garage and offices

[dochisholm]

hi everyone i need to supply a 3 car garage with a couple of offices above the existing installtion is a state so would like to install from new. I would like to do a 40 amp single phase supply about 60 meters from an isolator at supply end running into a 5way consumer unit in garage. Couple of questions. Would 10mm be ok. Would you take the earth from the supply end or tt the installation at the garage and would you do split load unit or just all on one rcd as it is only in a garage office. Any help thank you and any product ideas for a 40 a dp isolator with decent ip rating thank you.

 

[swindon spark]

Surely you can do the cable calcs and confirm the correct size of cable yourself?

It would depend on how the cable is run, what load is expected, the actual length, volt drop etc.

If you are not competent to do this simple calculation should you be considering doing the job in the first place?

 

[i=p/u]

(In) eaual or greater than (Ib)
(It) equal or greater than (In) over ca,ci,cg,cc

 

[i=p/u]

ooops In equal or greater than Ib

 

[dochisholm]

thanks swindon spark and i have done calcs it comes up as 16mm cause of volt drop more concenrened about the earth would you take earth from supply on a third core or would you tt the installation at the other end

 

[Paul.M]

I would tt it if your not going to get adequate earth. The Ze may be ok once installed but in a couple of years time???

 

[Dean182]

What are you using to protect the cable at the supply end? MCB? Switch Fuse Isolater with BS 88 fuses? You need to know to determine whether the ZS is going to be acceptable and meet disconnection times. Work out what the R1+R2 of whatever cable size your using is going to be(use GN3 for resistivity values) and use the worst case scenario ZE value for the type of supply, add them together to give an idea of your Zs, and see if it complies with your protection device!

 

[Dean182]

Obviously need to do other calcs, volt drop, current carry capacity etc. Above comment is regarding the earth situation.

 

[SirKit Breaker]

thanks swindon spark and i have done calcs it comes up as 16mm cause of volt drop more concenrened about the earth would you take earth from supply on a third core or would you tt the installation at the other end

If you have done the calcs and come up with 16mm why would you want to fit 10mm?.
without seeing the install, i couldn't comment on the earthing arrangements.
I can understand you not wanting to get involved with the existing if it is a mess.
Your intention to use a board with an RCD incommer is fine, but i would consider fitting an emergency light or 2, just in case the RCD trips in the winter when it is dark during office hours. If the office is to have a lot of IT kit you will need to think a bit deeper due to earth leakage current, which RCD's are not too keen on.

Cheers........Howard