connectors to support 63Amp rating

[mike67]

Hi there!
I need some advice; I am going to install a "sub" distribution box in an extension shed built in my garden. I have a 3 cores 6mm2 from the 63Amp fuse of the house's existing DB, this going to the extension shed; but my 3 cores 6mm2 is too short at about 3m to that extension shed cannot change the 3 core 6Sq because walls and ceilings are finished now.

I thought to buy single 6mm2 to do the last 3m because cables will be in a 25mm galvanised conduct (outside).
My question is assuming that everything I am planning to do is correct so far, how I could connect the 3 core 6Sq with the 3 single 6Sq. I planned to use a junction box IP55 and some connectors but all the connectors that I saw so far are rating 32Amp max, and me I want to carrying 63Amp.
Any advice, solution would be appreciated

 

[oharaf]

Henley block

Sorry again Rather tired didn't think about the cable size, you are probably going to need 16mm!!

 

[ricicle]

Stop what you are doing. 6mm² will not carry 63amps. If you need 63amps then you will need a bigger cable the size of which depends on the distance and installation factors

 

[mike67]

Ricicle!

I probably missed explain myself. I won’t need 63amp, the reason behind this 63amp is I have a complete DB that I am going to use and as on this DB you have the main fuse which is a 63amp ‘{but in any case I wont have a load of 63amp (7 lights of 60watt; 3 double sockets, 4 low voltage decking lights, that pretty much all I will have)} so I thought to connect this DB from the 63amp of the existing house.
Ok knowing that I cannot change the 3 core 6mm that I am talking about, what the max amp rating that you would put on this cable? I have about 28m distance to the shed.

Oharaf!
As you could see I don’t need 63amp I was trying to replicate the exact scenario of my house’s DB which has a 63amp fuse so I thought I have a full DB so I would take the power from the house’s 63amp fuse to power my shed DB. But if I can’t do that, I am open to any suggestion which will work with this 3core 6mm that I cannot change.

 

[Steve]

Ricicle!

I probably missed explain myself. I won’t need 63amp, the reason behind this 63amp is I have a complete DB that I am going to use and as on this DB you have the main fuse which is a 63amp ‘{but in any case I wont have a load of 63amp (7 lights of 60watt; 3 double sockets, 4 low voltage decking lights, that pretty much all I will have)} so I thought to connect this DB from the 63amp of the existing house.
Ok knowing that I cannot change the 3 core 6mm that I am talking about, what the max amp rating that you would put on this cable? I have about 28m distance to the shed.

Oharaf!
As you could see I don’t need 63amp I was trying to replicate the exact scenario of my house’s DB which has a 63amp fuse so I thought I have a full DB so I would take the power from the house’s 63amp fuse to power my shed DB. But if I can’t do that, I am open to any suggestion which will work with this 3core 6mm that I cannot change.

You'll be needing 25mm² singles in metal conduit then, if you really want to replicate the house CU.

Please, stop what you're doing. You have no idea whatsoever about circuit protection.

Even working on your theory that the circuit is protected by the lack of points served, 6 sockets = 78 amps potential draw. So this idea is blown out straight away.

 

[JohnD]

you could use a 32A MCB or lower in a small additional CU next to the main one in the house, feeding the 6mm cable. You will need an RCD for the shed. It can be fitted in the small CU in the house, or in the shed. If it is in the house it will give additional protection in case of cable damage, but might be inconvenient to reset if you are working in the shed at the time.

the MCB in the house should be less than the power rating of the cable, and no more than the maximum design load in the shed.

You can buy these small CUs with RCD and MCB quite cheaply, for sheds and showers. The cable join must be permanent, and weather-protected.

Are you legally permitted to do this work in your country, and are you sure you have the necessary skills?

 

[mike67]

Johnd,

Thanks for your reply; I can see clearly what I can do now.
About your question, I don’t think that it’s illegal to power your garden shed from your main CU providing that the work is done in line with the standards so why I am seeking for advices, about joining the cables I think I have a problem because as you said it should be permanent, I will have to find a solution for that one.
I don’t have the knowledge for sure but at the moment that I know where I am going (a clear scenario like the one you suggested to me) I have no problem to execute the work whatsoever.

Steve
I won’t argue with you but you don’t come with any suggestions only negative comments.
I don’t need to replicate the house CU because I don’t need the same load as the house, I was saying by replicate connect the CU as I can see the live one connected at the moment ( by the way the main is a 16mm2 in my house).
About the 78 amps potential draw, I live it to you but tell me, how come in my house the highest fuse is 63amp/40/32/20 and 16amp and I have to take an example 4 double sockets on a same 20 amp fuse with a 2.5mm2 twin & earth…
Going to the shop, it is suggested to connect 4 radial sockets and more up to 50 Sqm floor with a 2.5mm2 twins & earth from a 20amp MCB. Also it is suggested to connect 4 radial sockets and more up to 75 Sqm floor with a 4.0mm2 twins & earth from a 32amp MCB.
All this to say that you are loosing me, how come I cannot have 3 double sockets and 7 lights powered with a 3 core 6mm2 feeding a small CU?
A last thing before I go Is this forum electrical topic a DIY forum or not, people whom has the knowledge talk about it and give advices to those whom is seeking for it. Am I right or not?

 

[securespark]

You talk often of a 63A fuse.

Do you really mean that the isolators for your consumer units are rated at 63A? If so, there are no fuses in them. So if you exceed 63A, there is the possibility of damage, the degree depending upon the overload.

You should allow for flc on your 6mm² cable, so, if fed from a 32A MCB, the cable should be able to deliver this current to the garage without losing too much voltage or raising the EFLI value beyond accepted levels.

The longer the distance, the greater the csa needs to be. Or, in your case, as your cable size is fixed, if your garage CU is a long way from the main CU, you may need to derate the MCB to allow for the maximum loading on the cable at that given length.

 

[333rocky333]

You talk often of a 63A fuse.

Do you really mean that the isolators for your consumer units are rated at 63A? If so, there are no fuses in them. So if you exceed 63A, there is the possibility of damage, the degree depending upon the overload.

If you look at his other post, he may mean an rcd
//www.diynot.com/forums/viewtopic.php?p=962789#962789

 

[RF Lighting]

The same applies wether it is an RCD or a main switch.

Neither offer overload protection.

 

[securespark]

Please see my edit: I went off on one & added some stuff while you were posting!

 

[mike67]

Hi there!
Thanks all of you for taking your time to talk about my case. I’ve done some home works this weekend and have the final & following figures, if something is wrong just let me know:

The 63amp I am talking about is the available load of my CU (there is one main fuse 63amp & one RCD63amp)

The load I will need for lights is 1.6amp total including a 25% security. (5 lights X 35w bulbs & 2 lights X 65w bulbs)
Load for sockets: one double socket for general purpose 13amp
One single socket for 4 LV decking lights using a transformer plug max output is less than 1amp (I will have a single socket designated for this transformer/plug)
Socket = 14amp total need, I have 16amp & 20amp MBN/MCB 6000-3 from Hager at my disposal to fit in a small CU in the shed.
Lights = 1.6amp total need, I have 6amp & 10amp MBN/MCB 6000-3 from Hager at my disposal…

So the maximum load on my feeding cable could be 30amp or 26amp (20amp MBN/MCB + 10amp MBN/MCB or 16amp MBN/MCB + 10 amp MBN/MCB) let me know which one to use.


Design considerations

My feeding cable is EUPEN <VDE< 0250 NHXMH-J 3x6 length
For calculation purpose I took 30amp design load for the shed
Power is 6.9 KW (230V*30amp/1000)
Cable length 25m
Temperature in roof in summer 40° C
Cable partly in plasterboard, partly clipped on wall, partly in contact with other cable and in conduct.
So I have the following figure: (from web calculator with design information that I provided)
Cable size 6mm
Voltage Drop 5.48 volts
Percentage Drop 2.48%
Current load 30.0Amps
Max cable load 35.7Amps

So if I fit a 32Amps MCB in my house CU (there is plenty space in this CU) feed from the 63Amps RCD
At the other end in the shed, a 20Amps MCB/MBN (sockets) & 10Amps MCB/MBN (lights) for example.
Correct me if anything is wrong.

Few questions:
I don’t have any 20Amps & 10Amps RCD but only MBN from Hager for the shed CU will this do the job or I have to invest in RCD?

On the RCD, MCB, MBN there is some indications that I am not sure about like: 6000-3 or 3000-3 or 4500-3 is the first number the tripping time of the device in seconds and the 2nd number is it type? If the second is it type, then I am a bit confused, I thought that in domestic system we only use type 1 or B of RCD, MCB, MBN.
Also in a CU do you have to use all the RCD, MCB, MBN of the same type/indications (e.g. 6000-3) or you can have some 3000-3 with 6000-3 and 4500-3 regardless of the devices’ nominal current which will have to be selected with other considerations?

The single socket that I am going to have designated only for the LV decking lights, output from the plug/transformer less than 1Amps, can I feed this socket from the 10Amps RCB for lights in the shed? So that I could use a 1.5mm2 cable if not I will have to use the 2.5mm cable from the 20Amps RCD. Also these lights are supplied with a plug/transformer; the transformer will be in the shed with its designated socket, can I make this socket “switch able”? From inside the house so that I won’t have to go in the shed to plug/unplug. I know how to do it but I just want to know if for an electrician this is something which is acceptable to do.

Is there any web site where you could check the on-site guide BS7671 online, I don’t need to buy the all book?
It's a lot of literature but you better give all the info in order to allow whom wants to comment the case.