UK Diversity

[Mfctitch]

Morning Nearly said lads then thought better of it so morning everyone.

I've got to price for another shed job, I know you're thinking "so"

But question, as we have established with my previous threads I tend to go over board but this time I need some advice on diversity
For a change I know that there will be
at least
2 number 3Kw panel heaters
A radial circuit
A lighting circuit
A 10.5 Kw shower

This gives me 38.5 Kw over a 44 metre distance with at least 2 metres going under ground.
so I need to bring that figure down, I've watched the john Ward clip on cookers but can I use more diversity on the other circuits. I happy for you to point me in the right direction in the book if that would help.

One other question there anyway round the this calculation on going underground as the cable is only under the ground for two metres but it makes a big difference

Thanks

 

[ferg]

As a general rule of thumb when derating you should use the most onerous conditions.

For the shower and panel heaters no diversity is allowable in reality this may be different though.

I would say 2 panel heaters are very likely to be on at the same time.
A shed with a shower is also likely to have the heaters on at the same time as the shower as most people don't like cold showers or getting out of the shower in to the cold.

 

[Gavin John Hyde]

How about a priority device for the shower and heaters so the shower knocks the heating out for the short time its in use? when they step out of shower heaters come on,
Could look at some infrared heaters instead of panel ones, can work well where there is a shower or bath as its easy to direct

 

[marcuswareham]

Maybe you could set up a priority unit to turn off circuits when the show is on, etc

Like this:

View: https://www.youtube.com/watch?v=N_jTZbpUfTc

[automerge]1591785311[/automerge]

How about a priority device for the shower and heaters so the shower knocks the heating out for the short time its in use? when they step out of shower heaters come on,
Could look at some infrared heaters instead of panel ones, can work well where there is a shower or bath as its easy to direct

Doh, must have posted at the same time !

 

[bill01803]

How did you get 38.5kW?

 

[Mfctitch]

How did you get 38.5kW?

See top of page though I might be over stating things by adding the heaters rather than including them feel free to let me know
Thanks
[automerge]1591786838[/automerge]

See top of page though I might be over stating things by adding the heaters rather than including them feel free to let me know
Thanks

Don’t know where to add this but sorry I started this early this morning
I’ve confused my 16 and 6 amp breaker as Kw ??????
Will recalculate when I get home in a bit

 

[marcuswareham]

Shower = 43.7A
2x Heaters = 26A
Radial circuit on 16A MCB = 16A
Lighting circuit on 6A MCB = 6A

Total of 91.7A or 21.1kw

Seems a lot for a shed, I guess the lighting circuit will be way under 6A & if the 16A circuit is it used for sockets hopefully they won't need to plug any more heaters in! , but maybe a kettle

 

[Strima]

What is the intended use of the shed?

Permanent or temporary accommodation?

Used throughout the year or only in summer?

Using table A2 in the OSG:

Shower 100% as no diversity allowed. 45 amps.

Heaters 100% up to 10 amps plus 50% in excess of 10 amps. 16 amps.

Lighting 66% of total demand assuming 6 points @100w each. 2.6 amps.

Sockets 100% of current demand at largest point of utilisation plus 40% or current demand of every other point of demand. This is where it gets tricky and knowing the intended use helps a lot.

Say this is to be used as temporary lodging for family visits etc then you could safely assume no cooking taking place but kettle for tea/coffee etc. TV and DVD player, music system/radio. A couple of standard lamps/bedside lamps, phone chargers, WiFi repeater, small fridge etc etc etc

I would assume ball park figure for sockets taking the kettle as the largest load @3kW plus 40% of all other appliances at 1kW maximum. So 15 amps rounded up.

Total maximum demand = 78.6 amps. (18.08kW).

Bearing in mind this is maximum demand so heating, shower and kettle all on at the same time.

With the shower off, brew already made and all the lights on during a barmy summers evening you would be looking around 5 amps current draw.

Factor in LED lighting and energy efficient devices it would be even lower.

The OSG will give you a ball park figure to work from but looking at intended use and what could possibly be used there will get the numbers down a lot.

You also need to consider the main installation in on this, such as do they have an electric shower or instantaneous water heaters, electric heating etc as you need to consider the maximum demand for the whole installation from a supply perspective.

 

[Mfctitch]

Shower = 43.7A
2x Heaters = 26A
Radial circuit on 16A MCB = 16A
Lighting circuit on 6A MCB = 6A

Total of 91.7A or 21.1kw

Seems a lot for a shed, I guess the lighting circuit will be way under 6A & if the 16A circuit is it used for sockets hopefully they won't need to plug any more heaters in! , but maybe a kettle

back
and thank you
realised my mistake feeling very stupid.
but I get the feeling that it might be being used more often than not if you know what I mean.

 

[Ian1981]

Jesus, I live in a 4 bedroom house and I’m lucky to pull 30 amps!

 

[Mfctitch]

Jesus, I live in a 4 bedroom house and I’m lucky to pull 30 amps!

I know what you mean but this Shed is going to be used like a small bunglow/office and its the distance from the main house that causes the problem on the cable size
[automerge]1591792848[/automerge]
So with your help it looks we will be pulling about 88 Amps
so with the fact that some of it is going under ground and volt drop I have it down as 25mm 3 core SWA.
if I'm wrong please advise

 

[pc1966]

So with your help it looks we will be pulling about 88 Amps
so with the fact that some of it is going under ground and volt drop I have it down as 25mm 3 core SWA.
if I'm wrong please advise

It looks that way, but basically they are putting in most of what a house would likely have.

As already raised, that could be way too much if the main dwelling is trying the same, in which case you need to speak to the DNO about getting in a bigger supply.

If it is all too much then they need to ask themselves if they really need that much shower power as that is dominating the system. Coordinating it with the heaters and you drop it a bit, and if it could be coordinated with the house shower (if any) then it might bring it under the typical single phase limit of 80-100A.

You could fit 4-core SWA with 3-core supply for the out building and the other core as a "priority live" with a shower priority unit in the main house if they have a similar shower there.
[automerge]1591812708[/automerge]
You might need to use a TPN isolator switch in the out building though so they can tun off everything in one go. Typically they have 3 switched + bolted neutral, you could use the L1-L3 for PL/L/N of such a setup and ignore the bolted neutral.

 

[Mfctitch]

It looks that way, but basically they are putting in most of what a house would likely have.

As already raised, that could be way too much if the main dwelling is trying the same, in which case you need to speak to the DNO about getting in a bigger supply.

If it is all too much then they need to ask themselves if they really need that much shower power as that is dominating the system. Coordinating it with the heaters and you drop it a bit, and if it could be coordinated with the house shower (if any) then it might bring it under the typical single phase limit of 80-100A.

You could fit 4-core SWA with 3-core supply for the out building and the other core as a "priority live" with a shower priority unit in the main house if they have a similar shower there.
[automerge]1591812708[/automerge]
You might need to use a TPN isolator switch in the out building though so they can tun off everything in one go. Typically they have 3 switched + bolted neutral, you could use the L1-L3 for PL/SL/N of such a setup and ignore the bolted neutral.

They are having a new combi fitted so no need for an electric shower in the house.
We are asking the DNO to fit a new 100 Amp fuse
and I was going to fit a fused main switch in the house so that they can isolate the "shed" if need be plus to protect the cable.
[automerge]1591813224[/automerge]

It looks that way, but basically they are putting in most of what a house would likely have.

As already raised, that could be way too much if the main dwelling is trying the same, in which case you need to speak to the DNO about getting in a bigger supply.

If it is all too much then they need to ask themselves if they really need that much shower power as that is dominating the system. Coordinating it with the heaters and you drop it a bit, and if it could be coordinated with the house shower (if any) then it might bring it under the typical single phase limit of 80-100A.

You could fit 4-core SWA with 3-core supply for the out building and the other core as a "priority live" with a shower priority unit in the main house if they have a similar shower there.
[automerge]1591812708[/automerge]
You might need to use a TPN isolator switch in the out building though so they can tun off everything in one go. Typically they have 3 switched + bolted neutral, you could use the L1-L3 for PL/L/N of such a setup and ignore the bolted neutral.

might sound a silly question but is the main switch on the consumer unit not good enough?

 

[pc1966]

might sound a silly question but is the main switch on the consumer unit not good enough?

If you have just L&N fed to the out building then yes, the local CU incomer switch would be perfectly fine.

But if you had two lives (one permanent, one from shower priority unit) and one neutral you would have to isolate all simultaneously. As the out building would be considered a separate installation, it needs to have its own point of isolation, even though you could kill all by using a standard DP isolator in the house (assuming it is before any split for home CU and fused-switch to feed the out building).

Basically non-standard arrangements bring non-standard solutions!

 

[Strima]

As the out building would be considered a separate installation

Would it? Fed from one supply and on the same meter I would class as one installation.

 

[pc1966]

Would it? Fed from one supply and on the same meter I would class as one installation.

That is a valid way to look at it.

In the regs it just defines an installation as "An assembly of associated electrical equipment having co-ordinated characteristics to fulfil specific purposes" and my thought was a building with its own DB (i.e. coordinated set of circuits) would then require a safe point of isolation. More so when it is more-or-less a self-contained living quarters like this case (than say a garage with a couple of sockets and a light bulb).

 

[Mfctitch]

Would it? Fed from one supply and on the same meter I would class as one installation.

Not sure if I've confused matters but the supply for the "shed" will be 3 core SWA only one live this will be fed via a fused isolation switch in the house with a consumer unit in the "shed"