Sauna logistics

Hope to bottom this out here
from the main consumer unit at home I have a 80 amp RCCB with a B40 mcb connected to this I have a 16mm SWA going to a barn approx 15m away inside the barn I have another CU with 3 circuits a B32 mcb for 8 sockets (rarely used) a B6 for lighting and a B40 from which a 10mm SWA powers a static caravan.
I am extending the barn and want to include potentially a 6 KW sauna heater and potentially run a 6 KW shower,
Main question is the 16mm from the barn to the house sized ok for my plans.
I may have to put a sketch of the configuration as i do not think I have described it well enough, basically I can upgrade everything in the barn mcbs etc,
the sauna I have provisioned 10mm TE cable and the shower 6mm TE (i may have to go for a lower KW shower.
Hope i can get some help here....

16 mm² with a 40 amp overload seems OK to me, does not matter what loads are put in the cable if over 40 amp it will trip.

I would be more looking at distance and earth rods, I assume whole supply is TT, but I would want to look at the set-up to decide how many earth rods, and were, with a TT supply it is not really a DIY job.

morqthana said:

And potentially disappoint every user of that shower.

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I would have said that some years ago, however they were fitted here when I bought the house, and with the supplied rose, they seem adequate.

I look at the description 80A RCD - 40A MCB - 16 mm cable - CU - (32A MCB + 6A MCB + 40A MCB) the 40A feeds a caravan, so that feed can't be TN-C-S it is near impossible to work out if TN-S or TN-C-S so in real terms the supply needs to be TT.

A TT supply is good for supplying the sauna.

But all is off one 40 amp MCB, and for a caravan

Each socket-outlet shall be protected individually by an RCD

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and

NOTE: In the UK the ESQCR prohibit the use of a TN-C-S system for the supply to a caravan or similar construction.

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So it would seem that first 80 RCCB needs removing, and the RCD/RCBO's need fitting in the second consumer unit, and likely you will need more than 40 amps.

It would seem your OK to 60 amps, but the problem is the consumer unit, and can you get a 60 amp MCB, the old loadmaster went up to 70 amps, but most consumer units seem to stop at 45 amp.

You can get a larger moulded breaker, but the enclosures move into completely different range, and often the access has to be restricted to electricians due to being able to select tripping currents.

Then we also look at the supply, my home only has a 60 amp supply total, farms may has a split phase supply or three phase supply, but you are looking well beyond DIY.

I have two showers, never used together, two kitchens, so two cookers, two washing machines, so it does seem I do have a protential overload situation but my solar monitors the power used, and due to only being two of us, we never get near max load.

I would be looking at power showers from a hot water store, not instant electric. And also looking at current monitoring, so water only heated when the load is low, all can be done, but not really a DIY job.

I would want the caravan on a smaller MCB to the one supplying the whole CU, so if there is an overload, the MCB near to the caravan trips, not the main one in the house.

In other words it needs designing not everything forward and trust to the Lord.

A lot of this depends on potential usage.
For a start having 2 B40 MCBs in line is incorrect. As already noted remove the RCD from the supply end and go RCBO - (Imagine the inconvenience of teh caravan tripping the RCD while someone is in the shower late at night when the household has gone to bed) - and upgrade the supply to a 50 or 63A, I'd suggest a double pole (A quick google will reveal a whole plethora of them), or downrate the caravan to 32A. However what size is your main fuse - No point in having a 63A MCB if your supply is only 30A - I've seen exactly this for a woodworking workshop!
Despite what others will postulate a 6KW shower will be fine, and further; a 6 or 7KW is fairly standard when installed as part of the sauna. Anyone being 'disappointed' by it has a serious 'their problem'. At home we have a 10.5KW and we never run it on high power.
Earthing arrangements for caravans are supposed to be TT (and not within touching distance of other earthed equipment) and the same is true for pools/hot tubs so I'd not be surprised if the same is true for a sauna in it's own cabin.

As has been pointed out this really needs a full design applied to it and with the certification required I'm doing something I rarely do...
I recommend getting several estimates and advice from electricians.

Russanthonypowell said:

Hope to bottom this out here
from the main consumer unit at home I have a 80 amp RCCB with a B40 mcb connected to this I have a 16mm SWA going to a barn approx 15m away inside the barn I have another CU with 3 circuits a B32 mcb for 8 sockets (rarely used) a B6 for lighting and a B40 from which a 10mm SWA powers a static caravan.
I am extending the barn and want to include potentially a 6 KW sauna heater and potentially run a 6 KW shower,
Main question is the 16mm from the barn to the house sized ok for my plans.
I may have to put a sketch of the configuration as i do not think I have described it well enough, basically I can upgrade everything in the barn mcbs etc,
the sauna I have provisioned 10mm TE cable and the shower 6mm TE (i may have to go for a lower KW shower.escape road
Hope i can get some help here....

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If the 16mm SWA is already supplying the barn and the total load could increase significantly with a 6kW sauna and 6kW shower, why not calculate the maximum demand first before deciding whether the cable size is enough? If both are used at the same time, would the existing 40A MCB and RCCB arrangement also become a limiting factor?

morqthana said:

And potentially disappoint every user of that shower.

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Would whoever reported that please do me the courtesy of explaining why an opinion that users of a low-powered shower might be disappointed by its performance was so unacceptable to them that they wanted it removed.

morqthana said:

Would whoever reported that please do me the courtesy of explaining why an opinion that users of a low-powered shower might be disappointed by its performance was so unacceptable to them that they wanted it removed.

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Not recalling the context of the message which appears to be missing I don't know.

However if you are saying that a 6KW shower heater is unacceptable, perhaps you should explain why you have that opinion when there are possibly thousands of them in use and their users find them totally acceptable.

FWIW we currently have a 10.5KW heater and find it totally acceptable when run of low power which is 2 of the 3 heating elements so I'll assume they are 3½KW each. I'll go further and state the only time it has been used on high power has been when using it to fill a bath once when the boiler, DHW cylinder and bathroom walls were being replaced.

SUNRAY said:

However if you are saying that a 6KW shower heater is unacceptable,

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morqthana said:

And potentially disappoint every user of that shower.

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morqthana said:

users of a low-powered shower potentially might be disappointed by its performance

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SUNRAY said:

perhaps you should explain why you have that opinion when there are possibly thousands of them in use and their users find them totally acceptable.

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Because I've had 7kW-ish showers and I've had 10.8kW.

Because I know that a 6kW shower can raise less than 3l/min of water by 30°C.

Because I find a flow rate of <3l/min disappointing.

Because I know that at a more realistic (but still pretty poor) flow rate in the 5l/m area, a 6kW shower will deliver water at less than 30°C in winter, less still by the time it hits the skin, and I find that temperature disappointing.

I am entitled to have the opinion that a 6kW shower is going to deliver a disappointing flow rate and/or a disappointing temperature rise.

It is wholly reasonable to believe that I am not the only person in the world who has those opinions.

And that is why I think it is acceptable, and not warranting deletion, to say that a 6kW shower will potentially disappoint its users.

SUNRAY said:

FWIW we currently have a 10.5KW heater and find it totally acceptable when run of low power which is 2 of the 3 heating elements so I'll assume they are 3½KW each. I'll go further and state the only time it has been used on high power has been when using it to fill a bath once when the boiler, DHW cylinder and bathroom walls were being replaced.

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Assuming worst-case winter water temperature, a target of 40°C (probably too low given how much it will cool down during the filling process*), and a typical volume of 200l, that's 40 minutes to fill the bath.

OK.

* Imagine running the heater for 1 minute, and putting 5l of water at 40°C into the bath, then turning it off and leaving it. Come back in 39 minutes time, how hot do you think that 5l will be?

morqthana said:

Because I've had 7kW-ish showers and I've had 10.8kW.

Because I know that a 6kW shower can raise less than 3l/min of water by 30°C.

Because I find a flow rate of <3l/min disappointing.

Because I know that at a more realistic (but still pretty poor) flow rate in the 5l/m area, a 6kW shower will deliver water at less than 30°C in winter, less still by the time it hits the skin, and I find that temperature disappointing.

I am entitled to have the opinion that a 6kW shower is going to deliver a disappointing flow rate and/or a disappointing temperature rise.

It is wholly reasonable to believe that I am not the only person in the world who has those opinions.

And that is why I think it is acceptable, and not warranting deletion, to say that a 6kW shower will potentially disappoint its users.

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Fair enough. Personally I don't see the need for a mega flow of very hot water.

I'm away from home at the moment in an AirBnB with my 17 year old grandson and one of his interesting comments was how nice it is for the shower not stinging.

morqthana said:

Assuming worst-case winter water temperature, a target of 40°C (probably too low given how much it will cool down during the filling process), and a typical volume of 200l, that's 40 minutes to fill the bath.

OK.

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It certainly wasn't ideal but no it didn't take anywhere near that, more like 10-15 mins but this was 15 years back so details are not quotes.

Where does 200L come from?
Most DHW cylinders are 18" diameter x 3ft tall so about 30 gallons or 140L off the top of my head and I'd imagine a full cylinder would support 2 baths if drawn in quick succession, making 140L each allowing for 50:50 mix of hot and cold.
Without being able to measure I'll estimate a typical bath is 5*2.5*1.5ft ≈ 19ft³ ≈ 110 gallons ≈ 500L but the tub is more like 4*1.5*1.25 =7.5ft³ ≈ 45 gallons ≈ 200L to the brim. I'd guess being fairly big I'd use well under half of that; say 75L. a skinny person for a wallow may achieve the 140L.

I stress I'm making estimations while sitting at a table in a building without a bath and making mental calculations / conversions. I'll happily accept I'm making errors but somehow I doubt as much as 25% out.

Going back a lot of years, (> 65!) we had a gas geyser over the bathroom basin. If we hadn't lit the fire it was 4 or 5 buckets of hot water, allowing for 50:50 mix made it 16/20 gallons or 72/90L for a shared bath with my sister.

EDIT: Of course the easy thing to do is google:
From: https://www.easybathrooms.com/blog/what-is-the-average-bath-size-and-how-much-water-does-it-hold

Russanthonypowell said:

Hope to bottom this out here
from the main consumer unit at home I have a 80 amp RCCB with a B40 mcb connected to this I have a 16mm SWA going to a barn approx 15m away inside the barn I have another CU with 3 circuits a B32 mcb for 8 sockets (rarely used) a B6 for lighting and a B40 from which a 10mm SWA powers a static caravan.
I am extending the barn and want to include potentially a 6 KW sauna heater and potentially run a 6 KW shower,
Main question is the 16mm from the barn to the house sized ok for my plans.
I may have to put a sketch of the configuration as i do not think I have described it well enough, basically I can upgrade everything in the barn mcbs etc,
the sauna I have provisioned 10mm TE cable and the shower 6mm TE (i may have to go for a lower KW shower.
Hope i can get some help here.... free online games

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The 16 mm² SWA itself sounds reasonably substantial for a 15 m run, but I wouldn't say the installation is adequately sized just from the cable size.

The big issue is the 40 A MCB feeding the barn. A 6 kW sauna is roughly 26 A at 230 V, and a 6 kW shower is another roughly 26 A. Running both together would therefore be over 50 A before allowing for the existing sockets, lighting and caravan load.

So even though the 16 mm² cable may have plenty of capacity depending on installation method and voltage-drop calculations, the upstream 40 A protection currently limits what you can draw.

I'd get the complete installation assessed, including the house supply capacity, SWA installation method, voltage drop, earthing arrangement and the existing caravan load. Simply increasing the MCBs in the barn isn't the answer—the upstream protection and cable design need to be considered as a complete system.

For a sauna and electric shower of those ratings, I'd definitely have the final design checked by an electrician before making any changes. Both are significant continuous loads.

alexiaamber said:

The big issue is the 40 A MCB feeding the barn. A 6 kW sauna is roughly 26 A at 230 V, and a 6 kW shower is another roughly 26 A.

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If the loads are quoted at 240v, it would be 24A. Sounds picky but 4A might make a difference to a fully i's and t's dotted and crossed design.

alexiaamber said:

Running both together would therefore be over 50 A before allowing for the existing sockets, lighting and caravan load.

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What about a changeover switch? Can a sauna cope with no heating for the few minutes someone would be using the shower?

alexiaamber said:

So even though the 16 mm² cable may have plenty of capacity depending on installation method and voltage-drop calculations, the upstream 40 A protection currently limits what you can draw.

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Limits it possibly to half what it's capable of.

Have you looked at the trip curves for 40A B curve devices?

I?

Not recently.

Why?