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Joe Shmoe

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So sorry, I know these posts are tedious.

When I got my house rewired some 10 years ago, I knew I wanted a small music studio in the garden so I had the electrician install a spare cable running from the inside of the CU (left loose inside and not connected to a MCB) up into the loft, where it could then be connected to some SWA in the future. All the cabling is now covered/plastered over, so ideally I would like to use this spare cable he installed.

I've taken a look and it's only 2.5mm.

The 2.5mm run would go from a new MCB into the loft (1m length) before connecting to 10mm SWA running approx 30m into the studio.

Is this small run of 2.5mm upto the job, or will I need to remove all the plastering so some 4mm be installed perhaps?

Any pointers? Thanks.
 

flying haggis

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It depends on what you expect to power in the studio, heating lighting equipment and poss A/C. I think you will have to dig it out and replaster and I would install 10mm as that is what the swa is
 

sunnybob

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NO.
If you run 10 mm over 30 metres, and then 2.5 mm, it will burn.
Maybe not the first day, but it will burn.
 

colinc

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sunnybob":3r6ch8d7 said:
NO.
If you run 10 mm over 30 metres, and then 2.5 mm, it will burn.
Maybe not the first day, but it will burn.
Hi Bob,

What’s the logic? I can see that the thin cable is the weak link like a fuse, but why must it be destined to fail if the circuit is protected by an mcb rated for the 2.5mm cable. My schoolboy physics suggests that having a long run of thick cable just lowers the resistance and voltage drop.

Regards,

Colin
 

flying haggis

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could you run 6 or 10mm from the cu in conduit to save damaging the plaster too much and if the 2.5mm stays what were you thinking of fusing it at
 

porker

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Should calculate it properly taking into account voltage drop etc. you should be able to fuse with a 20A MCB, the limiting factor as said before being the 2.5mm cable. But if you have 10mm SWA it is probably worth upgrading the 2.5mm link also and getting the full benefit outside. I know that is easier said than done though.
Not sure why Bob thinks it will burn. If you can only draw the max rating for 2.5mm you won't have a problem.
 

sunnybob

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30 metres.

Have you ever tried to use a power tool rated for 13 amps, at the end of a 30 metre extension cable? it will blow the fuse time after time.
After a few resets the trip will go bad and the 2.5 mm will heat up.

Its bad practice and my opinion is that it is very dangerous.
 

colinc

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Isn’t that because the voltage drops over that length so the current rises for any given power requirement? A thicker extension cable ought to cure that.

With 10mm2 SWA I would have thought the drop would be substantially lower so the heating effect is reduced. Am not disagreeing that it seems daft to limit the current draw to the capacity of the 2.5mm2 cable.

I know that in my workshop the current carrying capacity of the cable was not the deciding factor in selecting the cable area, it was the permitted voltage drop.

I guess this is all pretty irrelevant anyway as this is an external installation and Part-P requires that it gets done and signed off by a competent electrician.

Regards,

Colin
 

colinc

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It’s probably worth mentioning that depending what your house earthing system is you will potentially (excuse the pun) need to separate the house earth from the studio and provide an earth rod or rods at the studio end. In a typical PME installation you might use the house earth conductor to protect the SWA armour all the way to the studio but would terminate the connection to it there. Also, RCD at the house end protecting the 2.5mm and/or SWA might need to be a higher trip current so that a fault in the studio trips at the studio and not at the house end.

Sorry, but these remote installations raise a lot of technical issues that are best discussed with a competent electrician who should be trained in how to deal with them and importantly have the gear to test the earth protection at the studio end which can be challenging to get right. Us amateurs are not the guys to bet your safety on.

Regards,

Colin
 

porker

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sunnybob":1lzfmd12 said:
30 metres.

Have you ever tried to use a power tool rated for 13 amps, at the end of a 30 metre extension cable? it will blow the fuse time after time.
After a few resets the trip will go bad and the 2.5 mm will heat up.

Its bad practice and my opinion is that it is very dangerous.
That is a different use case. Extension cables have a much lower CSA than a 10mm SWA cable usually 1.5mm or less. The trip you are referring to is a thermal trip on the reel and is usually caused because people operate with the extension reel still coiled up. That is bad practice (but we probably all do it). I have a 45m extension that is only rated at 720W (~3A) when coiled up and 3120W (~13A) when uncoiled.

Radial circuits fused at 20A are quite common in the UK wired in 2.5mm cable so it won't heat up. My only reservation would be that someone might see a 10mm SWA cable and not see the 2.5mm 'weak link' however this would be seen at the CU as a 2.5mm.

colinc makes a good point above. There are Part P rules in place now that cover outbuildings (I am not an electrician but am a qualified electrical engineer) so check with an electrician.
 

sunnybob

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Advice and opinions are separate things, but based on over 30 years as a breakdown and maintenance engineer, my advice is its dangerous.
I fully agree that a competent and qualified electrician should have the final say.
 

HappyHacker

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Agree with most of above, the existing earth method for the house needs to be determined and the earth fault loop impedance to ensure that a fault can trip the MCB in the house. Depending on the construction of the outhouse the house earth may be suitable for use in the outhouse or you may need a separate earth rod at the outhouse.

A metre of 2.5 T&E is going to limit your max MCB in the house to 25A. I have done the calcs and you may be able to use a C type MCB which will reduce the chance of a fault in the outhouse tripping the house MCB. Different value MCBs do not discriminate very well and the C type is less sensitive than the normal B type and should increase the chance of both of them tripping. I normally try to use a fuse at the origin of submains as they help with discrimination with MCBs.

Using 10 mm SWA at 30m long attached to the 2.5T&E will meet voltage drop and earth fault loop impedance assuming your house supply does not have a high value. 4mm would be too small and not meet the VD requirements.

Unless you plan to use lots of large equipment at the same time there should be no problems.

But hopefully you will be getting an electrician in and he will be able to make a recommendation based upon your supply, usage, local conditions and outhouse construction etc as well as testing and notifying under part P of building regs.

I normally recommend sticking an emergency light in so that if there is a problem you have light to see how to get out or check the consumer unit.
 

dzj

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You should get a licensed electrician to do this, but you can calculate the voltage drop by yourself (3-5% is permitted).
(e.g. 2.5mm2, 30m, 20A would have a 10.8V drop.)
 

RogerS

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sunnybob":30y9i45x said:
30 metres.

Have you ever tried to use a power tool rated for 13 amps, at the end of a 30 metre extension cable? it will blow the fuse time after time.
After a few resets the trip will go bad and the 2.5 mm will heat up.

Its bad practice and my opinion is that it is very dangerous.
I'm really struggling to get my head round what you're saying here.

I think you're saying take an extension cable ....like one of these ?



Unwind it fully. Stick a 13 Amp load on it. ?

That the 'fuse' will blow? What fuse and where ?

Or are you suggesting that the thermal trip will blow ? If so why ? There's a clue in the title...thermal. It's there to stop the heat building up inside the drum if it's not unwound fully and the maximum load is being drawn. It is not an over-current trip.

Where does the 2.5mm come in ? Are you saying put a bit of 2.5 sq.mm twin and earth into a 13 amp plug ?
 
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