Power to garage

[jswagger]

Hi,


I’m planning on putting in the electrics in my new garage and wanted to see if anyone had any advice/warnings. I have done quite a lot of home electrics in the past but have never done any outside electrics before or anything involving SWA cable. The garage is 25m from the house and will require 4 x double sockets and 4 x strip lights.


I plan to install a 2 way consumer unit in the garage, comprising 40A 30mA RCD, 1 x 6A and 1 x 16A Type B MCB. I intend to run 4.0mm SWA from the garage to eventually connect to a spare slot in the house consumer unit. As the house consumer unit is at the front of the house and the garage at the end of the garden, rather than run SWA all the way to the house consumer unit is it possible to run Twin +E from the house consumer unit to under the floor in a rear bedroom. Then the twin + E will be connected to the incoming SWA using a suitable adaptor (I’m assuming these are available). In the house consumer unit there is an unused RCBO, formerly used for an electric shower, would it be acceptable to connect to this?


Would 2.5mm SWA be ok? Also do I need to have a separate earth spike for the garage?

Thanks very much for your help.

 

[securespark]

You want an RCD at either end of the SWA?

I would fit a breaker in the house & an RCD in the garage: if the RCD trips, you don't want to have to go back to the house to reset. Either way you do it, you don't want two RCD's..

What size CPD are you using at the house? If you use the RCBO, I guess it is at least 32A, which is a bit tight for 4mm².

 

[davy_owen_88]

I’m planning on putting in the electrics in my new garage and wanted to see if anyone had any advice/warnings. I have done quite a lot of home electrics in the past but have never done any outside electrics before or anything involving SWA cable. The garage is 25m from the house and will require 4 x double sockets and 4 x strip lights.

OK, this is notifiable work so be sure to tell them before you start.

I plan to install a 2 way consumer unit in the garage, comprising 40A 30mA RCD, 1 x 6A and 1 x 16A Type B MCB. I intend to run 4.0mm SWA from the garage to eventually connect to a spare slot in the house consumer unit. As the house consumer unit is at the front of the house and the garage at the end of the garden, rather than run SWA all the way to the house consumer unit is it possible to run Twin +E from the house consumer unit to under the floor in a rear bedroom. Then the twin + E will be connected to the incoming SWA using a suitable adaptor (I’m assuming these are available). In the house consumer unit there is an unused RCBO, formerly used for an electric shower, would it be acceptable to connect to this?

What rating is the RCBO that the shower used to be connected to? You can run t&e for some of the length and connect SWA later on as long as it is equal in size and suitable for the load and voltage drop. The joint from t&e to SWA will have to be accessible though so can't be hidden under the floor.

Would 2.5mm SWA be ok? Also do I need to have a separate earth spike for the garage?

It is very unlikely that 2.5mm^2 will be suitable for connection to an RCBO that used to feed a shower, it is also unlikely that 4mm^2 will do but you might be lucky. What earthing system do you currently have?

Davy

 

[securespark]

Where's the emoticon with his tongue sticking out?

 

[davy_owen_88]

This one?

 

[jswagger]

You want an RCD at either end of the SWA?

I would fit a breaker in the house & an RCD in the garage: if the RCD trips, you don't want to have to go back to the house to reset. Either way you do it, you don't want two RCD's..

What size CPD are you using at the house? If you use the RCBO, I guess it is at least 32A, which is a bit tight for 4mm².

Thanks for the reply. This is the first house I've owned with MCBs etc so I'm more used to fuseboxes. What do you mean by CPD?

There is a spare MCB slot in the consumer unit so I could use that instead of connecting to the old shower one? If this is ok wht rating MCB would you recommend?

Thanks again.

 

[jswagger]

I’m planning on putting in the electrics in my new garage and wanted to see if anyone had any advice/warnings. I have done quite a lot of home electrics in the past but have never done any outside electrics before or anything involving SWA cable. The garage is 25m from the house and will require 4 x double sockets and 4 x strip lights.

OK, this is notifiable work so be sure to tell them before you start.

I plan to install a 2 way consumer unit in the garage, comprising 40A 30mA RCD, 1 x 6A and 1 x 16A Type B MCB. I intend to run 4.0mm SWA from the garage to eventually connect to a spare slot in the house consumer unit. As the house consumer unit is at the front of the house and the garage at the end of the garden, rather than run SWA all the way to the house consumer unit is it possible to run Twin +E from the house consumer unit to under the floor in a rear bedroom. Then the twin + E will be connected to the incoming SWA using a suitable adaptor (I’m assuming these are available). In the house consumer unit there is an unused RCBO, formerly used for an electric shower, would it be acceptable to connect to this?

What rating is the RCBO that the shower used to be connected to? You can run t&e for some of the length and connect SWA later on as long as it is equal in size and suitable for the load and voltage drop. The joint from t&e to SWA will have to be accessible though so can't be hidden under the floor.

Would 2.5mm SWA be ok? Also do I need to have a separate earth spike for the garage?

It is very unlikely that 2.5mm^2 will be suitable for connection to an RCBO that used to feed a shower, it is also unlikely that 4mm^2 will do but you might be lucky. What earthing system do you currently have?

Davy

Thanks for the advie.

The RCBO has 30ma on it. Is this the rating? The earth is as fitted when we move in, a stake going to underneath the floorboards in the hall.

Out of interst why should the join between SWA and T+E be accesible? The floor in the bedroom is easliy taken up.

Thanks again for your help.

 

[davy_owen_88]

What do you mean by CPD?

Circuit protective device. Ie what rating is the RCBO.

There is a spare MCB slot in the consumer unit so I could use that instead of connecting to the old shower one? If this is ok wht rating MCB would you recommend?

What is the expected total load in the garage? I.e what will you be using out there?

The RCBO has 30ma on it. Is this the rating?

That is the rating of the earth leakage section, it should also have Bxx written on it, where xx is a number stating it's overload rating. Most probably B32 or above.

Out of interst why should the join between SWA and T+E be accesible? The floor in the bedroom is easliy taken up.

All terminal joints need to be accessible for testing and maintenance. Under floorboards isn't really accessible as tiles/laminate flooring could be put down at any time making it very difficult to get at.


Let us know what rating the RCBO is, and what the expected load in the garage will be and we can offer you more advice.

Davy

 

[jswagger]

What do you mean by CPD?

Circuit protective device. Ie what rating is the RCBO.

There is a spare MCB slot in the consumer unit so I could use that instead of connecting to the old shower one? If this is ok wht rating MCB would you recommend?

What is the expected total load in the garage? I.e what will you be using out there?

The RCBO has 30ma on it. Is this the rating?

That is the rating of the earth leakage section, it should also have Bxx written on it, where xx is a number stating it's overload rating. Most probably B32 or above.

Out of interst why should the join between SWA and T+E be accesible? The floor in the bedroom is easliy taken up.

All terminal joints need to be accessible for testing and maintenance. Under floorboards isn't really accessible as tiles/laminate flooring could be put down at any time making it very difficult to get at.


Let us know what rating the RCBO is, and what the expected load in the garage will be and we can offer you more advice.

Davy

Hi ,


The former shower RCBO is a B40 so I take it that is 40 amps? Another poster said not to use it anyway as I don't want two RCBs on the SWA?

I don't plan to use the sockets in the garage to power any heaters, just power tools, mower, the electric roller door and possibly a MIG welder.

Incidentally I used an online cable size calculator and it said for a load of 4kw and a distance of 25m I can use 2.5mm SWA, not 4mm.

What do you think and what size MCB in the CU would be appropriate?


Thanks again.

 

[bernardgreen]

I don't plan to use the sockets in the garage to power any heaters, just power tools, mower, the electric roller door and possibly a MIG welder.

Roller door ? ? Have you got a second way into the garage if the MCB / RCD for the door is going to be fitted inside the garage ?

We have but I know a man who didn't......

 

[securespark]

The trouble is, you can't just say you want to use 4kW and stick an MCB on the end of the cable.

4kW = 17.39A @ 230V. You'd have to fit a 20A breaker, but then the cable needs cope with voltage drop.


By my calcs, you'd be losing 9V on that. VERY close to the 9.6V maximum.

 

[davy_owen_88]

The former shower RCBO is a B40 so I take it that is 40 amps? Another poster said not to use it anyway as I don't want two RCBs on the SWA?

You don't want 2 RCD's so either one at the house end or one at the garage end. Garage end is easier as you don't have to walk back to the house to reset it. You'll need to buy another suitable MCB for the house and fit the feed on the non-RCD side.

I don't plan to use the sockets in the garage to power any heaters, just power tools, mower, the electric roller door and possibly a MIG welder.

Incidentally I used an online cable size calculator and it said for a load of 4kw and a distance of 25m I can use 2.5mm SWA, not 4mm.

My point about 2.5/4mm^2 cable was that the cable you use has to be suitably protected. A 40A RBCO is too high for 4mm^2. If you chose an MCB/RCBO with a lower rating then you can use a smaller cable.

You say the total demand will be 4kW but you are going to use a 16A breaker? 4000W / 230V = 17.4A so I would suggest you use a 20A breaker for the socket circuit. Then what about lighting?

Voltage drop for 2.5mm^2 SWA:

18mV * 20A * 25m = 9V. Very very close to the maximum permissible level (4% of supply voltage). And that is without the rest of the installation. 4mm^2 is a better choice.

Davy

 

[jswagger]

I don't plan to use the sockets in the garage to power any heaters, just power tools, mower, the electric roller door and possibly a MIG welder.

Roller door ? ? Have you got a second way into the garage if the MCB / RCD for the door is going to be fitted inside the garage ?

We have but I know a man who didn't......

Yes there's a second door thanks.

 

[jswagger]

The former shower RCBO is a B40 so I take it that is 40 amps? Another poster said not to use it anyway as I don't want two RCBs on the SWA?

You don't want 2 RCD's so either one at the house end or one at the garage end. Garage end is easier as you don't have to walk back to the house to reset it. You'll need to buy another suitable MCB for the house and fit the feed on the non-RCD side.

I don't plan to use the sockets in the garage to power any heaters, just power tools, mower, the electric roller door and possibly a MIG welder.

Incidentally I used an online cable size calculator and it said for a load of 4kw and a distance of 25m I can use 2.5mm SWA, not 4mm.

My point about 2.5/4mm^2 cable was that the cable you use has to be suitably protected. A 40A RBCO is too high for 4mm^2. If you chose an MCB/RCBO with a lower rating then you can use a smaller cable.

You say the total demand will be 4kW but you are going to use a 16A breaker? 4000W / 230V = 17.4A so I would suggest you use a 20A breaker for the socket circuit. Then what about lighting?

Voltage drop for 2.5mm^2 SWA:

18mV * 20A * 25m = 9V. Very very close to the maximum permissible level (4% of supply voltage). And that is without the rest of the installation. 4mm^2 is a better choice.

Davy

I don't know that 4kw is the total demand, just a figure I got from the calculator but I would have thought that 4kw was more than enough. Do you think 3kw or less would be a more realistic figure? If I use 4 core (I want to put in a couple of two way switches) then the voltage drop (apparently) is 7 volts. I suppose I would be better off just going for 4mm then anyway?


Thanks to everyone who has replied.,

 

[davy_owen_88]

I don't know that 4kw is the total demand, just a figure I got from the calculator but I would have thought that 4kw was more than enough.

You can't just pull figures out of thin air for maximum demand and hope it will be sufficient, you need to work it out. Find out the ratings of the tools you will be using and then add a bit for lights and then some more for a radio/cd player and then a bit more if you want a security light and then a little bit more incase you forgot something.

Lowering your MD just so you can fit a less expensive cable is false economy when you find it isn't suitable for all your needs and you need to replace it.

Do you think 3kw or less would be a more realistic figure?

I think your going in the wrong direction.

If I use 4 core (I want to put in a couple of two way switches) then the voltage drop (apparently) is 7 volts.

The VD figures for 4-core cable are for 3-phase and even then the VD is 7.5V. What 2-way switching are you going to be doing on a sub-main feed?

I suppose I would be better off just going for 4mm then anyway?

I'd fit 4mm^2 at the very minimum and before you decide on that have a good think about everything you might have on at the same time and add them up.

Davy