Complicated SPUR problem for Studio

[leeuk321]

Hi, I was just wondering if anyone out there could help me with a bit of a complicated electrical problem?

At the moment I've got a 30amp radial circuit (4.0mm cable) in a garage/workshop type building, which goes about 10m from the consumer unit to the first single socket, then carries on for about another 12m to a junction box in another room, which then splits up to go to 2 sockets.

What I'm wanting to do is insert a spur at the first socket. I've got a ton of 2.5mm flex cable hanging around (but no 4.0mm mains cable) so I want to utilize this. Would I be ok to run a spur from the first socket on the circuit to a 20amp Fused Connection Unit (using 4.0mm mains cable) and from the FCU run the rest of the spur using 2.5mm flex cable? I just don't know if you can go from 4mm to 2.5mm cable (but figured that maybe an FCU would 'sort this out'?)

What I'm wanting to do on the spur is run 8 double sockets, about 2-3m apart each. Of course, Im not planning on plugging in 16 different appliances, it's just for the convenience because I'm building a studio, and I need to have a load of sockets around the top beams for plugging in 4-5 500w lights at any one time, and because the lights will be high up I can't just plug them into extensions.

Any help would be greatly appreciated, because like most people I don't want to fry myself or cause a big ugly fire.

Thanks! Lee.

 

[oharaf]

5 500watt lights is 2500w

2500/230 = 10.8amps so your 20a FCU is more than capable.

According to the schematic in the 17th Ed. pg 363 App. 15

A 30A Radial can have a 2.5mm spur using an FCU.

It states " Spur Circuit using an FCU - The number of socket-outlets supplied from a fused connection unit is dependant upon the load characteristics having taken diversity into account"

Also "NOTE: As a rule of thumb. the floor area served has been limited to 75mm for a 30/32A radial".

You will need check the entire floor area served by this radial (it sounds quite large) and apply diversity to your calcs


In theory sounds okay but you will need to look into this a little deeper I think


Hope this is of some help

 

[Pensdown]

Can you get a 20amp fused connection unit?

 

[oharaf]

Oh bllockers, not thinking there pensdown!!! Sorry

 

[oharaf]

YEP!!!

http://www.ekmpowershop2.com/ekmps/shops/vauxworld/0090386020--maxi-20-amp-fuse-334-p.asp

 

[Spark123]

Car fuse
Extend the radial in 4mm and there isn't a problem.

 

[leeuk321]

Thanks to everyone for your advice, and oharaf for your extensive post - I appreciate it. On the subject of a 20A FCU - I've dug up an MK FCU in the garage which says 13/20AMP 250V on it - does this mean that this FCU will accept a 13A or 20A fuse - or does it mean something else? I've attached a photo of it just in case it clears anything up.

Thanks for everyone's help!

Lee

MK 13/20AMP FCU

• leeuk321
• 4 Sep 2008
• 1

 

[streetlighter]

Would a small CU with a BS88 fuse holder and 20 amp fuse be aceptable as a 20 amp Fused connection unit ?

 

[Spark123]

By the time you have forked out for the price of a consumer unit etc you may have well spend the money and do it in 4mm
Don't know what the 20A is referring to on the FCU, the biggest BS1392 fuse available is a 13A

 

[skenk]

What about doubling up the 2.5? ie 2 lengths between each outlet, no need to fuse down?

Doesn't sound like a great idea to me but would it be allowed?

 

[oharaf]

What about doubling up the 2.5? ie 2 lengths between each outlet, no need to fuse down?



It amazes me how tight people can be and desperate to use up the 'odds and ends'

If you're going to do something do it properly or not at all

 

[Click-Sure]

Is running a new ring final with your 2.5mm2 back to the board not an option then?

 

[plugwash]

What about doubling up the 2.5? ie 2 lengths between each outlet, no need to fuse down?

Doesn't sound like a great idea to me but would it be allowed?

I'm pretty sure doubling up cable to increase current rating is allowed by the regs though it is rarely done.

 

[leeuk321]

Thanks for everyone's help. I think I'm going to fuse down to 13A and use the 2.5mm flex to create a spur. I've thought about the idea of extending the radial circuit back to the CU to create a ring circuit, but if you could see the mess of the room that this radial circuit ends up you'd understand why I wouldn't want to go down that road!

I also thought about maybe disconnecting the cable at the point where it goes to the other room and putting a junction box there (therefore making the 'other room' cabling a spur) and then creating a ring at the break point, but 1) I'd need to get a 50m roll of 4.00mm cable and 2) the actual CU is pretty old (I'm talking 30-50 years!) and bolted onto a big piece of plywood (with bolts that are now rusty!), so I don't want to get into rewiring that because I'd be out of my depth. And if you knew how much I'd spent on this studio so far you'd realise that I wasn't being cheap but necessarily cutting costs!

So I figure with 13A it'd give me 3120W which would be 5 500W lights (or 6 at a push) which would be plenty for my current needs. I'll fuse and earth all the way to the bulb so it'll keep it all safe.

Thanks again for everyone's help, I've appreciated it.