Ib of radial circuit supplying workshop

[scott-fitz]

Customer is wanting a feed to his workshop at the bottom of his garden, he has specified for 6 twin sockets and two 4ft fluorescent lights.

Going to run a SWA from his main CU in the house, then convert to pvc conduit inside the workshop feeding the sockers and feeding the two fluorescents from a FCU.

His main piece of equipment is rated at 1800w and he will be using a handful of power tools in there, obviously not all at once.

The run to the workshop from the house is 20m.

my question being what type of diversity would you use in relation to calculating the Ib

Thanks

 

[Deleted member 26818]

I would use the diversity tables in OSG.
I believe they recomend 100% of the main load, and 40% of the rest.

 

[Gordon]

Diversity isn't an issue as you would only require 1 power circuit, use I(n) protective device rating for your cable calcs. 6mm armouredcable with
design current of 32A will give volt drop of 4.7 volts over 20m which is within 3% limit for lighting. A lot of people fit a board for these type of installs
but a radial power circuit with a fused switch for lighting is perfectly acceptable and cheaper.

 

[Deleted member 26818]

Diversity is also applied to socket-outlets as well as circuits.

 

[thekingiam]

What is he going to be using shed for?
Have you thought of stroboscopic effect?

 

[scott-fitz]

There should be no issue with stroboscopic effect from the tools he has described to me, but I will be mentioning it if he's planning on having any rotating machinery ect, how would you get round the issue tho with it only being single phase? Yeh the main reason behind the one radial circuit is trying to keep the cost down, using the tabels in the OSG i got:

Ib=39.313 (13a + 5 x 6.5 for the sockets) + (0.313 for two 38w florrys)

In= going for a 40a 60898 mcb

Iz= 49a based on 6mm two core swa, with 7.3 mV/A/m, from appendix 4

so, 39.313 < 40 < 49

volt drop= 7.3 x 39.313a x 20m / 1000 = 5.7v which is within 3% for lighting

Anybody see anything wrong with this?

 

[scott-fitz]

forgot to mention it's clipped direct and not burried ^

 

[malcolmsanford]

I assume your fitting a local DB, then add up your final circuit protection devices in the sub board and multiply by 0.4 so if you had 32amp sockets and 6 amp for lights it would be 38*0.4 = 15.2 amps. Old rule of thumb and IMO still works.

 

[SimpleSimon]

Especially if power tools are going to be used, you'd be best following Malcolm's advice and fitting a DB for convenience.

 

[telectrix]

no mention of RCD as yet., in house or shed?

 

[Amp David]

I'd fit a 2 way CU aswell. 1 MCB and 1 RCBO. You can get the RCBO for about 12 quid.

 

[telectrix]

I'd fit a 2 wat CU aswell. 1 MCB and 1 RCBO. You can get the RCBO for about 12 quid.

unless he's coming off a RCD protected way in the house CU. ( don't mention TT or exported earths!)

 

[Amp David]

Well of course Tel. Just presumed that it wasn't thats all. The good old exporting earths discussions, don't you just love em! not

 

[ezzzekiel]

could you please explain about exporting earths?










mwahhahahahahahahahahahahhaha

 

[Amp David]

Right well it goes something like this........ blah blah blah!

 

[scott-fitz]

Sorry, RCD is fitted in the main CU, want to stay away from fitting extra board.

 

[malcolmsanford]

Sorry, RCD is fitted in the main CU, want to stay away from fitting extra board.

So your design for this guy in his workshop will be. RCD in the house 20m away, if something trips in his workshop, which is more likely in this type of enviroment than in the actual house, he plunges himself in darkenss in mid winter 20m from the house, he takes the whole house, or if a split board half of it anyways, out with it, he then as to find his way back in the dark, into probably a dark house to reset the RCD..................hmmmmmm

 

[SimpleSimon]

Sorry, RCD is fitted in the main CU, want to stay away from fitting extra board.

You could of possibly got away with it if the workshop wasnt at such a distance from the house and the customer wasn't using such high powered equipment which could be prone to nusciance tripping.
Are you sure you don't want a small two way DB? Fed from 4mm SWA perhaps with two RCBOs?............tempted?

 

[Dave_]

oh dear not again.....

 

[Dave_]

is the DB in the house split load with one side non-RCD protected with a spare way? would make life easier.... 40A CB in house DB, 6mm SWA feed to new DB as Simple Simon and others have quite rightly recommended with 30ma RCD in new CU in shed. No messing about with discrimination between RCD's. Well thats what i'd do if possible.