Spur off Immersion Heater circuit

[hawkeye]

I need to install a 3amp FCU for a new shower pump. Ideally I wish to situate this in the airing cupboard but the one (easily) accessible socket already has a spur feeding the burglar alarm control unit. Can I take a spur off the IH circuit (which is protected by 16amp fuse), or will I have to put a junction box in the mains ring circuit?

Any advice greatly appreciated.

Thanks

 

[Taylortwocities]

Strictly speaking any heater for a water heater vessel over a certain size (15 litres) should have its own dedicated circuit and double pole isolation.

If its possible you should try and run the shower pump as a 3A FCU spur off the ring final. I do see lots of immersion circuits with other stuff hanging off it (pump, central heating, intruder alarm, etc etc) shouldnt really be done but it is as its often the only available power source at that point.

If you take that route then do all the calcs you would normally do for a radial circuit.

Tayla

PS Check to see if the shower pump should RCD protected too.

 

[hawkeye]

Many thanks for the response. There is another option - I shall be removing the existing electric shower that has a dedicated circuit run with 10mm cable. Is it possible to connect the 10mm cable to a 30amp junction box and then run 2.5mm T+E to the FCU?

Please advise - nothing else will run off the circuit so I assume there will be plenty of capacity.

 

[didthathurt]

You must ensure you do not get in a situation where the 2.5mm T+E, followed by the JB, are the weakest links in the system - which they will be unless you change the circuit protection in the CU. Have you ever tried connecting 10mm cable into a 30A JB? Also, any JB used has to be readily accessible (I think is the phrase). As this is a radial cct and contains 2.5mm cable, the book suggests a max of 20amps if an mcb is used, but you may need to reduce this based on exactly what you are placing on the circuit - and, for safety's sake, ensure the reasons for doing so are indicated in or around the CU.

Also, this is in a bathroom so it is notifiable to LABC under part P.

 

[JohnD]

You say you have a socket with an FCU on it. You can connect another FCU to the load side of the first - that way, you will have a fused spur, whose load cannot exceed 13A because that is the max fuse you can put on it.

On this fused spur you can put as many loads as you like - FCUs or anything else. It is entirely permissible and cannot cause an overload.

 

[hawkeye]

Thanks for the quick response.

Just to clarify, after taking out the old shower, the only item I need to run off the circuit is the shower pump, via a 3amp FCU, so no risk of overloading. You're right about the 30amp JB - I guess a larger one may be needed to accomodate the 10mm - but is it still not appropriate to use 2.5mm from JB to FCU? If not, what other solutions are there that allow me to use the 10mm cable?

Many thanks

 

[JohnD]

I don't believe you need to use the 10mm cable. But if you feel you want to, I suppose you could entertain yourself by turning it into a submain with a small CU at the end of it feeding whatever socket, lighting or appliance circuits you wanted.

 

[hawkeye]

I don't particularly want to use the 10mm but as it will become redundant when I remove the electric shower, I thought it would be the simplest way of wiring up a new FCU. Put simply, what is the best way to use that circuit to feed a 3amp FCU?

I've no plans to install a new CU!

 

[didthathurt]

I'm with John D and would tap into the existing FCU in the described manner. Whilst using the 10mm is possible, it is a solution that has a number of boxes that you'd have to be absolutely sure you'd ticked to keep it safe and compliant with regs. A straight forward fused spur is, by far, the best of theses 2 options.

 

[securespark]

If you have a 10mm2 circuit, and wish to extend it, even for a small load, you must use the same size cable.

But, a solution would be to put a switched FSU on the end of the 10mm2 and run out to the pump. It is also advisable to have RCD protection, so if the circuit has none, fit an RCD spur.

 

[securespark]

You say you have a socket with an FCU on it. You can connect another FCU to the load side of the first - that way, you will have a fused spur, whose load cannot exceed 13A because that is the max fuse you can put on it.

On this fused spur you can put as many loads as you like - FCUs or anything else. It is entirely permissible and cannot cause an overload.

See what you're saying, but you need to ensure that each appliance has its own fuse.

IE Socket > Spur with 13A fuse feeding > BA spur with 2/3A fuse > Shower Spur with 5A fuse.

What you can't do is tap off the load side of an existing spur because that would mean everything being fused down to the value of the fuse in that spur. IE everything coming off that spur fused at 3A, or if you changed that fuse to 13A, then the BA panel would be incorrectly fused.