Sub-main query

[jonA]

Good morning!

Hoping to get some advice from someone more knowledgeable and experienced than I am.

I have a 100A PME supply to a garage.

I am providing a new sub-main to a new studio/workshop which is about 30m away from the garage CU. I have run the cable through underground ducting and already backfilled the trench. The cable is 16mm 3C SWA.

The plan was to EITHER split the meter tails and provide an independent supply via an 80A switch fuse OR fit an 80A MCCB to a spare way in the existing garage CU.

The studio will have a 100A DP isolating switch with all circuits protected by RCBOs.

The SWA cable has a maximum rating of 91A and I designed the installation on this basis. I realise now that the cable won't be able to take this load in practice and I think I will have to downgrade the design load from 80A to say 63A.

Needless to say I'd rather not do this unless it is strictly necessary.

Like I said, I'd be very grateful for some definitive advice as to what load the existing 16mm SWA cable can safely take.

Regards

J

 

[Evy]

i would say a 63 amp mcb in the exixting consumer unit would be a cheaper option. if you fitted a fused isolator you would have to consider the need of total isolation for the installation

 

[jonA]

Thanks Evy.

Yes, it would be cheaper and much easier too! However while 63A should be fine for now, I'd like to future proof the installation. Also with 80A protection in the garage I could provide discrimination with 63A in the studio.

My question really is, what is the maximum safe load for 30m of 16mm 3C XLPE SWA ducted underground?

Having laid the cable, I might as well design the installation for the maximum load possible.

Regards

J

 

[Flanders]

Some mccbs have an adjustable current ratting so you may be able to get a 100 amp mccb and turn in down to the max current capacity of the cable

 

[Flanders]

If you look im the back of the regs book it will give you the numbers you nead to do the cable calc

 

[Plonker 3]

Thanks Evy.

Yes, it would be cheaper and much easier too! However while 63A should be fine for now, I'd like to future proof the installation. Also with 80A protection in the garage I could provide discrimination with 63A in the studio.

My question really is, what is the maximum safe load for 30m of 16mm 3C XLPE SWA ducted underground?

Having laid the cable, I might as well design the installation for the maximum load possible.

Regards

J

Are you a electrician?

 

[Evy]

wouldnt go above 80amp fuse but you could tweak an mccb to get a little more after the cable calc has been done. discrimination like you have said needs to be considered

 

[Knobhead]

Are you a electrician?

Are either of them electricians?

The bind leading the blind.

 

[davesparks]

My question really is, what is the maximum safe load for 30m of 16mm 3C XLPE SWA ducted underground?

Having laid the cable, I might as well design the installation for the maximum load possible.

Regards

J

Quick, somebody shut the stable door, the horse has bolted!

 

[Knobhead]

Dave! I've got to clean the beer of my screen now!

 

[Octopus]

Are you a electrician?

Read his previous posts and the answer is contained within them.......

 

[Pete999]

I had a bad dream last night, and Ive just woken up to this nightmare.

 

[Knobhead]

Read his previous posts and the answer is contained within them.......

My apologies, I am not an electrician and am probably a bit out of my depth.

The DNO provided a TT supply to the garage because they deemed it to be a temporary builders supply. They said they would convert this to PME once the house was completed.

For some reason I can't quite fathom, I recall them saying that the house itself has to have a TT connection. I think they consider it to be a 'caravan' though it is no such thing! Something to do with equipotential zones?

Of course I'd rather rely on the DNO to provide the earth for both garage and house. But as I understand it, they will only do so for the garage once the house, which is on a submain, has a TT connection.

I might well have misunderstood all this and whether I have or not, I would very much appreciate your clarification!

Cheers, Jonathan

Cheers Murdoch

 

[SPARTYKUS]

If I was building a house, I'd build the whole thing first with any old bricks, plaster it and move in.

then and only then would I check that those bricks were any good.

 

[jonA]

blah, blah, blah ...

some of you really ought to get out more.