Is that right?!

[WetBehindEars]

Hi, this is probably the wrong section.

I am an apprentice, and recently started with a firm. Just your basic installs so far.

The guy im working with does some questionable things, with one specific job in mind.

We have been asked to add a sub board in a garden garage, for basic power ring and lighting circuit to run TV, sky box,  PC etc (a mancave) there was already a cable running from the front of the house to the back supplying just one socket (40A breaker) but it got condemned (hence the request) my guy is saying it will be ok to use the same cable (4mm) up until the garden, then junction it into (4mm) SWA? Then add the sub board and run everything from that. Does the sub board not need its own earth point because he is saying the main board's earth is fine? And is it ok to chops and change cable half way through a run?

Im a little confused?

Thanks 

 

[Tony S]

It’s called a breach joint, done right it’s OK. The armouring and if a CPC is installed have to be contiguous throughout.

I’ve done these joints in mainly industrial situations. There’s nothing to stop this method of power supply so long as the OCPD is rated for the smallest cable installed.

 

[WetBehindEars]

Thanks Tony, that makes sense. So would the CPC cover the earthing from the sub board to the main board?

 

[ProDave]

A 4mm is a pretty weedy cable to feed a man cave.  I would be wanting something much more substantial.

It certainly should never have been fed from a 40A mcb and hope at least now it is on something very much smaller.

What does the customer say?  Have they been led to believe they are getting a good supply to the man cave?

 

[WetBehindEars]

Customer doesnt have a clue. I think the breaker is a 40A RCD to the sub, which the main breaker is 100A.

Not 100% sure on those. Will have to check tomorrow morning 

 

[ProDave]

I would like to see your sparks justifying a 40A cn on 4mm.   Ask him and tell us the answer. 

 

[Evans Electric]

You'd need to know the length of the run  to work out the volt drop .

Then decide on the design  current for the garage  / man cave .  

The stuff you describe would give a design current of about 6A   say ,  so a 4mm  SWA  is looking OK  but there may be heating  to include .  

Your guy would need to reduce the size of the breaker to anything  between    10A    16A  or 20A   say.    

If he fits an RCD  you could just wire to a couple of plugs and a 3A spur off to a light    .     In my opinion a DP isolator should be fitted at the house to isolate sub feeds leaving the building . 

So to re-cap  you need to start with the design load  and the length of run and do the sums .  The breaker that you worried about needs to be a size that  protects the cable.  Nothing wrong with jointing a cable  that  is already there  .  Why  is it condemned ?      

 

[ProDave]

I am guess int was "condemned" as it was an ordinary cable (t&e?) buried in the ground?

 

[apprentice87]

Nobody going to ask what earthing arrangements the house supply has...

john..

 

[Evans Electric]

I typed it then deleted it TBH  .   Thought if I put anything else down  I may as well  go there and do the job .  I can only assume that  Wetbehindears's   sparky is at least checking it out  along with de-rating the MCB .   

 

[Tony S]

Thanks Tony, that makes sense. So would the CPC cover the earthing from the sub board to the main board?

It depends on the situation, a three core SWA with one core as the CPC added to the armouring will in most cases be more than ample.

A 4mm² to my mind and as Dave said isn’t really big enough for a “man cave” because the load will grow exponentially. (Even worse if the wife finds it and gets settled in.)

One thing I’ve learnt in 45 years of industrial electrical work, never under estimate the power required!

 

[phil d]

I remember putting my shed up, only a couple of lights and sockets, and it's only a short distance from the house, it still ended up with a 10mm swa from the main board. Likewise I wired a garage for a bloke, his house had been the show house for the estate, there was a 2.5 swa feeding the garage, did my calcs and over the distance it was good for 16A, guy said it was fine, threw a few sockets in and a couple of lights, "the sockets are only for a radio and a power drill" he assured me. The next thing was he rings up, "I've decided to turn my garage into a mini gym, a couple of electric keep fit machines and a tv, total load is about 5 amps, how many heaters can I run?" at this point I lost the plot, "just plug in whatever you want, when the breaker trips, you've reached your limit". I'm sick of people who think they can power a full estate on a piece of wet string!  

 

[ProDave]

I did one like that. The cable was already in place to the garage the other side of the road. He had molled under the road and put in a nice bit or purple 2.5 street lighting SWA Then tarmaced the drive from the road to the garage over the top of it, so clearly he was not going to change the cable.

Like you I put it all on a 16A mcb and waned not to "expect too much" of it.

 

[phil d]

I did one like that. The cable was already in place to the garage the other side of the road. He had molled under the road and put in a nice bit or purple 2.5 street lighting SWA Then tarmaced the drive from the road to the garage over the top of it, so clearly he was not going to change the cable.

Like you I put it all on a 16A mcb and waned not to "expect too much" of it.

It's all you can do, had another one tonight, several years ago a guy had a shed in his garden, wanted a single light and a socket for the lawn mower, I spurred off the ring through a switched fused spur and ran a 2.5 armoured the short distance to the shed. then last year he built a bigger shed and added another 4 sockets, he's been on the phone tonight, can I add some more sockets because he's built a summer house, " we can just feed off the shed can't we". It was more a statement than a question, to be honest, I told him with what he's doing now he really needs a 10mm from the board.He nearly had a coronary when I told him what the job would cost, then started questioning if it was really necessary, I gave up!