Mcb's And Rcd's For Shed

[sharpie78]

Hi all, I need some advice please.

I've just built a shed/observatory and have a consumer unit in there with a 40A RCD in it (it says it's a garage csu on the box).

It's got a 32A MCB and a 6A MCB in it.

32A MCB has 4 x 2G sockets on a ring main with a 13A fused/switched spur on a radial circuit off one of the sockets.

6A MCB only has 2 low wattage light on it.

Cable run from house to shed is 10metres. Cable used is 4mm SWA.

Load will only ever be computer/monitors/telescope/camera/occasional use of kettle and occasional use of a fan heater.

My sparky mate said all this will be fine but i just wanted to get a 2nd opinion please.

The other things I wanted to check was :

what goes in the csu in the house please ? 40A MCB or 40A RCD or 32A MCB etc etc ?

does the shed/observatory need it's own earth spike in the ground ?

any advice/help greatly appreciated

 

[Murdoch]

Hire your mate to do it correctly?

 

[Evans Electric]

 I'd say a 32A  MCB in the house would be fine .     4mm cable is fine .    You could go 40A but you'll never use all 32A  from what you've said.

Earth spike ?     You need to know what sort of supply you have . You could be opening a can of worms with that one  TBH .  

Do you have a earth loop impedance reading for the shed  ?     

 

[Badger]

id change the shed mcb from 32 to 20 even on ring main on sockets, 6 a for lighting and 32 in house end for supply to 4mm / shed.

 

[SPECIAL LOCATION]

32A MCB has 4 x 2G sockets on a ring main with a 13A fused/switched spur on a radial circuit off one of the sockets.

Load will only ever be computer/monitors/telescope/camera/occasional use of kettle and occasional use of a fan heater.

Sounds like a bit of overkill to me....

Typical DIY overengineering..

16A or 20A radial would be more than sufficient for a few sockets in the average shed.

:C

 

[sharpie78]

ah yes but it's not an average shed. it's an observatory in reality. 

My mate calculated my total load would max out at 21A on the ring main when everything in use. that's why i went for the csu with the 32A MCB in it but from what you guys are saying it sounds like I could reasonably go down to a 25A MCB in the shed.

heater and kettle apparently spike the load by 10A (2kW each when in use). all the other equipment running would only be around 11A nominally, there's quite a bit of kit in there. 15 different pieces of electrical kit including the computer and monitors.

I won't be doing the connections, my sparky mate will be, I'm just running the cables. not doing a diy job. I can wire a plug/socket/light fitting/build a basic circuit/fix electronics etc but when it comes to csu's and mains feeds etc I aint taking any risks. plus he's gonna certify it for me.

thanks for your input guys

regarding the earting side of things.....I'll leave that can of worms closed and just leave it down to my mate to sort that out....I was just trying to get ahead of the game so all he had to do was terminate everything and test it.

 

[SPECIAL LOCATION]

ah yes but it's not an average shed. it's an observatory in reality. 

My mate calculated my total load would max out at 21A on the ring main when everything in use.

Max out @ 21A...

Therefore one assumes with a bit of diversity it will be a fair bit under 21A..????

Kettle will only be on for a very short period of time ....

i.e. it will not be running for hours and hours in one go..

Try and redo you calcs over a sample typical time period of say 6hours...

what proportion of that 6 hours will you be running..

a) > 20A

b) >10A <20A

c) <10A.

If a majority of the time you ARE actually running >20A then you could well need a circuit greater than 20A.. 

BUT..

I would still guess for the majority of the time you are actually running well under 20A..??

Diversity is not a strict set of rules set in concrete..

The circuit designer will calculate based on their specialist knowledge of the circuit they are installing..

I may have missed a key point that you are aware of..

BUT..

I would still be betting that a 20A radial will do your requirements perfectly adequatley...

Try another exercise...

Work out your max load for the rest of your whole house....

Then go look at the incoming supply cable and main fuse..

Bet your calcs would suggest they are both too small!

Guinness

 

[steptoe]

^^^^^^

I'd agree with wot he says, cos he is clever like,

my own shed,

10mm SWA [cos I had some kicking around] on a 40A radial, total overkill, !!!!!!!

apart from the odd occasion I do a bit of welding, and its only on the strike it makes any difference,

I reckon I'd get away with a 4mm on a 20A C type and let the lights flicker if they wanted too,

earthing, well , Im a bit strict in that respect, and wanted it done properly,

it will all be down to your electrician, and how much he REALLY understands earthing concepts,

 

[sharpie78]

Thanks for the replies guys

When we initially worked out Loads etc we did it roughly over the phone, I told him what I would be having in there, how many sockets I needed etc. The main reason for having a ring main with a radial off it is because the observatory is partitioned in 2 halves.

One side is a warm room (with computers etc running on the ring main) and one side houses only the telescope and attached equipment (running off the radial circuit with a 7A psu on switched/fused spur). I wanted to have the ability to isolate the telescope etc with the spur without killing the power to the computers etc and I only had a 2way csu. I do appreciate the spur could have gone at the end of a radial circuit and still have isolated it but we opted for the ring main with radial option. It physically seperates the observatory into 2 circuits (kinda).

I have done as you suggested and re-done some calculations, myself, as my mate aint here right now and I wanna get it all right so he can just do the final connecions..........

majority of the time it will be running at 9A and will increase to 11A when the telescope is slewing to different areas of the sky (it draws an extra 2A when doing this according to the manufacturers specs).

During the winter while I'm in the observatory at night I would expect to run the 2kW heater for an hour on/half hour off etc(I like to be warm), which would draw an additional 5.03A.......so as long as my ohms law calculations are correct.......14A total constant load with it increasing to 16A for short periods of time (telescope slewing).

It is unlikely I'll be boiling the kettle while slewing the telescope so I doubt it will ever actually spike up to 21A.

Based off this and the info you guys have given I'm gonna swap out the 32A mcb for 20A mcb.

Earthing.....my mate has got his 17th edition, Part P, whatever else you need to be fully qualified etc etc and is able to certify so he knows what he's doing. He's said it will probably need a seperate earth as he has done work in houses similar to mine but hasn't actually had the time to look at the csu in my house yet as he's so busy so doesn't know for sure what type of earth I already have. My reason for asking on here was so that it was all ready to go when he gets here. I've done my own reserach and I'm 99% certain I have TN-C-S.

any holes in my thinking.....please point them out !!

Thanks again for your responses

 

[speedster]

Hi I was just reading your post.

A couple of simple points which may help you out.

,

If you or a friend or colleague is a NAPIT member,once the member logs in, there is a download section containing technical advice.

One of the topics discusses supplies to sheds/ outbuildings in great detail. Included is advice on running supplies to a shed/ workshop from properties supplied by any of the three main types of domestic supply, i.e. TT  TN-C-S and TN-S.

Regarding calculating the load at the shed end, the electrical safety council has produced tabulated information for calculating maximum load, including considerations for diversity.

If you want to find out the maximum load for a distribution board in a property, why not try measuring it ?

Have a word with the owner about their usage of equipment such as washing machine/ cooker/ tumble-dryer /electric shower/s, what  and when are they in use,

and measure the load using a clamp ammeter at the meter tails.

Obviously you need to find out at what time of day most equipment is in use.

Hope this is of some help.

Speedster.

 

[apprentice87]

"During the winter while I'm in the observatory at night I would expect to run the 2kW heater for an hour on/half hour off etc(I like to be warm), which would draw an additional 5.03A.......so as long as my ohms law calculations are correct.......14A total constant load with it increasing to 16A for short periods of time (telescope slewing)

heater and kettle apparently spike the load by 10A (2kW each when in use)"

Think you need to re-visit Ohm's law....

john..

 

[OnOff]

Just finishing off the telescope pier in my brother's garden for him. A long job as I don't get over his way too often. This is it all mocked up before the steel went for blasting and painting. He's just finished the mass concrete plinth so hopefully all in before or over Xmas. Hoping for an invite for the star gazing & beer etc!



http://s1185.photobucket.com/user/c...001_801360676254860326_n_zps2feff5f5.jpg.html

He currently only has power to the garage on a bit of 6mm SWA which I at least managed (over the phone) to get him / his builder to run in a proper trench when his new drive was done. Now wants power taken onto the shed for light/heat/pc with a run out to the plinth for a socket there - I welded a plate on for this.

Trouble is we have differing opinions on how to do things. I'm a "dig a 2'  deep trench, line, cover with tiles, back fill with warning tape" type but he's more "...slip it under the turf in blue water pipe......I'll know it's there"  :facepalm:  Easier to steer clear sometimes and let someone else do it. Saying that the local bloke he got to connect the 6mm to the garage has removed the old board and fitted a new split load BG board (on the pi$$) with 30mA RCD protection to the garage...........where there's another board with another 30mA RCD. No thoughts to discrimination etc.

 

[sharpie78]

update....... my mate came and connected it all up today and it's all working perfectly. currently sitting in my obsvervatory with the heater on taking images of the orion nebula he tested it all using his meter and all the readings were spot on. dunno why I was worrying....should of known he'd get it all sorted.

regarding the heater ohms law comment ......I = W/V.....2000/230 = 5.02A. Is that not right John ? kettle and heater both 2kW so 2 x 5.02 =10A of load when in use ??

regarding telescope pier......nice pier.....I did mine in one solid concrete form. if i had a pic i'd upload it but my camera is busy pointing at the stars

 

[Andy™]

if you get 5.02 from 2000/230, i think you need a new calculator...

 

[sharpie78]

hadn't thought about checking it on a calculator......I just went on this website......csgnetwork com/ohmslaw2 html and assumed it would be right.

(I think the saying is....... "assumption is the mother of all f*** ups")

you'd have thought a website for doing ohms law calculations would work it out properly eh. lol. now used my calculator 8.69A

figured out why I got 5A......I hadn't unchecked the 3-phase box.....Doh !!

 

[OnOff]

update....... my mate came and connected it all up today and it's all working perfectly. currently sitting in my obsvervatory with the heater on taking images of the orion nebula he tested it all using his meter and all the readings were spot on. dunno why I was worrying....should of known he'd get it all sorted.

regarding the heater ohms law comment ......I = W/V.....2000/230 = 5.02A. Is that not right John ? kettle and heater both 2kW so 2 x 5.02 =10A of load when in use ??

regarding telescope pier......nice pier.....I did mine in one solid concrete form. if i had a pic i'd upload it but my camera is busy pointing at the stars

We looked at various options - was even considering a CARDBOARD column former for casting a solid concrete one or doing a slip form one etc. Went for steel in the end as I'm comfortable with it. Shot blasted, zinc flame sprayed and finished in 2-pack epoxy. Studs in the base and for levelling are all stainless steel as are all the fixings. I basically cheated and COPIED this Altair one that my brother wanted but didn't want to pay that much for. I quite literally measured from the photo with an improvised scale then stuck it on CAD:





Saw AFTERWARDS some astronomy bod on YouTube slating the Altair design saying there's no need for the top part to be adjustable. Just hope it all works............ ray

Thinking once it's bolted onto the base to fill with pea shingle etc to "dampen" any vibrations down. What scope have you got, my bro has the Celestron 9.25" SCT.

It's funny, if his shed and greenhouse were swapped over we'd have done a sliding roof on the shed for that proper observatory feel! He is though now on about some Wi-fi  / Cat-5 box you can get to control the scope from indoors.

 

[sharpie78]

I was gonna copy the altair design too but doing it in concrete was quicker and cheaper When I was planning to do it in metal I was told to use sand to dampen the vibrations. I don't know which would work better.....sand or gravel. In my mind I'd think sand would be the better option but don't take my word for it.

oh yeah.......don't listen to the bod on youtube. My pier adapter is 2 brake discs from a corsa bolted together with 20mm bolts. They're 300mm long so I've got loads of adjustment there if I need it and it's rock solid.

I've got a Skywatcher Quattro 8S newtonian and an ST80 for guiding mounted on it and I haven't had any issues at all.

I ran cat5 back to the house so I can control it from indoors when the kids are in bed. I've always prefered hardwired networks to Wifi.

There's a good website called "East Midlands Stargazing" if ya want a good place for info. I rate it higher than "stargazers lounge". They're all really nice people on there and always willing to give advice. I've got a post on there about my observatory build if ya fancy a read. the post is called "obsy build in norfolk". I'll warn ya.....it's quite long. lol. Maybe I'll see ya on there sometime