Please advise

Sav · Nov 25, 2009

Hi again guys,
Just a quick question.
I have just wired an office and store room built in the back of a 44 metre garden.
It is brick built and split into office and store room.
I have installed a 17th edition RCD split load board 10 ways ( only 5 circuits being used ).
A 16mm armoured cable has been run from inside the house ( 25mm tails into henley blocks ).
My question is can i get away with using a Wylex 60amp isolator (25mm tails in supply side,16mm armoured in load side), to isolate the office power, or do i need to put in an RCD isolator ??? The consumer unit in the office has a 100amp main breaker ( which I am replacing for a 60amp one).
The armoured comes out of the side of the house, along the bottom side kitchen ( about 3 metres), the buried in the ground ).
Sorry to be so specific, but any advise / help would be great.
Thanks again guys,
Sav

supasparxs · Nov 25, 2009

Sav said:
Hi again guys,
Just a quick question.
I have just wired an office and store room built in the back of a 44 metre garden.
It is brick built and split into office and store room.
I have installed a 17th edition RCD split load board 10 ways ( only 5 circuits being used ).
A 16mm armoured cable has been run from inside the house ( 25mm tails into henley blocks ).
My question is can i get away with using a Wylex 60amp isolator (25mm tails in supply side,16mm armoured in load side), to isolate the office power, or do i need to put in an RCD isolator ??? The consumer unit in the office has a 100amp main breaker ( which I am replacing for a 60amp one).
The armoured comes out of the side of the house, along the bottom side kitchen ( about 3 metres), the buried in the ground ).
Sorry to be so specific, but any advise / help would be great.
Thanks again guys,
Sav

Are you designing the garden office to run the whole 60amp?

If so the cable maybe a bit small?
Are you sure that the garden office need that much power?

If you do need that much power do the volt drop cals (they are in my regs and not to hand atm.
The 100amp isolator that yopu are planning on downgrading to a 60amp......
why?
do you have a spare 60amp?
remember that it's a main switch not a breaker so it will not protect against over current.

Good luck

Richard

Deleted member 9648 · Nov 25, 2009

You would only need an RCD isolator for the distribution circuit at the supply end if the earthing system is TT.
The installation will need a full cert and regeresting with building control.
As Richard has said the main switch in the DB is just that and not overload protection therefore the existing 100a will be fine
60a overload protection for the distribution circuit must be provided at the supply end via a 60a fuse in your isolator.

mechelec · Nov 25, 2009

wirepuller said:
You would only need an RCD isolator for the distribution circuit at the supply end if the earthing system is TT.

Why?? the RCD would be better at the remote end as if it trips at the origin you have a long walk to reset it. The new supply is armoured cable so no worries about buried in walls less than 50mm etc. please explain.

Sav · Nov 25, 2009

Thanks for all your help guys with this.
I was going to leave the 100amp main switch on, but a numbnuts fiend of mine said you have to change that as well !! Thanks for clearing that up.

One final thing, does anyone have a quick way of working the volt drop / cable size required. I keep flicking the BS7671 and on site guide for the formulas, but wondered if any of you had a quicker way to work these out.
Thanks again for all your expert help.
Best site ever.
Sav

supasparxs · Nov 25, 2009

Sav said:
Thanks for all your help guys with this.
I was going to leave the 100amp main switch on, but a numbnuts fiend of mine said you have to change that as well !! Thanks for clearing that up.

One final thing, does anyone have a quick way of working the volt drop / cable size required. I keep flicking the BS7671 and on site guide for the formulas, but wondered if any of you had a quicker way to work these out.
Thanks again for all your expert help.
Best site ever.
Sav

to work out the VD I need the following information
voltage =230v
cable run =44meters
Design current =????

what is the design current?

Sav · Nov 25, 2009

Design current is 43amps.
Does this help or do you need further info?
Thanks again,
Sav

supasparxs · Nov 25, 2009

Voltage Drop Calculator

10mm cable will have a volt drop of 8.42 amps 3.7%

based on a non-lighting circuit

you say that a 16mm SWA has already been pulled in well the volt drop on that cable has a VD of 5.6volts (2.3%)

Deleted member 9648 · Nov 25, 2009

mechelec said:
wirepuller said:

You would only need an RCD isolator for the distribution circuit at the supply end if the earthing system is TT.

Why?? the RCD would be better at the remote end as if it trips at the origin you have a long walk to reset it. The new supply is armoured cable so no worries about buried in walls less than 50mm etc. please explain.

Click to expand...

Explanation....if i'ts a TT system RCD protection will be required at the supply end of the distribution circuit as the Zs will be too high on the earth electrode for the required disconnection time of the distribution circuit to be met by the 60a fuse alone. Remember the armouring of the SWA will (or should) be earthed and disconnection must take place within the required time in the event of a fault on the cable. Tripping at the supply end in the event of a fault in the building on the final circuits will not happen, as the RCD at the supply end of a properly designed system will be a 100ma or 300ma time delayed type,with the final circuits in the building protected by 30ma RCD's which would trip before the time delayed RCD at the source of supply.

Edit...QUOTE "You would only need an RCD isolator for the distribution circuit at the supply end if the earthing system is TT"
just noticed my original post can be read two ways.....I'm not saying that if the system is TT the only RCD required will be at the supply end....but that an RCD will be needed at the supply end as well as in the building.