Repurposing a 2.5mm radial

[sunny71]

Hi,

I'm looking at installing an immersion but have a problem getting a separate circuit back to the consumer unit. I currently have separate feed for the boiler from the cu off a 32A mcbvia a 2.5mm wite. Can that be used to to add a small consumer box near the boiler to split 2 loads one for the boiler and one for the immersion?

Thanks

 

[oldbutnotdead]

Your bit of 2.5 t & e should be protected by a 20A fuse or breaker, not a 32. And since you are adding to that circuit it'll need RCD (or RCBO) protection as well- from the CU (to protect the cable).
No need for a sub board, a pair of FCUs on that radial (3A fuse for boiler, 13A for standard 3kw immersion) will be fine.

 

[ericmark]

If the 2.5 mm² is less than 3 meters from the consumer unit then two FCU's will drop the current max to 26 amp which is getting close to limit but likely within it, The current rating depends on insulation method, type of cable, and size of cable and it is unlikely 2.5 mm² will comply, mineral insulated cable may, but unlikely you have mineral insulated cable.

I note my boiler uses around 200 watt, it note my back ground usage raises about that much when boiler is running, so a 3 amp FCU and a 13 amp FCU would protect both.

My immersion heater can take between around 30 to 3000 watt depending on how much I am exporting, so one has to consider it may use 3 kW so 13 amp fuse. If the immersion is near the bottom of the tank it can run for an extended time, so we are told it should have a dedicated circuit, normally it would have a 16 amp MCB, but you could likely use a 20 amp with no problem.

I note my boiler is not RCD protected, it is the only thing in the house not RCD protected, And it was connected up about two months ago to that supply, and I have a compliance certificate, I have queried the non RCD protection and am told it is OK as cables surface mounted and sockets from same supply are type A RCD protected. Reading my old 2008 version of the regulations it seems they are correct, although personally I would have given it RCD protection in fact until solar panels fitted it was RCD protected.

For the cost difference between a RCBO and MCB I would have always used a RCBO, but in my case the boiler is supplied from the UPS, so there is no breaker between the inverter and the FCU.

 

[EFLImpudence]

Wow - a lot of spurious advice.

I'm looking at installing an immersion but have a problem getting a separate circuit back to the consumer unit. I currently have separate feed for the boiler from the cu off a 32A mcbvia a 2.5mm wite. Can that be used to to add a small consumer box near the boiler to split 2 loads one for the boiler and one for the immersion?

If you reduce the MCB to 20A then all you need is a 3A FCU for the boiler (which you probably already have).

The immersion can be connected to the circuit with just a switch or timer needed to control it.


A circuit with concealed cables if fitted nowadays should be covered by an RCD but that is for the protection of people; not the cable, boiler or immersion.

 

[Murdochcat]

Is the 2.5 mm cable the only cable in the mcb?

 

[ebee]

Wow - a lot of spurious advice.


If you reduce the MCB to 20A then all you need is a 3A FCU for the boiler (which you probably already have).

The immersion can be connected to the circuit with just a switch or timer needed to control it.


A circuit with concealed cables if fitted nowadays should be covered by an RCD but that is for the protection of people; not the cable, boiler or immersion.

Yep,
The immersion is better with a 20A DP switch if needed (no fuse) and the boiler (assuming gas not electric of course) only needs a 3A fuse (SFCU) and not much of that 3A will actually be drawn.
So you shouldn`t need a consumer unit , 2.5 T & E radial on a 16A or 20A MCB should suffice (or even 15A fusewire in a rewireable fuse would do it)

I am, of course, assuming you immersion heater is 3KW max.