Supply to a garage with TT to house

[cbx040]

I need a to get some estimates for the wiring of a new garage, but I want to check what might me needed before I do.

The house is wiring is TT single phase and was been installed about 15 years ago.
The garage is 35m away from the CU.
I was thinking of a small radial and lights.

What I think needs to be quoted for is

Installing of 4mm or 6mm SWA earthed only at the house
A new ground spike installing at the garage

A 40A isolator without rcd on the incoming supply to protect the cable

A cu in the garage with a 32A mcb going to
6A mcb - lighting
16A rcbo - sockets

The cabling in the garage to be done radial with conduit or trunking and metal clad sockets.

The garage also has a metal outside tap supplied by a plastic pipe, this may need earth bonding to?

The supply can be isolated at the meter.
If the trench is already dug will this job need about 2 days labour?

Is there anything else required?

 

[AndrewDavie]

I had a very similar thing done.
The trench took me ages to dig (not very fit and several tree roots in the way!), but the electrician took less than half a day to do all that you mentioned.
I put 6mm SWA through plastic conduit (I put cat5 and fibreoptic in the conduit so I could put all my noisy servers down there), but apparently smaller would have been fine for 32A.

I doubt the outside tap will need to be bonded.
The interior metalwork of the garage was bonded.
I didn't get an additional earth spike, but that will depend on the quality of your earth?

I'd estimate you'll be paying #350-450 ballpark.

 

[cbx040]

Thanks for the reply Andrew.

I think it will need 6mm and another earth because of the distance.
fortunatly I know someone with a JCB and there is good access.

How many servers do you need?
Why did you run fibre and cat5?

 

[JonoPashley]

Thanks for the reply Andrew.

I think it will need 6mm and another earth because of the distance.
fortunatly I know someone with a JCB and there is good access.

How many servers do you need?
Why did you run fibre and cat5?

Dedicated Server line and interneal network at a guess

 

[RF Lighting]

As you have a TT supply, you will need to RCD protect the SWA supply at the house end and earth the armouring

This is best done by installing a 100mA type S RCD at the house end, and then a local 30mA RCBO at the shed end to provide protection for the sockets. This RBCO needs to be double pole. A standard SP RCBO is not permitted on a TT supply.

You will not need an earth rod at the shed end. There is no benifit from installing one.

You could install an erth tape in the trench if the reading from the existing rod is not adequate.

 

[cbx040]

Thanks RF

 

[AndrewDavie]

I think it will need 6mm and another earth because of the distance.
fortunatly I know someone with a JCB and there is good access.

How many servers do you need?
Why did you run fibre and cat5?

*calculates* Your maximum usage is 16+6=22A, so the voltage drop for 4mm over 35m is 9.24V (>6.083V allowed for lighting circuit), so yes, 6mm is required, where the voltage drop would fall to 6.083V.

I have fileserver, mail server and firewall. I used fibre for gigabit and cat5 as a 100Mb backup.

 

[ban-all-sheds]

Is there anything else required?

Yes.

A convincing explanation as to why you want to go into such incredible detail of what materials you need, and how to install them, when all you really have to do is to say to your electrician "I want sockets & lights in that garage".

 

[cbx040]

Sorry ban-all-sheds I missed your reply.

From my original post

"I need a to get some estimates for the wiring of a new garage, but I want to check what might me needed before I do. "

In the past I have dealt with some less than honest tradesmen.
So before I got some quotes I wanted to check what might be reasonable to do the job.

 

[ricicle]

. A standard SP RCBO is not permitted on a TT supply.

.

Care to quote a reg. no. on that one Rob ?

 

[ericmark]

531.2.9 Where, for compliance with the requirements of the regulations for fault protection or otherwise to prevent danger, two or more RCDs are in series, and where discrimination in their operation is necessary to prevent danger. The characteristics of the devices shall be such that the intended discrimination is achieved.
NOTE: In such cases the downstream RCD may need to disconnect all live conductors.

 

[ricicle]

Wouldn't apply on an all RCBO set-up though would it. (assuming no up-stream TD RCD)

 

[ColJack]

I can't find a reg anywhere about TT specifically, but for "Devices for protection against the risk of fire", an "RCD shall switch all live conductors".

so in a wooden building, the RCD / RCBOs would need to be double pole..

 

[Spark123]

Wouldn't apply on an all RCBO set-up though would it. (assuming no up-stream TD RCD)

You can't really get away from having an upstream RCD as the feed to the outbuilding will be in SWA or similar.

 

[sparkyspike]

I put 6mm SWA through plastic conduit...

The interior metalwork of the garage was bonded...

What did you use to bond the metalwork?