size of cable and consumer unit help Ask questions and find answers to many subjects relating to electrics and electrical work

[danny001]

hi,
iam interested in running a power supply to a shed i have
i will want only 1 light and 2-3 sockets i will have a grinder and a table saw in the shed,and more want to had later

my question is what size cable should be used i have read a few post and one says 4mm is not enough for 2x 32amp + a 6amp and then one says it will be enough i will not be using that much power, it there a chart that shows whats - what

i have seen a 5way mcb with a 100a will this be ok

also when the cable is run from my consumer unit i know it has to run from the rcd side but what size mcb does it have to be conected to
will i have to install a 100a in my house board to feed the 100a in the shed

thanks

[sparx]

hi Danny, leaving aside the part p requirements, which I know you are aware of.
If only one person working in there at a time then can only use 1 tool at a time so allow load for biggest plus heater in winter plus 2Amps for lighting. say total load 25A 4mm should be ok if not too far, mcb to protect 4mm max say 32A .
It doesn't matter what size switch on garage unit 1000 A can only draw max 32A chears Sparx

[danny001]

hi,
thanks for the reply

but does the garage unit need to be rcd type
reason i ask is that i have a spare 32amp mcb in my home unit and its on the rcd side can i run the 4mm cable of this 32amp mcb and to the garage unit to the 100amp
or will the fact that its already rcd on my home unit ok

also i have access to a new 17thed book but iam not a electricain - is they a page in the book that give what mcb to what cable sizes
ie how do you know a ring main in 2.5mm is on a 32amp

thanks

[ericmark]

The IET do a magazine called “Wiring Matters” Issue 16 Autumn 2005 has an article “Electrical installations outdoors: a supply to a detached outbuilding” this will help I am sure it is available on the IET web site as a PDF down load.
Cable size is worked out using volt drop and current carrying capacity in the main as these are normally the most limiting factors.
So 4mm max current around 32 amp depending on reference method for installation and drops 9.5 mV/A/m so for example if feed with 25 amp MCB you can run about 25 meters or at 32 amp about 19 meters. This is working out at the 6.9 volts allowed for lighting.
2 x 32 amp would be too much but I would not think you will draw over 1 x 32 amp.
Using a mini consumer unit with a 25 and 6 amp mcb for power and lights should be enough the RCD can either be in mini consumer unit or in main one in house if using SWA I would fit in shed if not then in house. The MCB feeding in house needs to be bigger than largest in shed so if 25 amp in shed then 32 amp in house.
The 100 amp on consumer unit refers to maximum the switch can handle and there is no way you would want a 100 amp feed to shed I would guess 32 amp max.
Most sheds would only need a 6 and 16 amp mcb in shed and 20 in house feeding it.
Earthing is covered with IET document in general if not feeding anything outside the shed and the shed is wood then the house earth is normally good enough but if there is metal on outside of shed or power from shed is used for garden then more earthing considerations will need to be made but this will be all covered with the Part P paper work when that is completed and I am assuming you are Part P registered to be doing this work or under LABC watchful eye so I will not expand on this complex subject.
Eric
PS what happened to pump?

[danny001]

hi,
thanks
so just so iam getting this right if the swa is wired into a 32amp mcb in the house it can then be feed to a smaller consumer unit in the shed but must be smaller then 32amp

but does it not have to be feed into the main switch which is say a 63amp which then inturn feeds the 3 x mcbs
( this is the consumer unit i have seen )

if it has to be feed into the main switch which is say 63amp and has 3 x mcb spaces
will the mcb in the house it comes from have to be over 63amp

or am i missing something
thanks

[danny001]

hi sorry the pump is still on going he is going to send me some info on the size

thanks

[ericmark]

If you use a 32 amp mcb in the house you would in the main want the mcb’s in the shed to trip so they would be smaller than 32 amp although using 2 x 25 amp is still OK as we would not expect it to be fully loaded on both circuits at all times. It would be normal to include a main switch on any consumer unit which would in turn feed the mcb’s this main switch could be a RCD the amps at which the main switch or RCD is rated must not be lower than mcb feeding it but of course can be higher.
What we are looking for is in the main the closest device to any overload will trip first no point in having to walk to house every time there is an overload one wants the shed one to trip out first.
The only exception to this seems to be caravans where a 30ma trip is required in both the caravan sites supply and the caravan but this is really in case some caravan site has not been up-graded. Sometimes one is left with no alternative for example we jump from 10 amp to 16 amp with mcb’s so if we wanted to use an old 3Kw electric fire we could not use a 10 amp mcb so if the cable would not take 20 amp next size up than we may need to fit 16 amp at both ends this again is common with caravans in the same way as 13 amp fuse in extension lead plug and in item plugged into extension lead but where possible we try to avoid this as it doubles up where we need to check in case of faults.
The big thing is to see the difference in what an item can handle without damage i.e. the 63 amp printed on RCD and the amount of current which will cause automatic disconnection like B63 printed on MCB the first we look for something bigger than required but the second we select the maximum we want to flow.
So a B63 MCB will trip if 63 amp passes through it for a extended time or if 195 amp passes through it for milliseconds the B in front of the number means it will trip nearly instantaneously with 3 to 5 times the rated current there are in fact two independent devices built into the same package.
So returning to caravan example using a C16 MCB on caravan site and B16 MCB in caravan would give some discrimination between the two devices it’s all maths really which is why doing a degree in any engineering subject has as many hours of maths as nearly all other subjects combined.
All best Eric

[danny001]

hi,
thanks for the info its been a great help

[TOPSPARK]

Aside part p regulations. What needs to be done is that as the shed is classed as an outbuilding,under current regulations you take a 6mm twni and earth out to the shed and into a 2way garage consumer unit.Off the 2way consumer unit you take a 10mm earth outside the shed and then put an earth rod in the garden making the shed consumer unit a tt system.Also when connecting this cable in the consumer unit in the house DO NOT CONNECT THE EARTH CABLE IN THE TWIN AND EARTH. This is the correct way of doing it as this was a question that came up earlier this month when i had my annual part p surveillance vist from my part p provider

all the best
topspark

[sparx]

Sorry Danny but must TOTALLY disagree with statement by 'top spark'.
This is another fallacy put about usually by NIC people.
It is not REQUIRED to make an out building TT with an earth rod in pit at garage end, it is an OPTION if the house supply is of certain types but even then the cable supplying the outbuilding MUST be earthed at the house end in order to protect it from harm if cut through, just like the supply to a caravan pitch.
If doing it this way then you can use whatever size cable will carry the load, subject to volt drop calcs. there is no minimum size, and the earth to the rod need only be 6mm2 as your RCD will be off 30mA rating to comply with TT requirements,
BTW ERICMARK is spot on,
and it's a pity some of the organisations are apparently giving misleading info to their 'domestic Installers'.
I read the IET article in Wiring Matters Mag. & a similar one in NAPIT's Competent Person Mag. recently and they both agree.
Don't want to get into arguement on here but it is important to give out correct advice,
regards SPARX

[ericmark]

If your not sure what to do read http://www2.theiet.org/Publish/WireRegs ... tdoors.pdf this tells you all you need to know and since published by IET can be taken as correct. The 17th Edition did come out after this was published but that has not really changed what it says.
As to TT or TN-S if shed is non metallic then no need for TT supply. If supplying a greenhouse with metal holding glass which was away from the house by some distance I may consider a TT supply but in most cases a TN-S supply i.e. earth taken from house is no problem.
Anyway read yourself and see what you think. Sorry should have given that link ages ago.
I will not say Top Spark is wrong but as usual what Sparx says is correct. It would be unusual to require earth rods. And believe me with Sparx's hobby he knows all about earthing.
All best Eric

[danny001]

hi,
i have abit more info now as went to see him last weekend

the pump going in is also a type of pool heater and it has a current of 30amp

i had a look at this consumer unit in his house and this is how it goes
off the main consumer unit is a seperate 20amp wylex fuse this is on its own and runs to the shed

in the shed is a small consumer unit which comes from the 20amp fuse in the house it has a main 32amp breaker and off this is 2 x 20am 1 x 10amp and 1 x 6amp all series e c type mcbs

there is space for some more

i understand that the cable need will be 4mm
but can a 32amp mcb be added to the shed consumer unit and the 4mm run of this to the pump which will be no more then 3 metres away

will i need to replace the 32amp breaker which is already in the shed consumer unit to something higher like 40 or 50amp

any help would be great he has had a electrian in
but got confused with it all as he just needs to know whats what so he does not getting ripped off

thanks again danny

[ericmark]

You have to do some careful measurements. I would say the volt drop is often the most limiting factor on shed runs but also cable must be able to take max load.
4mm² SWA cable PVC reference method D (direct in ground) has maximum current of 37 amp you have talked about 63 amp so you would need at least 10mm² which is rated at 60 amp. Using thermosetting cable rated at 90°C instead of 70°C will raise these figures to 43 (4mm²) and 71 amp (10mm²) but from what I understand cable to shed is already laid?
With such a heavy load it would be easy between shed and house to exceed the 100 amp incoming supply and blow supply fuse.
If a 40 amp supply was used from house then you would be most likely near enough to the 37 amp max not to really matter but with pump running that’s only 10 amp for everything else.
I would be seriously looking at other heating methods.
You say he has an electrician on site I hope he is registered under Part P. If so then once complete he will submit all the test results to his overseeing organisation who will in turn submit them to LABC so with three people looking at data there should not be a problem.
Someone on site will see things we will not and at end of the day it is his signature on all the paperwork so up to him how he does it.
All best Eric
PS I will not answer new post as once answered people tend not to add their thoughts so I will leave and see what others think.

[danny001]

Hi
thanks for the reply
sent him your info and just got asked

so what he is looking at is replacing the cable with 10mm

and putting in a new consumer unit in the shed with say a 80amp breaker and have the 2 x 20amp 1 x 10amp 1 x 6amp and the 32 amp mcb for the pump/heater running off this
and in the house where the separate shed fuse is can this be upraded to 80amp also or will it need to be lower

thanks danny

[ericmark]

I have given examples I can't work it out for you. How it is run and where it is run and how far it is run all make a difference to final selection and we just don't have enough information to do it for you that needs to be done by someone on site. Too easy to miss some vital fact and get it all wrong. Who ever is going to sign the design section of the installation certificate must do the design.
Eric