Garage/Shed switchover, swa still suitable?

[trevw]

Hi,

During a recent extension I laid in 6mmsq 3 core swa from the CU under the stairs to the future location of my garage and 2.5mmsq 3 core to the expected shed location dug to required depth with tape etc. to ensure I was covered for supply

Unfortunately we decided to switch the two around. Obviously the shed should still be 'just about' ok if I decide to get that done.. But what are my options in the garage.

The 2.5 swa to the garage is under the floor for 9m, by my calculations of voltage drop it just about scrapes in at being okay for 22A, could I therefore have a new CU with 16/6A mcbs for ring/lights running off this cable? I am not expecting to use serious power tools in the garage but am concerned this would be easy to trip

If not, the garage is attached and there are sockets in the lounge where the ring could in theory be extended in to the garage, the SWA could then just be used for the light circuit..?

I have a last resort option that I would rather avoid due to cost and appearance - it may be possible to run a cable up into the loft from the CU then back down the outside of the house into the garage (possibly entering through soffit), total length of run at least 15m - I am assuming the cable down the outside of the house would ideally be SWA but could probably be terminated in the loft and connected to T+E?

any thoughts?

best regards

Trevor

 

[OwainDIYer]

The lighting load in a garage should be negligible provided you use fluorescent lights and not loads of 500W halogens.

Therefore the load will be down to the power tools you use. 20A should cope with any single domestic power tool being started, you can use a Type B MCB or even, if you can prove by calculation it still offers disconnect times, a Type C which will be slower to trip on motor start loads.

Install a battery backed up emergency light unit so if the 20A in the house does trip, you are not left in complete darkness with power tools spinning down.

Also note some fluorescent lights can have a strobe effect so rotating tools or engine parts may appear stationary.

 

[trevw]

So, are you saying that it is the responsibility of the mcb in the main CU to safeguard the swa cable, therefore you would look at a 20A mcb there and 6+16/20/32 in the garage to 'protect' the lighting and ring cable in the garage? The whole system would then tend to trip at the house in an overload situation?

the lighting will just be flourescent tubes.

 

[OwainDIYer]

So, are you saying that it is the responsibility of the mcb in the main CU to safeguard the swa cable, therefore you would look at a 20A mcb there and 6+16/20/32 in the garage to 'protect' the lighting and ring cable in the garage?

Yes. Probably a 16A in the garage for power.

The whole system would then tend to trip at the house in an overload situation?

Not if the MCBs have correct discrimination which is why you might need to use Type B or C on the 20A for the cable. You need to check the time/current curves for the particular devices.

If you have 16A MCB for power then if there's 22A load on the power circuit hopefully the 16A in the garage will trip before the 20A in the house.

However an emergency light might be useful 'just in case'

In terms of power use I forgot to mention - heating and kettle.

 

[DetlefSchmitz]

If you have 16A MCB for power then if there's 22A load on the power circuit hopefully the 16A in the garage will trip before the 20A in the house.

If a bulb blows and short circuits the lighting circuit, then either or both MCBs will trip. Discrimination between MCBs (to a short circuit) is almost impossible. Only a fuse at the feed end is likely to help.

 

[ban-all-sheds]

So, are you saying that it is the responsibility of the mcb in the main CU to safeguard the swa cable, therefore you would look at a 20A mcb there and 6+16/20/32 in the garage to 'protect' the lighting and ring cable in the garage? The whole system would then tend to trip at the house in an overload situation?

You seem to have taken it upon yourself to design electrical installations.

How much do you know about it?

 

[trevw]

Sorry, should probably have kept my ill informed musings to myself. Was just trying to understand the inital reply. Certainly know very little about discrimination etc. other than what I've picked up on this forum.

I'd be interested in your thoughts?