New Garage Circuit

[rcampbell]

Hello wise ones! My 1st post so please be gentle

Before I beg you for your advice i'll give you a quick resume. Was in IT for 20 years and then made redundant in April. Decided then to follow my boyhood dream and become an electrician.. A hunt around for an electrician's mate position did not bear any fruit so I decided to go down the paper electrician's route. Not much else I could do. So, I have now passed the 2330 level 2, 17th regs, Part P and PAT testing exams. 2330 level 3 training starts in 1 week which will be finished by xmas. Done some electrical work here and there as well. So, my soul having been revealed, here are my questions.

For my Elecsa part P assessment, I intend to install a new circuit in a detached garage, radial to twin socket, max usage 3kw and a single light. So, standard 2 way RCD board in the garage, 1 x 6A and 1 x 16A. The detached garage is only a doors width from the house. The total length for the cable from the garage to the main CU(split RCD) in the house is 18m. What I need some advice on is the following.

In regards to getting the cable over to the garage, i will have to go overhead. Is it acceptable to run the cable through some pvc conduit between the buildings or will i have to go with catenary? The reason i suggest conduit is that i will be running conduit round the building to get to the garage, so if i used conduit to bridge the buildings I could run the singles all the way to the board in the garage.

The rating of the MCB for this circuit on the main CU in the house. I have a spare 20A, will this be ok to use? Considering my design current for the circuit will not exceed 15A.

The csa of the cable.. 4mm single pvc has a 32A current carrying capacity so would be more than adequate for the job, however I was wondering if 2.5mm ( 24A current carrying capacity ) would be ok as it seems in theory it would. As design current (15A) will be less than the device current(20A) which will be less than the current carrying capacity (24A).

Look forward to hearing from you,cheers!

 

[Evans Electric]

Welcome Rcampbell , Good to hear from you , a 2.5 would be my choice , 16A or 20 A breaker . PVC conduit would also be fine , just wondering about the crossover to the garage , PVC not best for spanning a gap in mid-air on its own, suggest you fix a timber strut or simlar .

Or change to a short length of galvo, PVC would be drooping in the hot sun for a start without someone swinging on it.

Deke

 

[batty]

Welcome to the forum Roger opening a thread on seperate garages may not be a good idea exporting earths etc but we sure can help out.

 

[rcampbell]

Thanks Deke and Batty for your quick responses. Saggy PVC and pesky kids I had not thought about! Will look down the galvo route.

Enjoy the rest of your weekend (all 4 hours of it)

 

[PC Electrics]

Welcome.

Are you trying to copy me? I did exactly the same thing for my assessment job.

Have you considered using swa? I think swa would be more cost effective than conduit and singles because it will go up quicker. Also you can continue the swa run straight across a catenary wire with no breaks (PVC conduit would not be self supporting).

Also, for cost effectiveness, there is no need for a mini CU if the circuit is 20A at the CU. Simply come out of the CU with your swa core and straight to the first socket. Then on to your other sockets in t&e (unless you consider that the usage of the garage necessitates extra mechanical protection for the cables, in which case you could conduit the t&e up to the wall plate and then clip round). Fit an FCU somewhere with a 3A fuse for the lights.

You can get your RCD protection by either using an RCBO at the board or use an RCD socket in the garage or fit an RCD in a 2 module enclosure after the fcu but before the first socket (you could use a metal fcu and gland your swa into it). Using armoured gives these options because the cable itself does not require RCD protection.

If the cable is surface all the way and at a reasonable height, you could use 3 core H07RN-F. Edmundsons (amongst others) sell this by the metre.

Your suggestion was okay though except for the overhead pvc conduit. At 18m you'll be okay using 2.5 all the way.

 

[Lurch]

Only issue with the suggestions above is discrimination. If there is a 30mA RCD protecting the spare way or if you need to fit one to cover the cable from the house to the garage then the downstream one in the garage is effectively redundant.

 

[rcampbell]

Hi PC, thanks for the reply. Lots of food for thought! I originally discounted SWA due to the cost but I suppose once you start adding up the cost of singles and conduit etc.. Will get the calculator out and see whats what. Also, as you say, SWA wil be a lot quicker to install. Thanks!

Lurch, I was going to use a spare way in the main CU that is RCD protected. Does this then mean i do not need any local RCD protection in the garage itself?

Cheers, Richard

 

[batty]

Hi PC, thanks for the reply. Lots of food for thought! I originally discounted SWA due to the cost but I suppose once you start adding up the cost of singles and conduit etc.. Will get the calculator out and see whats what. Also, as you say, SWA wil be a lot quicker to install. Thanks!Lurch, I was going to use a spare way in the main CU that is RCD protected. Does this then mean i do not need any local RCD protection in the garage itself?

Cheers, Richard

I hate having RCD in house better to have it in garage less walking when testing, you will need to put double pole switch in house because if you have a fault in garage it will knock house out also. I personally would put small consumer unit with main switch in house as long as its not TT and have garage consumer unit in garage. When are we going to talk about earthing.

 

[a1spark]

Hi Richard ; I would go for Armoured cable supply with either.

1. Suitable cb at house c.u and 2-way RCD c.u. at garage....or..

2. Rcbo at house c.u and 2-way main isolator c.u. at garage.

What type of Earthing system has the house ?

Is there any extraneous conductive parts in the Garage ?

a1spark.

 

[Lurch]

Lurch, I was going to use a spare way in the main CU that is RCD protected. Does this then mean i do not need any local RCD protection in the garage itself?

That would be correct. However, it could be an inconvenience as it may be quite likely that power used for things in the garage would be likely to cause the RCD to trip. If the RCD is in the house and covers other circuits also it is an inconvenience due to it being in a separate building. I would try to arrange it so the RCD is at the garage end of the circuit, but I wouldn't get too hung up on it as either way is acceptable.

 

[PC Electrics]

In your second paragraph you said:

For my Elecsa part P assessment, I intend to install a new circuit in a detached garage, radial to twin socket, max usage 3kw and a single light. So, standard 2 way RCD board in the garage, 1 x 6A and 1 x 16A. The detached garage is only a doors width from the house. The total length for the cable from the garage to the main CU(split RCD) in the house is 18m. What I need some advice on is the following.

So I assumed from this that you were planning to use a non RCD way at the house. This would be prefered, to minimise inconvenience in the event of a fault, but if you're planning to use an RCD protected way then this is acceptable and you won't need another RCD in the garage.

If you do use an RCD protected way, be prepared to be asked why.

 

[Apache]

That would be correct. However, it could be an inconvenience as it may be quite likely that power used for things in the garage would be likely to cause the RCD to trip. If the RCD is in the house and covers other circuits also it is an inconvenience due to it being in a separate building. I would try to arrange it so the RCD is at the garage end of the circuit, but I wouldn't get too hung up on it as either way is acceptable.

But in reality an RCD should be like a car airbag - protection IF we need it but hope we don't? If you cut the cable once in 10 years on the lawnmower or hedge trimmer fine, but there is something badly wrong if you regularly trip the RCD!?

No?

Occasionally our 6A light MCB trips when a bulb goes, but can't think I have ever tripped an RCD under a fault. I do test mine a couple of times/year.

 

[a1spark]

I hate having RCD in house better to have it in garage less walking when testing, you will need to put double pole switch in house because if you have a fault in garage it will knock house out also. I personally would put small consumer unit with main switch in house as long as its not TT and have garage consumer unit in garage. When are we going to talk about earthing.

I just did Batty, but the OP has gone off-line. maybe he will return soon to discuss further.

a1spark.

 

[PC Electrics]

When was the last time you drove any car with only one airbag?

 

[Apache]

When was the last time you drove any car with only one airbag?

our old Polo has no airbags!

 

[Lurch]

But in reality an RCD should be like a car airbag - protection IF we need it but hope we don't? If you cut the cable once in 10 years on the lawnmower or hedge trimmer fine, but there is something badly wrong if you regularly trip the RCD!? No?

Yes, hence the preferable method if easily achievable, perfectly fine if the only RCD covering the garage is in the house though.

 

[m4tty]

Wonder what earthing system he's got there?

 

[ADS]

Wonder what earthing system he's got there?

Yeh, it's been a while since we've had a 200 post thread. :slap :innocent

 

[rcampbell]

Thanks all for your input on this. Very much appreciated.

Batty: earthing. I have a TN-S system. Is it ok to export the earth to the garage as its only 18m away? At that distance the (R1 + R2) should be fine?

A1spark: The garage has no extraneous conductive parts. Am now going to use SWA all the way, RCD protection in the garage and MCB in the house. One potential problem is running the SWA into the house CU. Its plastic and very flimsy. I do not think i will be able to terminate the SWA into it. Can i terminate the SWA into an adapter/junction box then run T&E into the CU? Similarly, at the other end into the garage CU. I assume this will be also plastic and flimsy as well. If I did do this, i.e. add some pvc cable into the run from the house to the garage, would I then need to RCD protect this run as i know SWA does not need to be RCD protected, but PVC does?

Richard

 

[PC Electrics]

Thanks all for your input on this. Very much appreciated.Batty: earthing. I have a TN-S system. Is it ok to export the earth to the garage as its only 18m away? (1) At that distance the (R1 + R2) should be fine?

A1spark: The garage has no extraneous conductive parts. Am now going to use SWA all the way, RCD protection in the garage and MCB in the house. One potential problem is running the SWA into the house CU. Its plastic and very flimsy. I do not think i will be able to terminate the SWA into it. Can i terminate the SWA into an adapter/junction box then run T&E into the CU? (2) Similarly, at the other end into the garage CU. I assume this will be also plastic and flimsy as well. (3) If I did do this, i.e. add some pvc cable into the run from the house to the garage, would I then need to RCD protect this run as i know SWA does not need to be RCD protected, but PVC does? (4)

Richard

(1) Carry on. It would be my recommendation to use 3 core swa.

(2)Obtain a 80x80 galv terminal box (like this one http://www.screwfix.com/prods/19806/Electrical-Supplies/Cable-Accessories/Junction-Boxes/Adaptable-Box-4-x-4-x-2). Strip the outer sheath and armour back about 600mm and place your gland here. The inner of the swa is rather like a heavy duty flex. Punch out 2 holes in the box opposite each other. Mount the galv box directly next to the cu and put a hole in the cu to line up with one of the holes of the box. Secure the gland into the box and pass the swa inner through the box directly into the cu and terminate in the usual way. Don't forget to earth the box (and hence the armour) with a fly lead. If aesthetics at the cu are a concern use one of these instead http://www.screwfix.com/prods/14556/Electrical-Supplies/Cable-Accessories/Junction-Boxes/IP65-Adaptable-Box-80-x-80-x-52mm, don't forget to use the banjo to earth the armour if you do.

(3) Use the same technique as (2) above at the garage end. Although you may be able to terminate the swa directly into the garage cu as these are often tougher, being thicker plastic (like this one http://www.screwfix.com/search.do;jsessionid=MRYB4DI4LG05KCSTHZOCFFI?_dyncharset=UTF-8&fh_search=33248)

(4) If you use the technique above you won't need to add in any t&e. In any case it is only buried cables (<50mm) that need RCDing, t&e surface mounted does not.