Caravan diversity question

[Pretty Mouth]

What allowance for diversity, if any, would you apply to caravan hookup sockets?

The circuit will have just one hookup point, with two 16A sockets. It's a fair distance from the CU, so voltage drop is going to be significant. It would be good to keep the cable CSA down if feasible.

 

[UNG]

Is this for touring caravan hookup or for a static van

 

[Pretty Mouth]

Is this for touring caravan hookup or for a static van

Touring

 

[westward10]

Surely two sockets is two hook ups.

 

[Pretty Mouth]

Surely two sockets is two hook ups.

Probably. Never done caravans before

 

[westward10]

Each socket should have individual rcd protection and individual circuit protection. See Section 708.

 

[Pretty Mouth]

Each socket should have individual rcd protection and individual circuit protection. See Section 708.

These are supplied at the hookup unit.

18th Edition Caravan hook up 2 × 16A 2p+e switched interlocked outlet c/w 2 × 16A DP 30mA RCBO

www.yesss.co.uk

I'm fairly happy with the regs side of the job, I'm just trying to fathom whether I treat the circuit feeding the hookup unit as a 32A load, or if I can apply some diversity to it

 

[UNG]

Most touring hookups are often limited to 10 amps it's very rare to find 16amp in the UK, in Europe a lot of sites only offer 6 amp

 

[brianmoooore]

I'm in the design stages/site preparation of something similar, but for four points.
Each point has a 16 interlocked, switched socket protected by a 10A type C RCBO, and I've allowed 24A as the design current.
The only long term heavy load likely to be drawn by a touring caravan is the electric space heating, which is usually 2kW max.

 

[westward10]

These are supplied at the hookup unit.

18th Edition Caravan hook up 2 × 16A 2p+e switched interlocked outlet c/w 2 × 16A DP 30mA RCBO

www.yesss.co.uk

I'm fairly happy with the regs side of the job, I'm just trying to fathom whether I treat the circuit feeding the hookup unit as a 32A load, or if I can apply some diversity to it

That is fine each outlet is individually protected.

 

[westward10]

What is the distance of cable you need and where is the TT, at source or at the hook up point.

 

[Pretty Mouth]

Thanks guys, this is really helpful info.

Each point has a 16 interlocked, switched socket protected by a 10A type C RCBO, and I've allowed 24A as the design current.

Is the type C (rather than type B) RCBO important? Might make selectivity difficult if I use C's at the business end.

 

[Pretty Mouth]

What is the distance of cable you need and where is the TT, at source or at the hook up point.

Just shy of 100m. I intend to TT at the hookup, and use the PME earth to protect the armour of the cable.

 

[westward10]

Creating TT at the pitch means you are going to have to enclose the gland/armour but not impossible. That distance you are looking at 10.0 for 20A and 16.0 for 32A and this is for volt drop and doesn't take into account fault protection. You will also have selectively issues if you use a circuit breaker at source.

 

[James]

Not done the calculations but have you considered 4 core 6mm? Parallel conductors may help reduce vd without costing a fortune.

 

[brianmoooore]

Is the type C (rather than type B) RCBO important? Might make selectivity difficult if I use C's at the business end.

The problem arises when someone decides to boil an electric kettle when the toaster/microwave/space heating is already on.
A 10A type C will likely survive this, whereas a 10A B might trip on the magnetic.
This is based on many years of caravanning, rather than anything theoretical, and not everyone on holiday carries the wide selection of electrical cabinet keys that I do.

 

[Pretty Mouth]

Creating TT at the pitch means you are going to have to enclose the gland/armour but not impossible.

I'm planning to bring the SWA into the hookup enclosure via plastic compression gland, cut the armour flush with the sheath, and SA tape over the tips of the armour.

That distance you are looking at 10.0 for 20A and 16.0 for 32A and this is for volt drop and doesn't take into account fault protection.

I should be able to meet disconnection times for the 10mm 2C SWA, if I use an upstream B32 (using R1+R2 of 0.00783 at 20deg).

You will also have selectively issues if you use a circuit breaker at source.

I'm no expert on selectivity, it's almost never an issue for the type of work I usually do, but I can see the need to get it right in this instance. The general rule of thumb I was taught was 2:1 for breakers, so I figured a B32 upstream, and B16 downstream would do it. Do you think this won't work?

 

[Pretty Mouth]

Not done the calculations but have you considered 4 core 6mm? Parallel conductors may help reduce vd without costing a fortune.

Actually, I can just about scrape through on voltage drop if I use 3 core 6mm, and double up on either the L or the N. I have no idea if this is permitted by regs though, doubling up on just one of the live conductors?

I'll probably use 10mm anyway, as it gives me a large VD allowance, and there's talk of additional lighting and an electric gate.

 

[Pretty Mouth]

The problem arises when someone decides to boil an electric kettle when the toaster/microwave/space heating is already on.
A 10A type C will likely survive this, whereas a 10A B might trip on the magnetic.
This is based on many years of caravanning, rather than anything theoretical, and not everyone on holiday carries the wide selection of electrical cabinet keys that I do.

This is very useful to know.

So if I use B16's to protect the sockets, the lower limit for current to operate the magnetic trip should be 3X16 = 48A. For C10's, 5X10=50A. Pretty close, both should survive similar short term overload/inrush?

But for upper limits, B16 is 5X16=80A, vs C10 10X10 =100A. I can see selectivity with the upstream device being an issue if I use C10s?

 

[brianmoooore]

I'm also working on a completely separate caravan project, that needs a single 32A supply. Cable calcs showed that 10mm2 wasn't quite big enough, but 6mm2 4C, doubled up, would just be adequate.
In the end, for the sake of just under £200 extra, I went for 16mm2 and have done with it.

 

[timhoward]

But for upper limits, B16 is 5X16=80A, vs C10 10X10 =100A. I can see selectivity with the upstream device being an issue if I use C10s?

Could you consider using fuses upstream?

 

[mainline]

These are supplied at the hookup unit.

18th Edition Caravan hook up 2 × 16A 2p+e switched interlocked outlet c/w 2 × 16A DP 30mA RCBO

www.yesss.co.uk

I'm fairly happy with the regs side of the job, I'm just trying to fathom whether I treat the circuit feeding the hookup unit as a 32A load, or if I can apply some diversity to it

Why do you need a double outlet ?
All the touring hook up sites i have seen have been single outlets with 30ma rcd and 16amp mcb.

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Also, all the Consumer units in the caravans / motorhomes i have seen has a 16 / 6amp MCBs and 30ma rcd.

 

[brianmoooore]

There's been regulation changes lately, and interlocked switched 16A sockets are now required with individual RCDs, which are usually RCBOs.
The usual set up is dual units, mounted on the dividing line between two pitches, or quadruple units mounted on the + between four pitches.
16A MCBs are OK, but 10A limits the possibility of customers taking the *iss with continuous heavy loads.

 

[Pretty Mouth]

Could you consider using fuses upstream?

Yes, I guess....

I'll be honest, I'm struggling with working out whether selectivity is guaranteed or not, whatever device I use. Looking into it further, it seems to be a very complicated subject involving comparing obscure manufacturer's data. There's not really any answers in BS7671, or the OSG, or any of the other books I own, that I have been able to find anyway. I watched a youtube on the subject, but I'm pretty sure that the guy trying to explain it is explaining it wrong. I did find some useful info in some of Hager's literature.

Trying to find this out feels like it's above and beyond the call of duty, and is making me look seriously at fast-track plumbing courses.

 

[Pretty Mouth]

Why do you need a double outlet ?
All the touring hook up sites i have seen have been single outlets with 30ma rcd and 16amp mcb.

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Also, all the Consumer units in the caravans / motorhomes i have seen has a 16 / 6amp MCBs and 30ma rcd.

As @brianmoooore said, it's 2 pitches, side by side

 

[mainline]

As @brianmoooore said, it's 2 pitches, side by side

Is there likely going to be other hook-ups in the future ?

Most caravans know not to draw too much, and a 2kw heater in a van will warm the van up in a minute or two.

 

[brianmoooore]

a 2kw heater in a van will warm the van up in a minute or two.

My son, who was banished to one over the winter of 2020-21, as a Covid precaution, would probably strongly disagree.
The 2kW heater in mine is only described as an auxiliary to the gas heater, which has an output of over 3kW.

 

[mainline]

Is there likely going to be other hook-ups in the future ?

Most caravans know not to draw too much, and a 2kw heater in a van will warm the van up in a minute or two.

Selectivity is always going to be a problem with having two 30ma rcds one at the hook up point and one in the van, even if using 100ma S type or similar at the source.

 

[Pretty Mouth]

Is there likely going to be other hook-ups in the future ?

Most caravans know not to draw too much, and a 2kw heater in a van will warm the van up in a minute or two.

The customer doesn't think there will be

 

[brianmoooore]

The customer doesn't think there will be

As we all know, the customer doesn't usually have a clue.

 

[mainline]

My son, who was banished to one over the winter of 2020-21, as a Covid precaution, would probably strongly disagree.
The 2kW heater in mine is only described as an auxiliary to the gas heater, which has an output of over 3kW.

You should close the windows then, or get a van that's insulated.

 

[Pretty Mouth]

Selectivity is always going to be a problem with having two 30ma rcds one at the hook up point and one in the van, even if using 100ma S type or similar at the source.

True, but what goes on in the caravan is out of our control. Regs say we have to provide RCD protection for the socket.

 

[brianmoooore]

Selectivity between the caravan RCD and the hook up point RCD isn't important. Selectivity between the hook up point nd the source of the power is.

 

[mainline]

True, but what goes on in the caravan is out of our control. Regs say we have to provide RCD protection for the socket.

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I would Time delayed rcd at source 10mm 2 core swa and rod at the Hook up.

 

[brianmoooore]

Can't have an S type for the 16A socket. There's no guarantee that the 'van has any kind of RCD, or it might be just a 13A extension lead to a tent plugged in.

 

[mainline]

Selectivity between the caravan RCD and the hook up point RCD isn't important. Selectivity between the hook up point nd the source of the power is.

Its still selectivity.

 

[mainline]

Can't have an S type for the 16A socket. There's no guarantee that the 'van has any kind of RCD, or it might be just a 13A extension lead to a tent plugged in.

I said at the source 30ma at the hookup.

 

[brianmoooore]

As Pretty Mouth says, we have no control over the van's installation, and the van owner has no control over what he plugs into.
Another potential problem that's beginning to appear, is that Motorhomes (often DIY made) often have petrol invertor generators built in, and who knows what the changeover arrangements are for them.

 

[brianmoooore]

Selectivity is always going to be a problem with having two 30ma rcds one at the hook up point and one in the van, even if using 100ma S type or similar at the source.

I said at the source 30ma at the hookup.

What's the point if the supply cable is SWA? No harm, but little point either, since there already should be two 30mA RCDs between the source and the customer.

 

[mainline]

Can't have an S type for the 16A socket. There's no guarantee that the 'van has any kind of RCD, or it might be just a 13A extension lead to a tent plugged in.

I said at the source 30ma at the hookup.

As Pretty Mouth says, we have no control over the van's installation, and the van owner has no control over what he plugs into.
Another potential problem that's beginning to appear, is that Motorhomes (often DIY made) often have petrol invertor generators built in, and who knows what the changeover arrangements are for them.

As you have just said, “no control over the vans installation”. So not a problem.

 

[brianmoooore]

As you have just said, “no control over the vans installation”. So not a problem.

Might be, if they somehow manage to put a potential on the cpc to the adjacent van.
Never underestimate a DIYer.

 

[mainline]

What's the point if the supply cable is SWA? No harm, but little point either, since there already should be two 30mA RCDs between the source and the customer.

I assume its pme you may not get the disconnection time.

 

[mainline]

Might be, if they somehow manage to put a potential on the cpc to the adjacent van.
Never underestimate a DIYer.

They maybe a lot of things, but it's their installation.

 

[brianmoooore]

Electrocuting innocent caravanners is NOT good for business.
If the RCD at source is needed to meet disconnection times in the event of a cable fault, then fine. That's all down to design.

 

[mainline]

Electrocuting innocent caravanners is NOT good for business.
If the RCD at source is needed to meet disconnection times in the event of a cable fault, then fine. That's all down to design.

Ok so are you saying that the OP needs to install into the design a circuit that detects flow into the source ?.

Maybe have some sort of sprinkler system as well as a fire alarm and security just in case they have fire arms

 

[brianmoooore]

Ok so are you saying that the OP needs to install into the design a circuit that detects flow into the source ?.

Not as yet, since it's relatively new potential problem, that the powers that be haven't cottoned onto yet. Watch this space. Separate earth rods for each point?

Maybe have some sort of sprinkler system as well as a fire alarm and security just in case they have fire arms

If you've ever seen a caravan burn, you'd know that there's little chance for fire fighting once it gets going.
We burnt one at our barbecue a few years ago. Drove everyone10m back across the garden and onto the top of the hedge from the heat.

 

[westward10]

This is getting way out of control from what @Pretty Mouth asked can we keep this to the original query.

 

[mainline]

What allowance for diversity, if any, would you apply to caravan hookup sockets?

The circuit will have just one hookup point, with two 16A sockets. It's a fair distance from the CU, so voltage drop is going to be significant. It would be good to keep the cable CSA down if feasible.

If the specification says 16A per caravan for every caravan, then you can't apply diversity, what does the spec actually say?
it is primarily up to the designer to establish with the client the types of loads and thus supplies needed.
I would guess each would pull no more than 10Amps for any significant duration.

I would also say you could get away with 5% voltage drop as the lighting is 12v

 

[Pretty Mouth]

If the specification says 16A per caravan for every caravan, then you can't apply diversity, what does the spec actually say?
it is primarily up to the designer to establish with the client the types of loads and thus supplies needed.
I would guess each would pull no more than 10Amps for any significant duration.

I would also say you could get away with 5% voltage drop as the lighting is 12v

There's no specification. The customer has told me they want 2 caravan sockets, and told me where they want them, the rest is up to me.

 

[mainline]

There's no specification. The customer has told me they want 2 caravan sockets, and told me where they want them, the rest is up to me.

100 mtrs Buried 10mm 2 core swa should give you around 25 amps 4.9% voltage drop.

Like you said, divorce the tncs armour at the hook-up and rod it.

I would also 100ma S type rcd at source if only to protect against the hook-up being damaged and the live supply coming into contact with the TT.