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.

 

[scott-fitz]

Sorry fellas realised I'd been very vague with this post,

I suggested a small 2-way board when viewing the job, the customer was adement that he didn't want one (for whatever reason).

Theres a dual RCD CU within the house and my plan was to come off the board with a radial fed in SWA to the outbuilding were I would pick up the sockets, and feed the two lights from a switched FCU. worked out (within reason) the most he would have on in there at once, allowing for his main tool, the two lights a heater and a radio, was coming out at just below 20A.

Going back to malcom's post, the board is a dual RCD and to avoid unwanted tripping I would have to ad an rcbo, not fed from either of the rcd's, which ment converting the board, so the plan was to install on the RCD with the upstairs lighting on, so if it did trip, the downstairs lights wouldn't be effected, limiting the danger of walking around in the dark to find the CU. Even if I installed a RCD board in the outbuilding I would have the same problem unless I installed a time delayed RCD upstream to the outbuildings new board.

After all the messing about I went back to the customer to "talk him around" to having a small board in the outbuilding which he's now agreed on after a little debating

New plan of action is to install a small metal clad board in the outbuilding with a 63A main switch, 20A mcb for sockets and a 6A mcb for the lighting, fed from a 32A mcb on the opposite side of the house CU to the downstairs lights (32A mcb still being protected by a 30mA RCD) fed in 6.0mm SWA.

Anybody see any problems with this, open to suggestions

Thanks

Scott

 

[malcolmsanford]

Scott Henly block the tails and take 10mm tails into an enclosure something like this


Buy Wylex Switch Fuse, Metal, 45A, 1 Way - 106RED

or there are so many you can choose from.

Then take your SWA from that to your garage and fit sometihng like this in the workshop

Builder / Shower Supply Units again there are many different types you can get.

IMO this is the best installation you can give to the customer and it's not going to be that expensive.

Also not trying to start another thread, but if your going to TT that workshop then it is advisable not to fit a metalclad CU as you will have to protect it with a 100mA front end RCD and then 30mA your final circuits. Stick with insulated.

 

[TPES]

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.

I thought it was add up all circuits in the CU except the highest rated MCB, then x 0.4, then add to that figure the highest rated MCB in the unit

Example: 3 x 32a, 3 x 6a, 1 x 16a, 1 x 20a
Take off one of the 32a MCB's. Then add rest up = 118a, then x by 0.4 = 47.2a then add on your 32a you took away at beginning 47.2 + 32 = 79.2a
Although not 100% accurate Is this not the rule of thumb?

 

[malcolmsanford]

That is the way the OSG does it in for socket outlets in Table 1B section 10 and final circuits 9.

The way I do is an old way the DNO lads use to do it. That add up the total of all the protection devices and *0.4 so mine would be 150*0.4 = 60amps. Whch is a normal domestic installtion which your scenario seems to be.

Remember that the DNO lads not only dealt with domestic but commerical and industrial. so they devised this rule of thumb 0.4 for domestic, 0.6 for commerical and 0.8 for industrial. It's to be used purely as rule of thumb and I would not say that mine is the right way to do it and yours might be a tad to high. Or yours is the right way as mine is to low.

 

[i=p/u]

just feed from mcb in house cu, to outbuilding, 2 way board rcd protected, tell the wife no tv while im in the shed

 

[WDMDL]

You can get around any stroboscopic issues by using HF luminaires. They needn't be more expensive than bog basic orange DIY shed switch-start ones if you shop around, and are much more efficient too.

This is what I measured on my little power meter for comparison:
Electronic HF: 104 lumens per circuit watt (5ft 58W T8 Triphosphor, 5200lm, 50 circuit watts, PF0.99)
Switch-Start Non-PFC cheapie: 55 lumens per circuit watt (5ft 58W T8 Halophosphate, 4800lm, 87 circuit watts, PF 0.61
Switch-Start PFC corrected: 68 lumens per circuit watt (5ft 58W T8 Halophosphate, 4800lm, 70 circuit watts, PF 0.94

Simon.

 

[Engineer54]

You can get around any stroboscopic issues by using HF luminaires. They needn't be more expensive than bog basic orange DIY shed switch-start ones if you shop around, and are much more efficient too.

This is what I measured on my little power meter for comparison:
Electronic HF: 104 lumens per circuit watt (5ft 58W T8 Triphosphor, 5200lm, 50 circuit watts, PF0.99)
Switch-Start Non-PFC cheapie: 55 lumens per circuit watt (5ft 58W T8 Halophosphate, 4800lm, 87 circuit watts, PF 0.61
Switch-Start PFC corrected: 68 lumens per circuit watt (5ft 58W T8 Halophosphate, 4800lm, 70 circuit watts, PF 0.94Simon.

Yep quite agree with you on the efficiency thing regarding electronic/HF ballasts!! ....Just a shame that they don't tend to last that long, whereas the old switch start ballasts would normally last the life of the fitting and beyond. ...haha!! Seen 40 year old fittings still going strong with the original big black ballasts...lol!!!