Some advice ahead of running wiring to shed

[ac505]

3 phase supply into the property, 1 phase to flat upstairs, 1 to mine, 1 redundant

 

[ban-all-sheds]

But is it a TN-C-S supply?

 

[ac505]

yes, I believe so, but will have to check

 

[JohnW2]

For future proofing I'll wire this up as a ring however keep the 16A RCB in for the moment

Why are you doing design work when you don't know what the Wiring Regulations say about ring final circuits?

Now there's an interesting one

Assuming that the intention is to wire the ring in 2.5mm² cable, and that the op means 16A MCB or RCBO (not RCD), what do the regs say about a 2.5mm² RFC protected by a 16A OPD?

Common sense suggests that it should be compliant. After all, it would be nothing more than a compliant radial circuit, branching at its origin, with the ends of the two branches joined together.

Kind Regards, John.

 

[ac505]

okay, daft idea on the ring, I can drop that and go radial. Working on the assumption that max power I can theoretically have is going to be 5.28kW, a 16A and 6A RCB will be fine.

Calcs for cable:
Cable Type SWA
Application :underground
Circuit Type
Ambient Temperature °C 15
Number of Circuits 1
Power 5.5kW
Voltage Volts
Length metres
Required Cable Size 2.5 mm
Voltage Drop 10.8 Volts.
Percentage Drop 4.7%
Current Load 23.9 Amps
Max Cable Load* 25.2 Amps

With that in mind, I can use a 6mm cable to give some further margin for error and a 30A switchfuse (the 6mm cable in this application can handle this with ease).

Metal clad consumer suggested above is fine also.

Am I now on the right track? Thanks

 

[JohnW2]

Required Cable Size 2.5 mm
Voltage Drop 10.8 Volts.
Percentage Drop 4.7%

That would not be acceptable if you had any lighting - for which the maximum permissible voltage drop would be 3% (6.9V). However...

Required With that in mind, I can use a 6mm cable to give some further margin for error ....

Are you saying that, having presented the figures for 2.5mm² cable, you intend to use 6mm² (which would probably be OK for lighting)?

Am I now on the right track?

It's a little hard to tell.

Kind Regards, John.

 

[ac505]

sorry John, a little unclear on my part. Using calculator on TLC direct site. If I change to lighting circuit on drop down and 3% drop, then I get the following:
Required Cable Size 4 mm
Voltage Drop 6.82 Volts.
Percentage Drop 3.0%
Current Load 23.9 Amps
Max Cable Load* 30.0 Amps

So if I use 6mm, a 30A fuse protects the cable run sufficiently. Then can use RCD +RCB or RCBO's accordingly.

 

[JohnW2]

sorry John, a little unclear on my part. Using calculator on TLC direct site. If I change to lighting circuit on drop down and 3% drop, then I get the following:
Required Cable Size 4 mm
Voltage Drop 6.82 Volts.
Percentage Drop 3.0%
Current Load 23.9 Amps
Max Cable Load* 30.0 Amps
So if I use 6mm, a 30A fuse protects the cable run sufficiently. Then can use RCD +RCB or RCBO's accordingly.

That's obviously better, and the calculator seems to suggest (haven't checked it myself) that 4mm² cable would be adequate - so, if that's correct, 6mm² certainly should be OK.

Kind Regards, John.

 

[Electrifying]

One thing no-one seems to have mentioned is that this work is notifiable - as you intend doing most of the work yourself, who's the daft spark who will sign this off and notify for you?
Or have you informed BC of your intentions?

Then there's the TN-C-S issue - if anything in the shed needs 'bonding', you better forget your 4mm & 6mm plans and get back to BAS's 10mm.

Yes, the forum is handy for advice - but not to tell someone nearly every detail of how to do a job - because the couple of details left out could kill them.

 

[ban-all-sheds]

One thing no-one seems to have mentioned is that this work is notifiable

There's a reason why nobody has mentioned that.


ac505

Joined: 26 Apr 2011
Posts: 12
Location: Dunbarton,
United Kingdom
Thanked: 0 times

 

[Electrifying]

One thing no-one seems to have mentioned is that this work is notifiable

There's a reason why nobody has mentioned that.


ac505

Joined: 26 Apr 2011
Posts: 12
Location: Dunbarton,
United Kingdom
Thanked: 0 times

Nice one, BAS.

I need to pay more attention.

 

[ban-all-sheds]

We've all done it at one time or other.

 

[ac505]

no need for bonding so assume 6mm okay. Glad I've been educating myself on all this as I've noticed by CU in the house has no RCD protection. So will be calling in a spark to add a split load board or maybe add RCBO's if space allow.s

 

[londonboy]

You cannot export a TN-C-S supply.

You will need to isolate the armouring at the shed end, and fit a plastic CU with RCD main switch, and earth electrode.

 

[Electrifying]

You cannot export a TN-C-S supply.

You will need to isolate the armouring at the shed end, and fit a plastic CU with RCD main switch, and earth electrode.

Care to quote some regulation numbers or other references for this little gem??