New garden office, Cable selection and method...?

[srl-8]

Hello Guys,

I will be installing an electrical supply to a new garden office and I want to run my thoughts about my installation past you all, to see if I have got this right in term of cable selection and load, or if anyone can suggest another way of doing this…

The Office will use the following load, I have rounded all amounts up to be safe.

3 x 1.5 kw heaters = 4500w = 20Amps

8 x 50w down lights = 400w = 2 Amps

4 x computers = (allow 250w per machine) =1000w = 5 Amps

Possible additional usage (microwave/printer/phone charge/fan etc) allow approx 1000w = 5amps

TOTAL AMPS = 32



I plan to install a 10mm PVC multi-core cable from Fuse board, (Protected by a 40 or 45 amp MCB or RCBO) through the house, until outside, then change to 10mm SWA up to the new installation. (Approx 15 meters away)

A new garage board will be installed with rcd in the new office, protecting the relevant new circuits.

(It is yet to be decided if client wants to have a new main fuse board in the house or install a separate unit fed from the Henley block.. The old fuse board is re-wireable fuses and non-rcd. I believe the earth arrangement is PME at this stage.



I would like your opinions and suggestion please

Thanks!

 

[malcolmsanford]

Hello Guys,

I will be installing an electrical supply to a new garden office and I want to run my thoughts about my installation past you all, to see if I have got this right in term of cable selection and load, or if anyone can suggest another way of doing this…

The Office will use the following load, I have rounded all amounts up to be safe.

3 x 1.5 kw heaters = 4500w = 20Amps

8 x 50w down lights = 400w = 2 Amps

4 x computers = (allow 250w per machine) =1000w = 5 Amps

Possible additional usage (microwave/printer/phone charge/fan etc) allow approx 1000w = 5amps

TOTAL AMPS = 32



I plan to install a 10mm PVC multi-core cable from Fuse board, (Protected by a 40 or 45 amp MCB or RCBO) through the house, until outside, then change to 10mm SWA up to the new installation. (Approx 15 meters away) What type of cable would this be? would depending on the cable and erection method, need RCD additional protection. Is the CPC the same size as the LIVE conductors?

A new garage board will be installed with rcd in the new office, protecting the relevant new circuits. If the multi core cable in the house needs additional 30mA protection will you have an issue with RCDs in series

(It is yet to be decided if client wants to have a new main fuse board in the house or install a separate unit fed from the Henley block.. The old fuse board is re-wireable fuses and non-rcd. I believe the earth arrangement is PME at this stage.



I would like your opinions and suggestion please

Thanks!

Will this office have services, such as a bathroom and therefore a water supply, is the construction of the building metallic at all, What type of earthing system do you have?

 

[srl-8]

No no services at all, no water supply, Earth is unknow at this stage, I believe it is PME.. It is defo NOT TT.

 

[malcolmsanford]

ok and the other points about additional protection etc

 

[srl-8]

Do you mean, "burried cable etc" , what are you getting at malcolmsanford?

 

[srl-8]

I would probs install 100MA RCD at House end and 30Ma RCD at garage end. This should allow garage to trip first, and not the house RCD

 

[malcolmsanford]

The cable going through the house, what kind of cable is it, and if is buried anywhere will you need to give it RCD protection?

If it is T + E and 10mm your CPC will be 4mm and your SWA CPC will either 10mm extra core or the armour, will that 4mm if T +E be man enough?

 

[Andy78]

Is it feasible to take the SWA right back to the CU ?

 

[srl-8]

Is it feasible to take the SWA right back to the CU ?

No Andy, it is impossible to run the SWA back to CU, I will have to make a connection somewhere on the outside wall etc.

cable for inside the house will be same as typical shower cable 10mm twin & earth
Cable for outside 10mm SWA

 

[telectrix]

as above comments, and, if using 10mm T/E in the house, run a separate 10mm earth with it.

 

[srl-8]

Can someone please confirm or correct me, that my cable sizing is correct, based on my info in my first post. I dont have my books to hand, so cannot confirm 10mm cable rating....

 

[RISElectrical]

10mm cable is fine, it's the cpc of the 10mm that will be the problem as in twin and earth the cpc will not be 10mm. Hence why you were advised to run a separate 10mm cpc with the T+E or use 3 core swa for the whole cable run.

 

[Dave 85]

Out of interest, is that a reg requirement, the upgrading of the T+E earth?
I would imagine the 10mm T&E would prolly not be chased in a wall so wouldn't need RCD right?

I take it the office is a wooden shed type building?
I've done few of these and usually have the sockets fed from external black PVC conduit at the back of the shed and the lighting surface clipped T+E with conduit/trunking for the switch drops. Gives a nice balance of cheap/neat/protected.

 

[malcolmsanford]

Out of interest, is that a reg requirement, the upgrading of the T+E earth?
I would imagine the 10mm T&E would prolly not be chased in a wall so wouldn't need RCD right?

I take it the office is a wooden shed type building?
I've done few of these and usually have the sockets fed from external black PVC conduit at the back of the shed and the lighting surface clipped T+E with conduit/trunking for the switch drops. Gives a nice balance of cheap/neat/protected.

Upgrading of the T+E CPC would be required under regulation 543.1.1, if the CPC size was too small for the fault current.

To be honest you may find the 4mm CPC large enough, the only problem is that because it seems to be a TNC-S system, you may just want to do the adiabatic to make sure

 

[TPES]

To be honest you may find the 4mm CPC large enough,

this is what I would have thought.. I'd of thought 4mm would be enough to carry the fault current

 

[srl-8]

The 10mm will be run under floorboards in the house and the swa will be burried.
I think i will run 10mm 3 core swa, and this will give me the 10mm earth to be sure.

good advice

 

[Engineer54]

The 10mm will be run under floorboards in the house and the swa will be burried.
I think i will run 10mm 3 core swa, and this will give me the 10mm earth to be sure.

good advice

It won't, not if you are still going to use 10mm T&E....

 

[srl-8]

It won't, not if you are still going to use 10mm T&E....

Er.. why not ? why can you not run 10mm T& E under floorboards or have i miss understood??? Same as a shower circuit etc...

 

[RISElectrical]

Because the cpc in 10mm T+E isn't 10mm

 

[srl-8]

Oh right, yes of course, but if I ran 3 core + Earth I would have 3 x 10mm cores... right.

 

[malcolmsanford]

You won't be able to get a 3 core 10mm T+E though.

Can I ask if your an electrician?. Because as I mentioned in post #14 you may find the 4mm CPC is good enough, but you need to do the adiabatic equation to make sure

 

[RISElectrical]

3 core and earth? You would just need the 10mm T+E and then a separate 10mm earth. Basically just ensure that the size of the earth is always at 10mm for the whole run. If that makes sense