determine if the SWA armour has enough capacity to be used as the sole earth?

Only other question that I think I know the answer to but wanted to check, I don't have to enclose exposed wiring in conduit inside the garage do I? I think it's ugly stuff, I'd rather clip directly to the wall.

Thanks!

 

[333rocky333]

A 40 amp rcd is fine on a 32 amp supply, as you know 32 is less than its 40 amp rating.
However what confuses some people is that most Cu's only have 80 amp rated Rcd's and are suitable for 100amp supplies

Personally I think clipped cable is rough, depending on what cable your clipping it may need mechanical protection in a garage

 

[winston1]

Do you actually need a CU in the garage? Do you need an RCD there or is the submain already protected by the house RCD?
The SWA should be 3 core one of which is the earth. Don't rely on the armour.

In the garage you can wire sockets on 4mm cable or make a ring using 2.5mm. Garage lights via a switched FCU with 5 amp fuse using the FCU switch as a light switch.

Wiring does not need to be in conduit but is a good idea for the lower cables.

 

[333rocky333]

The SWA should be 3 core one of which is the earth. Don't rely on the armour.

Why

 

[JohnW2]

Why

That's another one which ought to be answered by reference to countless past discussions!

The short answer is that "opinions (often quite strongly polarised ones) vary" !

Kind Regards, John

 

[JohnW2]

However what confuses some people is that most Cu's only have 80 amp rated Rcd's and are suitable for 100amp supplies

It's not the theoretical capacity of 'the supply' which counts but, rather, the possible loads (i.e. currents potentially going through the RCD) which matter.

I would think that a shower, cooker and a few other things, if on the same RCD, could theoretically get one over 80A for short periods of time. I suspect that the 80A rating is a 'continuous current' one - any idea what the peak short-term rating is?

Kind Regards, John

 

[ChrisZx]

Do you actually need a CU in the garage? Do you need an RCD there or is the submain already protected by the house RCD?
The SWA should be 3 core one of which is the earth. Don't rely on the armour.

In the garage you can wire sockets on 4mm cable or make a ring using 2.5mm. Garage lights via a switched FCU with 5 amp fuse using the FCU switch as a light switch.

Wiring does not need to be in conduit but is a good idea for the lower cables.

Submain is not on an RCD and I don't really have space to put it on one. Maybe I could get away without a CU but at £30 I wouldn't be saving myself much!

Now I think of it, might be worth putting the sockets on an rcbo so if they go I'll be able to see the angle grinder I've got wedged up inside somewhere still spinning

 

[JohnW2]

Submain is not on an RCD and I don't really have space to put it on one. Maybe I could get away without a CU but at £30 I wouldn't be saving myself much!

To have an RCD only in the garage is, in fact, most people's idea of the best arrangement.

Now I think of it, might be worth putting the sockets on an rcbo so if they go I'll be able to see the angle grinder I've got wedged up inside somewhere still spinning

Separate RCBOs for both sockets and lighting would, indeed, be idea (for the reason you state).

Kind Regards, John

 

[John D v2.0]

theoretically get one over 80A for short periods of time. I suspect that the 80A rating is a 'continuous current' one - any idea what the peak short-term rating is?

I always assumed 80A or 63A was the breaking and making capacity.

 

[Taylortwocities]

Can I install a CU with 40A rcd with 32A+6A mcb's on a 32A submain?

You can, but it would be a bad design. Essentially the upstream protection device must be larger than the downstream one and should also meet the discrimination tables for the devices.

If you have a 32A MCB in the house and another in the garage CU then there's no telling which device would trip in the event of a fault.
It is very hard to discriminate using two MCBs and the better design is to have a FUSE at the source end with an MCB at the other.

 

[ChrisZx]

I

To have an RCD only in the garage is, in fact, most people's idea of the best arrangement.
Separate RCBOs for both sockets and lighting would, indeed, be idea (for the reason you state).

Kind Regards, John

I actually think lights should not be on an rcbo so if they go the sockets go as well! So in the angle grinders case it would stop spinning when I can no longer see it. Small ones lock on so you can hold a separate part of the body so could be dodgy if you can't see it to find the switch again.

 

[ChrisZx]

You can, but it would be a bad design. Essentially the upstream protection device must be larger than the downstream one and should also meet the discrimination tables for the devices.

If you have a 32A MCB in the house and another in the garage CU then there's no telling which device would trip in the event of a fault.
It is very hard to discriminate using two MCBs and the better design is to have a FUSE at the source end with an MCB at the other.

I get that, but it's not actually much of a problem in my case is it? Ultimately I don't really care which one trips, so long as one of them does!

But I guess I could put the sockets on a <26A rcbo then it's all gravy?

 

[Taylortwocities]

I guess I could put the sockets on a <26A rcbo then it's all gravy?

Even a 20A MCB won't discriminate against a 32A MCB. Look at the tables.
Discrimination and Back-up tables • Moulded Case Circuit ...

You could follow Winston's suggestion.
32A MCB in the house
The SWA terminating on an RCD as the incomer/ isolator at the garage end.
4mm² Radial round all the the sockets
FCU with a 3 or 5A fuse in it as the supply for lights.

Job done.

 

[JohnW2]

I actually think lights should not be on an rcbo so if they go the sockets go as well! So in the angle grinders case it would stop spinning when I can no longer see it. Small ones lock on so you can hold a separate part of the body so could be dodgy if you can't see it to find the switch again.

Whilst I understand your concern, I can't see how youi can have it both ways - in other words, I can't see (without the use of a relay or similar) how you could arrange that if the lighting circuit 'failed', the sockets one would be guaranteed to do so as well.

You could ger part of the way there (in terms of just earth leakage faults) by not having any RCBOs, but having both lighting and sockets protected by the same, single, RCD (with separate MCBs for lights and sockets) - but the downside of that would be that if the sockets power failed (due to RCD trip) then so would the lights. In any event, if the lighting circuit were to fail because the MCB tripped, the sockets circuit would then not die unless you had included a relay to bring that about.

Kind Regards, John

 

[John D v2.0]

Well, if you're going to be doing light critical tasks with angle grinder in there you need to invest in maintained lighting. Separation of circuits is to minimise inconvenience and I don't think they class loss of fingers as an inconvenience. If there's a power cut it'll all go off anyway unless it's backed up