Immersion Heater cable

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I often find 2.5sq mm Butyl. But why?
Isn't 1.5 sq mm ok, and pvc high temp stuff has the same 85° rating and there's no oil about...??
 
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Thermal insulation factor for a heap of fluffy towels? :LOL:
 
majority of times it would be on it's own 16A or 20A breaker fed with 2.5 and isolated with a double pole switch.. no fusing down, so it should carry on in 2.5..
 
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An easier way is to fit a 13amp DP switched fuse-connection unit next to the immersion heater, and run the final length between FCU and Element in 1.5.mm HR flex, the cost differential I have found is that 1.5. flex is half the cost of 2.5.flex, yet under normal circumstances will handle 12.5. amps safely, if fused at 13amps in the FCU.
 
ColJack said:
majority of times it would be on it's own 16A or 20A breaker fed with 2.5 and isolated with a double pole switch.. no fusing down, so it should carry on in 2.5..
Eh? But it is fused down? And even if it weren't, and 1.5 were used, the CCC doesn't change so surely it wouldn't need de-rating. :confused:
 
what i'm saying is that if it's a 20A circuit fed via a double pole switch in 2.5, then it's not fused down from 20A protection so 1.5 flex is under-rated..
 
I apologise - I misread your post, and was thinking that there was an FCU involved, i.e. limiting current to 13A. Sorry.

PS Sorry.
 
Is this any different to a twin socket spurred from a ring circuit though?

In that scenario you have a cable rated at 27A supplied by a 32A MCB, so the cable is too small for the OCPD, but as the cable only feeds a twin socket, the maximum load that can be drawn on the single 2.5mm² is 26A, which is ok.

With the imm heater scenario, there may be a 20A supply to the 20A DP switch fed in 2.5mm², but as the imm heater is a fixed load of 3kW (around 13A), then the most current that can flow on the 1.5mm² flex is 13A, which is ok for the CCC of the flex.

Or even like the DNO distribution with downstream protection of the 16.0mm² cable to you house being provided by the 100A cutout, even though there is a much bigger fuse at the sub station. (I know this is slightly differend, with being goverened by the ESQCR)
 
Softus said:
I apologise - I misread your post, and was thinking that there was an FCU involved, i.e. limiting current to 13A. Sorry.

PS Sorry.

Softus. In the circumstances, I feel it's the very least you can do to apologise. Come on.

;)
 
RF Lighting said:
Is this any different to a twin socket spurred from a ring circuit though?
I think so.

In that scenario you have a cable rated at 27A supplied by a 32A MCB, so the cable is too small for the OCPD, but as the cable only feeds a twin socket, the maximum load that can be drawn on the single 2.5mm² is 26A, which is ok.
The double socket isn't rated at 26A, so it isn't ok; although I take your point that it's ok for the cable, but these loads are attached to fused plugs, so even a fault current in an appliance couldn't exceed the cable CCC.

With the imm heater scenario, there may be a 20A supply to the 20A DP switch fed in 2.5mm², but as the imm heater is a fixed load of 3kW (around 13A), then the most current that can flow on the 1.5mm² flex is 13A, which is ok for the CCC of the flex.
But this load is unfused (if wired to a DP switch only) so a fault current could exceed the flex CCC.
 
It is more usual to fuse Immersion Heater circuits at 16A.

p.s. I don't approve of a single appliance (e.g. immmersion heater) with two overcurrent devices (e.g. FCU and MCB) protecting it which do not reliably discriminate. So a switch,but not an FCU, is correct..
 
Thanks. Just realised I should have writ flex not cable... :rolleyes:

But what about the insulation. I don't think there's any advantage in butyl over HR PVC? Both 85º.
 

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