Feed to Garage

[ryan111]

Currently our garage is powered from a fused spur from the back of one of our plugs.

I have been doing some calculations recently and work out that I am very very close to the 13a limit of the spur.

We have a 3 Phase Swimming Pool Pump (rated at 4.2A @ 240v), I also have a large chest freezer, fridge and a LOT of PA equipment. When running full tilt, the PA equipment draws around 1500watts or 6.25A.

What I want to do;

We have a free 32A or 40A breaker on the main CU (RCD side) I want to run a length of SWA cable from the CU out to the garage, then place a smaller CU out there with 3 separate circuits;
1 ring main for approx 6-8 double sockets - 32A?
1 light ring (couple of strip lights) - 6A?
1 outside feed (Power the Pump and a couple of double sockets - using SWA cable) - 16A?

What I want to know is; What size SWA cable will I need between the CU and the garage, and also between the garage and the pump/sockets

The distance between garage and CU is approx 20m, and between garage and pump is around 15m

Before anyone asks, I will be getting a electrician to fit all this, but I will be putting the main SWA cable from the CU to the garage in because it is a awkward and time consuming run!

Thanks

 

[ryan111]

I have been doing some googling and found this:

These are for a 25m run of 3 Core SWA XLPE

Cable Size 4 mm
Voltage Drop 7.61 Volts.
Percentage Drop 3.3%
Max Cable Load* 42.0 Amps


Cable Size 6 mm
Voltage Drop 7.39 Volts.
Percentage Drop 3.2%
Max Cable Load* 53.0 Amps

So basically, if I want to run 40A, I only need 4mm between the CU and the garage?

Also, can I use 2.5mm between the garage and the pump/outside sockets?

 

[Electrifying]

Before anyone asks, I will be getting a electrician to fit all this, but I will be putting the main SWA cable from the CU to the garage in because it is a awkward and time consuming run!

Thanks

Best check with your electrician first, before you go doing this, as he has to sign the job off as complying with all relevant regulations.

In order to answer your question more info is required.

What is the supply earthing system to your house?
Are there any extraneous conductive parts in your garage? (water/gas pipes, steel work)
What is the garage constructed of - wall, floor, roof?

 

[ryan111]

I'm not sure what is supplying the earth to the house, presumably a earth rod? Its a detached house.

There is only a water supply to the garage, it uses the Blue plastic piping.

The walls are made of blocks, the roof is wood beems with tiled roof, floor is solid concrete

 

[Electrifying]

I'm not sure what is supplying the earth to the house, presumably a earth rod? Its a detached house.

There is only a water supply to the garage, it uses the Blue plastic piping.

The walls are made of blocks, the roof is wood beems with tiled roof, floor is solid concrete

Sorry, I should have made the questions easier.

You need to know what 'earthing system' your house electrics utilises - this is important.

It could be a 'rod' - this would make it a 'TT' system.
It could also be a TN-S system - where the supplier provides a seperate earth for you to use.
Or it could be TN-C-S - where the supplier combines the function of the neutral and earth into one conductor.

You cannot presume what it is - you need to know.

The other important aspect is the metalwork that I mentioned.......is there any metal pipework, strips, supports etc that come into the garage from outside? (that's about as simple as I can put it. )

You see, depending on the 'earthing system' and the presence of 'extraneous conductive parts', your choice of conductor may be restricted to 10mm or bigger (certainly for the earthing/bonding).....so it's very important that we know.

As for the volt drop calcs - I don't know where you got your figures, but your only allowed 3% - and that's from the origin of the supply to the last light in your garage.

 

[ricicle]

Ask your electrician - he needs to sign the design part of the certificate (as well as the other sections)

 

[Taylortwocities]

.......and I would not run the feed from the RCD siode of the board.
Any trips due to pool pump, etc will trip the whole house supply.
Best to run from non-rcd side and fit rcd CU in teh garage, but your electrician will tell you this.

Your best move is to engage an elecrician, get him to agree the size of SWA cable, the n you can run it and call him back to do the rest.

 

[ryan111]

Untitled

• ryan111
• 17 Jan 2012
• 1

Does that mean my earth is supplied via the black cable? The black cable is the feed to the biuse, the black box to the right then has two meter trails running to the electricity meter, then in to my CU

The only pipes or anything going in to the garage is one blue plastic water pipe that is under the concrete floor

 

[Taylortwocities]

Cannot see quite from your photo.

I guess the earth cable goes into the black box, and that the twisted conductor comes out of the box and then connects to the metal outside sheath of the incoming cable?

If so, you probably have a TN-S supply. But needs closer inspection, really.

 

[Aragorn84]

Having asked a similar question recently on here, it was pointed out to me that even on the non-RCD side of the board (if it has one), there wouldnt be enough discrimination between the 40A submain MCB and the 32A garage ring MCB to meet the requirements of the regs.

As such the recommended approach would surely be split meter tails and use a switchfuse, feeding the garage board with appropriate cable as required.

 

[ryan111]

That was another option, would it be better to.completely split the garage from the house? Different meter trails?

I will get some better pictures of the earthing soon

 

[ricicle]

Ask your electrician

 

[stillp]

If your garage is fed from the back of a socket-outlet, you can't be running a 3 phase pump!

 

[ryan111]

Okay, ill get in contact with the electrician and ask him.

I think its 3 phase, it has a large capacitor on it I think to make it 3 phase maybe? Anyway, it draws a lot of power

 

[Aragorn84]

your supply is only single phase...

So unless you have a phase converter sitting between the supply and the pump its a single phase pump!