Boiler wiring

[Dazz0i]

A recent job has a combi boiler which is being swapped out for an ideal logic system boiler and tank. It's currently supplied by a 2.5t&e on a 6amp mcb. The boiler itself requires a local 3amp fuse. Which isn't s problem. But obviously now there's the tank to contend with.

Can I swap out the mcb for a 20A. Locally isolate the power and also "split" the local supply, one 3A for the boiler and a 13A for the tank? Or do I have to break it to them that I have to destroy some of their house to get a separate feed to the cupboard for the tank.

Cheers.

 

[xr4x4]

16 amp radial to immersion switch, from supply side to a FCU with a 3 amp for boiler.

 

[EFLImpudence]

Can I swap out the mcb for a 20A.

If it is a dedicated circuit, yes.

Locally isolate the power and also "split" the local supply, one 3A for the boiler and a 13A for the tank?

I would do this.
You will be told that an Immersion must be on a separate circuit but this means separate from the sockets or other substantial loads.
Sharing with a boiler will be no problem electrically.
It will, however, be inconvenient if one fault takes out all heating but that's what they have now isn't it.

 

[ericmark]

The Reference Method 100# will allow 21A with 70 deg twin and earth but are you sure it's installed to Reference Method 100#?

If I was doing the work I would carefully inspect and consider if the cable run was within the description of Reference Method 100#.

Second is the impedance. Both line - earth and line - neutral need to be to a value where the MCB will trip and the volt drop will remain within the 5% allowed. So for a B20 MCB you would need 2.3 ohms or lower.

Although you may see this as a minor works I would say you are taking full responsibility for the work when you are drawing 4 times the originally design current so you should really fill in a new installation certificate. Without sight of the old certificate you have to be very sure there are no errors that you are signing for including the operation of the RCD.

Would I do the work? Likely yes, but if I found items in the installation I was unhappy with then maybe I would not want to take over some one else's work.

I will be interested to see what others say on this and also what was the ELI at the original 6 amp supply.

 

[EFLImpudence]

As said above,
16A probably better because if Voltage was a bit high it may get up to 14A and as recent thread stated this would not be good for 13A fuse and FCU.
You will still need DP isolation for immersion.

 

[EFLImpudence]

It would have to be an exceptionally long cable and installed in the worst case environment for eric's worries to cause this to be unsuitable.

After all this is how the vast majority of immersion heaters are connected.

 

[Dazz0i]

House isn't too old. Ceilings appear to be clipped direct and 100mm of insulation.

Distance is roughly 10M being generous.

 

[holmslaw]

..

 

[Dazz0i]

Swap out as in change to. Although as suggested 16amp would be a sufficent.

The owners are talking of UF heating in the rear kitchen extension which was the reason for asking about a 20amp, to allow for power required to supply that too.