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Posted By Topic: Caravan Cables in polystyrene Insulation

Oct 28 2019 19:07

Hi all

I understand new caravans need to be wired in flex or x amount of stands from memory.

The question is when the caravan has polystyrene insulation does this have any effect on cable we select?



Oct 28 2019 19:42

You can't put standard TPS or flex with polystyrene. It would have to be non-migratory or in conduit.

Oct 29 2019 08:07

So there is no cable suitable for caravans with polystyrene insulation.

I have a 70's lightweight caravan completely stripped down. Doing a complete re-wire. Originally it had standard TPS with polystyrene Insulation.

Any reason I can't put it back as it was?

I generally I like to improve safety were possible but running conduit seams a bit over the top.


Oct 29 2019 08:30

Yes there is cable. Non-migratory TPS. Usually purple. It should be available in 1.5mm2 7 strand.

Oct 29 2019 08:42

Non-migratory TPS is probably the best option; but any method that avoids the sheath coming into direct contact with the polystyrene will meet the requirements.

Oct 29 2019 09:03

Suggest that you check out the new Nexans new generation standard TPS(light green sheath) as this cable is thought to be OK for use in contact with polystyrene insulation.

You will need to check with the cable manufacturer to get confirmation.

Oct 29 2019 09:53

Thanks guys

That seams like a reasonable solution.

I don't like lighting circuits in 1.5mm though. Not fun to wire

Oct 29 2019 16:58

expensive and my wholesaler will only sell 100 meters, better to sleeve the whole lot, a little fiddly but not really a problem

Oct 30 2019 06:45

My night school tutor used to say wrap standard tps in masking tape since conduit, duct tape, electrical tape are all pvc

Oct 30 2019 08:53

None of them are just PVC. It's the plasticisers in insulation & sheath of TPS that give it its flexibility; and difference with conduit is it doesn't have plasticisers. They can't migrate out and make the conduit any less flexible.

Equally the loss of plasticiser over time from TPS only matters if / when the cable is moved - as long as it's left alone, the fact that it has lost (some of) its flexibiltiy can't cause a problem.


Oct 30 2019 18:40

Of course, the fact that it's in a caravan and we have to use 7+ stranded cable indicates that some vibration and movement is to be expected...

I wonder whether flexible conduit has plasticisers? Probably not enough to pose a concern.

Oct 31 2019 09:37

The migration process is gradual; takes a long time to get to stage of brittle sheath, let alone brittle core insulation.

Oct 31 2019 18:09

Yeah, that's what I seem to be hearing. It also seems to be temperature linked; i.e. the colder it is, the slower.

This (quite old) article, for example, suggests that keeping temperatures below 50-60C should prevent any problems for ~50 years:

So perhaps just use 1.5 on a 6A breaker for the lights, and consider either 2.5 purple or 4mm for the sockets?

Oct 31 2019 18:20

For a standard caravan on a 16A inlet, there shouldn't be any need to use anything larger than 1.5mm2 7 strand. Which would be rated at 15/16A partially surrounded. Certainly doesn't need 4mm2. But use 2.5mm2 for power if you want to.

As for the TPS plasticiser problem, I wouldn't trust too much old information. Nothing was commonly known about it in the 80/90's. And the effect can be reasonably quick in warmer temperatures (like a metal caravan in the sun). I've seen cables cut through 50mm insulation in a matter of a year or two. Flexes as well as regular TPS.

There's a few wholesalers that will sell in smaller quantities than a full roll. You might just have to shop around.

Or it's just as cheap to use corrugated conduit anyway. And that gets you around the running through holes in metal problem as well.

Oct 31 2019 18:21

And you don't need to use "a 6A breaker for the lights".

One 16A RCBO is all you need for all the power and lights, wired in 1.5mm2 7 strand or flex.

Nov 01 2019 18:12

My point is that, if that article is accurate, we can use standard non-anti-migratory TPS simply by using larger cable to achieve a lower temperature rise.

That 1.5 is going to be sitting pretty close to 75C if you're using the better part of 16A. Go up a size or down a current rating, and you could be halving the temperature rise above ambient - say, to 55C ish.

Nov 01 2019 20:44

Usually I use 2.5mm2 to the inlet, 1.5mm2 to everything else.

And I wouldn't use standard TPS in polystyrene without protection. It's not just the loss of flexibility, but also the reduction in insulation properties of the TPS and conductor insulation, and damage to the insulation.

It's certainly not recommended by the manufacturers.
For the sake of a few more dollars for non-migratory - it's not worth the trouble.

Nov 01 2019 23:25

The website I previously reported is

Nov 02 2019 10:25

Not about caravans but I've been to quite a few modern hot water cylinders with foam insulation where the flex has been against the
foam and and migrated into it. Been sleeving with fibreglass but it's a bit of a pain as wholesalers only seem to sell the packs of multi size and no longer by the meter.

Nov 07 2019 12:00

Just a comment on Doug's Post

"For a standard caravan on a 16A inlet, there shouldn't be any need to use anything larger than 1.5mm2 7 strand. Which would be rated at 15/16A partially surrounded. Certainly doesn't need 4mm2. But use 2.5mm2 for power if you want to."

This may be true in some cases such as a standard van with no insulation ( how many of those are out there ) , but using 1.5mm in a caravan or connectable installation built of Poly panels requiring NM cable would require a max 10A MCB or RCBO as it would generally be classed as completely surrounded by Thermal Insulation , even if it is enclosed in a conduit in the panel, it is still completely surrounded.
so in order to just use a 16A RCBO you would need to do everything in 2.5mm if all wiring is embedded in the panels
We normally use 1.5 for lighting and 2.5 for plugs.