Wiring a cabin

[cralph]

Hi,

I have used this site before and found it very informative so thought Id ask another question.

I'm installing a log cabin for use as an office in my garden which obviously needs power. To facilitate this, I am installing 6mm 3 core SWA connected to the main consumer unit via a 40amp mcb with a 40A 30mA RCD in the cabin consumer unit which will run a lighting circuit at 6A (1.0mm twin and earth) consisting of two flourescent tubes and a ring circuit at 32A (2.5mm twin and earth) with approximately 8 double sockets.

The SWA cable will be buried approximately 600mm with a total run of approx 30 meters.

I have been measuring my current voltage and ampage draw for a number of months and they are very low so my question finally is, have I done my calculations properly and chosen the right cable and MCB/consumer unit setup?

Thanks in advance for your advice,

Chris

 

[securespark]

Your calculations are close!

Your 40A circuit, using 6mm² on a 30m run would mean a VD of 8.76V.

OK, so you're allowed 4%, but this is very close to the mark. I would use 10mm².

 

[cralph]

Thanks, I'll increase the cable to 10mm in that case.

chris

 

[ColJack]

8.76VD?

that's not even down to the 230 you're supposed to start with..
and then it's +/- %10 from there ( 17th edition.. )

if it's an office what eqipment will be on the sockets? if it's lots of computers then they have high earth leakage and need to be wired special..

double earth in the backs of the sockets, one earth wire to each.. and each earth at the board needs to be in it's own screw..


EDIT:

having said that I would put 10mm² anyway since it's a certainty that at some point you're going to think..
"hey, I can put a socket in for my lawnmower, I think I'll put a fountain out front of my office, I can do with some garden lights" or some other little extra that will need more power..

better to spend a bit more now than to have to dig it back up again in a few years to uprate it..

I'd also put it in ducting and run a seperate duct from the house for phone/data/whatever else while you have the trench open.. maybe even a water line and gas ducts.. ( you never know.. small gas heater or tea making facility.. )

 

[Lectrician]

You can only drop a maximum of 4% of the supply volts - weather that is 230, 234, 238, 240 etc.

If you want to comply with section 607 you would deffinantly need 10mm as the submain.

 

[cralph]

I am installing LSF crush resistant conduit to in the trench to carry CAT6 network cables and telephone lines but I thought that using SWA meant that I didn't need to put it in ducting?

Installing the double earth is not a problem so I'll make sure thats done but I'm intrigued about the high earth leakage on computer systems. Most of my equipment is standard home/office stuff (desktop PC's, laptops, printer etc) although there are a couple of commercial rack mount devices. Do you have any additional information on this?

Thanks

Chris

 

[bernardgreen]

I'm intrigued about the high earth leakage on computer systems. Most of my equipment is standard home/office stuff (desktop PC's, laptops, printer etc) although there are a couple of commercial rack mount devices.

The power supplies of most modern equipment have input filters on the mains input with capacitors between Live and earth and Neutral and earth. These capacitors produce an earth current of between 1 and 5 milliamps when ever there is mains power at the power supply input. The equipment need not be operating, the filter is always on line.

These earth currents add up and about 10 PCs will trip a 30 milliAmp RCD.

Best option is to have the ring main NOT protected by an RCD. Divide the equipment into clusters of about 5 units and each of these has a local RCD protected socket(s) feeding the equipment.

A rack can be considered as a cluster with local RCD.

Some equipment with heavy filtering may not work with an RCD as the filter has an earth current high enough to trip the RCD. ( office equipment is UN-likely to fal into this group ) These may require special consideration.

 

[Lectrician]

The more current that leaks to earth, the more of a fault 'touch' voltage would be present on the earthing arrangements should an earth become disconnected.

In a normal radial or ring circuit, if the small 1.5mm earth falls out of the terminal due to beeing loose or severed by the screw (does happen), then you have an open circuit earth on one or more sockets - this open circuit earth will allow volts to float, expecially due to these leacky bits of equipments. There is a real shock hazard.

To over come this, the regulations state a minimum of 10mm/sq earth used - try dropping that out of a terminal by mistake!

OR OR OR

The circuits earth must be wired in a ring format - with each conductor in a seperate terminal (so two earth terminals on a socket). This means if ever a single earth does pop out for some reason, all socket remain earthed from the other direction.

To achieve this ring of earths you can either:

- Wire the sockets as a ring, and terminate the earths seperately. This is the easiest way.

- Wire a radial circuit, taking the last sockets earth back to the cu on a single core earth cable. This single core earth cable could be a 4mm single, the amour of a cable, the metal conduit/trunk etc.

- Wire two radials, and link the two last sockets earths together with a piece of 4mm singles.

The submain is treated as a radial circuit, so unless you provide a looped return on the earth, you need to use 10mm or above for the earth on the submain.

Don't confuse clean earth with this high integrity earthing - too many people do! A clean earth is an earth path from socket back to the mains, which doesn't share with any non-it kit. An example of this would be a socket with a fridge plugged in in an office. A clean earth socket commonly has a special plug to prevent other kit being plugged in, and will have two earth terminals, one for the clean earth, and one for the main earth (for the back box, and rest of the circuit). Funnily, these clean earth sockets will not comply with sec 607 as they do not have dual earth terminals for the clean earth. (the ones I use don't anyway).

Clearer? Or more confused!

 

[cralph]

Wow! This information is extremely useful although to clarify my usage requirements - I'll be running no more than 3 1u rack mount servers at any one time (these are the only commercial grade pieces equipment that I have but even so they are no more than a fairly standard computer in a small form factor), a PC, 3 monitors, 3 laptops with a couple of switches and 5 firewalls (no I'm not paranoid I test them for a living) and if it gets really hot in the summer, a portable air conditioner. All of this equipment is currently in use in my existing office (a spare room) which is running an rcd protected 32A circuit which has not tripped in over 6 years.

The advice for double earth and submain requirements has been very useful and I'll be changing my installation accordingly but is it necessary for me to do anything else or just advisable if I were going to submit the circuits to near full capacity?

Chris

 

[ColJack]

nominal voltage is stated in the regs as 230v +10% / -6%.

permited voltage range of 216.2 - 253.0V

it is proposed that as of january 2008 the tollerance be altered to +10% / -10%..

giving a permited voltage range of 207.0 - 253.0V



as for the SWA in the trench, no you don't have to have it in a duct, but for the cost of 30 meters or so of black drainpipe ( must be black to signify power ) it saves a lot of hassel should you ever need to replace the cable for any reason.. ( gets damaged where it emerges from the ground, small fire in the shed melts it a bit .. or any number of unforseen occurances.. )

 

[Lectrician]

nominal voltage is stated in the regs as 230v +10% / -6%.

permited voltage range of 216.2 - 253.0V

it is proposed that as of january 2008 the tollerance be altered to +10% / -10%..

giving a permited voltage range of 207.0 - 253.0V

That is fairly irrelevent, as the regs state a 4% max drop from origin to final point of utilisation - regardless of supply volts.

 

[ColJack]

yeah i saw that just after i posted, it does however say NOMINAL volts, which i took to mean 230V which is what is used in the calcs in the regs....

hence my original statement of

that's not even down to the 230 you're supposed to start with..

how can we be expected to comply with a set of suggestions that seemingly contradicts itself at every turn..????

 

[dw3101]

or so of black drainpipe ( must be black to signify power )

I cant quote the regs but that sounds bad to me.. use black 4" waste pipe -no cable marking tape and someone cuts the pipe with a stihl saw to add a drain..

Spend the money and buy proper cable ducting.

David

 

[davelx]

This may be of interest/help:

http://www.pavingexpert.com/ducting.htm

 

[Lectrician]

The info in the link is a little incorrect.

Low Volts Lecky (as in 240v) is a BLACK duct
High Voltage is RED
Orange is traffic lights
Purple is street lighting / sign lighting (or black.....)
Yellow is gas
Green is CCTV/fibre