Workshop RCD / MCB help

[jacko555]

Hello.

I have some questions regarding RCD & MCB selection & placement for my garage / workshop.

The Main Consumer Unit

The house is supplied with a 100Amp consumer unit, about 20 years old and containing the old fuse types (the ones where you screw in a length of the correct amperage fuse wire),and no RCD or MCB's in it. There are also NO free slots in the CU.

The Spur

The power to the workshop is supplied with a 1.5cm T&E spur from a 30a ring main socket. The cable run is around 4 meters from the spur to the workshop. The 'spurred' socket is next to the CU.

The workshop

The Spur ends with two double sockets. From these sockets I am running (not all at once though!) :

1200 ish watts tools (saw, drill, sander etc)
4 x 40watt fluroscent light fixtures = total 160watts.
1 x 50watt pond pump ( a long cable on the pump out the window into pond).

Each device is plugged straight into the sockets (ie no extension cables / double-plugs)

So the total load is around 1500w (1.5kw) at any one time.

Questions:

Is this layout safe? It has been fine so far (about 2 yrs), and the spur looks like its been there years...

I would like to add a mini CU before the sockets in the workshop for safety while working. I was thinking of adding an RCD and a MCB in the mini CU.

Although the total circuit works at 6.25 amps (240v, 1.5kw), I understand the tools, pond pump and lights are an 'inductive' load rather than resistive and so the startup amp load is about twice the rated load.

This would mean a total possible load (if everything was turned on at once) of about 12.5 amps? Is this correct?

So I would have to select a type 'C', 16amp MCB. Type 'C' being used for inductive loads.

The closest RCD to my needs is a 63 Amp, 30mA, 2 Pole. I know not to wire the lights into the RCD in case it trips while working...

Does putting the mini cu inline with the spur sound ok?
Is there any adice as to my mcb / rcd selection?

I don't really want to pay a sparkie to do this, as I assume I'll need a new house CU for space for a line to the workshop, which will also need a CU etc etc etc.

I haven't the money to re-do it all. It works fine already and I just want the extra bit of safety an RCD / MCB will provide.

All help is appreciated.

nb The price of a mini CU, RCD & MCB (listed above) is 25 quid total. The price of a 'plug in' RCD is 15 quid. Thats why I'm considering the mini CU option.

 

[andy]

its not safe. the 1.5mm cable from the ring should be at least 2.5mm. this should either be replace with 2.5mm or a FCU should be added where it is spurred from the ring.

best thing to do to make it safe is to add the FCU. that way the cable gets protection. RCD FCU's are also available.

if you were to put a mini CU in, i would NOT recommend spurring from the ring to feed it. give it its own feed. also, you do not need C MCB's. altho these could be used, youll have to make sure the resistance is low enough to allow it

 

[jacko555]

I thought the 1.5mm was on the thin side...

So I'll replace the spur with 2.5mm t&e, with a Fuse Connection Unit (FCU), leading to the RCD, then the workshop sockets.

Do you think a MCB will still be necessary with the FCU?

I wish I could give it its own feed, but my house cu is full...

Thank you for your help

 

[andy]

I thought the 1.5mm was on the thin side...

So I'll replace the spur with 2.5mm t&e, with a Fuse Connection Unit (FCU), leading to the RCD, then the workshop sockets.

Do you think a MCB will still be necessary with the FCU?

I wish I could give it its own feed, but my house cu is full...

Thank you for your help

use an RCD FCU. 1.5 can be used, altho i would recommend 2.5mm. MCB is not needed since it will be protected by the fuse in the FCU

 

[Big_Spark]

Jack, before you do anything to your Workshop supply, which is an accident waiting to happen, you should change your main CCU to a new type.

This way you can have the benefits of a board with spare ways in it, the electrical security of a safe means of disconnection should a fault occur (fuses are safe, but MCB's safer still).

Further this main unit could then be used to provide a dedicated supply to your workshop. This circuit could also be protected by an RCBO.

The supply as you have it is certainly unsafe. The Lights should not be on sockets, two sockets, or even one one socket, fed by 1.5mm cable is simply lunacy. As Andrew has stated, this should be corrected as a BARE MINIMIUM.

 

[jacko555]

I am glad I checked here first... As the spur and light were already there, I assumed it was safe.

Although replacing the main CU would be ideal, I do not have the funds to have it replaced... Of course if I MUST, i will.

Possible improvements...

In the hope of saving some money while increasing safety how does this sound?

1) Replace existing spur to 2.5mm t&e, with a FCU and RCD feeding the sockets.

2) Run a second 2.5mm spur, from a second 'unspurred' socket,. with a FCU inline to feed the lights.

I thought about branching from either of the two house 'lighting' radials. However, these are both 'full'...

Does this sound better? Or for safety's sake, should I start saving for a larger house CU and sparky? (Bear in mind the existing wiring will stay until I've saved up, 2 months at least..)

Again, Many thanks for your help

Jack

 

[andy]

get the RCD FCU and wire it with 2.5 for the sockets. and for the lighting circuit, what do you mean by 'full'?

 

[jacko555]

Both light circuits have 5amp fuses (Downstairs / upstairs)

There are 7 light bulbs on each five amp circuit, so a max of 700wats each(3 amps ish)

I thought this was about as high as it should go for 5amp fuse.

Will adding the workshop lights (4 x 40 watt fluroscents) be too much for the downstairs light circuit?

thanks

 

[ban-all-sheds]

Is the workshop joined onto the house? Could you not simply extend the ring main (rather than spurring) and the lighting circuit into it? (The extra load on the lighting circuit will be fine, BTW).

If it's not joined on, then you shouldn't be using twin & earth cable....

 

[jacko555]

I cannot easily extend the main - there is only one plug socket in the basement (the room where the CU is situated)

The only way would be with junction boxes (not allowed anymore) or extending the main from the CU itself.

Looks like I'm gonna use a RCD FCU coming off the spur (re wire to 2.5mm) for the workshop, and branch from the downstairs light radial for the lights.

Many thanks for all your help!

nb Yes the workshop is joined on - the cable goes through a hole in the wall...

 

[ban-all-sheds]

I cannot easily extend the main - there is only one plug socket in the basement (the room where the CU is situated)

So where you currently have

[CU]===[BASEMENT SOCKET]==>{onwards}

could you not do

[CU]===[WORKSHOP]===[BASEMENT SOCKET]==>{onwards}

?

The only way would be with junction boxes (not allowed anymore)

Yes they are - why do you say that? True they are not ideal, but they are allowed.

or extending the main from the CU itself.

As in [CU]===[WORKSHOP]===[BASEMENT SOCKET]==>{onwards}?

 

[jacko555]

Quote:
The only way would be with junction boxes (not allowed anymore)

Yes they are - why do you say that? True they are not ideal, but they are allowed.

I thought they were not allowed - the 'How To' section of this site says

N.B. This method is now excluded from the IEE Wiring regulations, but remains for reference.

//www.diynot.com/pages/el/el014.php

Does the IEEE reg's count for the UK?!?

As you've suggested, extending the ring in the following fashion would be best
[CU]===[WORKSHOP]===[BASEMENT SOCKET]==>{onwards}

I just didn't want to wire into the CU if possible.


Thanks

 

[andy]

junction boxes can be used. but they must be accessable (i.e not plastered over)

 

[plugwash]

Quote:
The only way would be with junction boxes (not allowed anymore)

Yes they are - why do you say that? True they are not ideal, but they are allowed.

I thought they were not allowed - the 'How To' section of this site says

N.B. This method is now excluded from the IEE Wiring regulations, but remains for reference.

//www.diynot.com/pages/el/el014.php

Does the IEEE reg's count for the UK?!?

the IEE is NOT the IEEE
http://en.wikipedia.org/wiki/IEEE
http://en.wikipedia.org/wiki/IEE

however what is said there is NOT correct

junction boxes are fine provided they are accessible. The exact definition of accessible is not mentioned in the regs and is quite widely argued among sparkys.

the best option is crimps. theese do not have to be accessible. however if you use theese make sure you use a proper ratchet crimper and enclose them in a chockbox.

of course this rule is totally unenforceable because an unaccessible junction by definition cannot be inspected.

As you've suggested, extending the ring in the following fashion would be best
[CU]===[WORKSHOP]===[BASEMENT SOCKET]==>{onwards}

I just didn't want to wire into the CU if possible.

you could cut the wire coming out of the CU put a socket on it next to the CU and wire from there if you don't want to work on the CU (which with some of those old wylex boxes i can understand).