reusing a disused shower circuit

[jpbunce]

I have a 6mm shower circuit on a 30 amp fuse that is and will be no longer used, can i use this to fit 2 switched FCUs outside the bathroom that will run to 2 flex outlets in the bathroom for a 1KW a wall heater AND a 500 Watt towel heater.
Thanks PB

 

[kai]

Yes, it should be safe, provided that the existing circuit is in good condition, and there is 30mA RCD protection, note the breaker MUST be swapped to a smaller Type B one rated at 10amps.

 

[jpbunce]

circuit is good no RCD though, can i use RCD spurs, or fit and RCD next to CU
PB

 

[Taylortwocities]

note the breaker MUST be swapped to a smaller Type B one rated at 10amps.

Why would he need to do that? The 30amp fuse is there to protect the 6mm cable. So no need to change.
It sounds like its a rewireable fuseboard and i never did see 10A fuse wire for a BS3036.

----------------------

The circuits into the bathroom must now be RCD protected so, yes, RCD spurs will do the job. The existing circuit remains as is and regulated under its (old) regulations.

Do remember that the new work that you do in the bathroom must be notified to the local authority before you start work..

 

[Goldberg]

The 30amp fuse is there to protect the 6mm cable. So need to change.

I'm sincerely hoping that you don't work as an electrician with this approach to selecting circuit protective devices.

You might find the relevant Wiki topic illuminating:

//www.diynot.com/wiki/electrics:fuses_and_miniature_circuit_breakers:mcbtocable

 

[SUNRAY]

The 30amp fuse is there to protect the 6mm cable. So need to change.

I'm sincerely hoping that you don't work as an electrician with this approach to selecting circuit protective devices.

You might find the relevant Wiki topic illuminating:

//www.diynot.com/wiki/electrics:fuses_and_miniature_circuit_breakers:mcbtocable

I dont understand the 'problem' here surely the cable must be protected by the 'fuse', at the end of the cable a couple of FCU with fuses rated at 5A & 3A would be spot on, as long as the RCD is there somewhere and the cable will fit into terminals of of the FCU's.
Am I missing something?

 

[Goldberg]

I dont understand

Imagine a radial circuit with a design load of 5A - let's make it easy and say the conditions make it acceptable to use a type B 6A MCB and 1.0mm² cable.

If I then chose to 6mm² cable, for my own secret personal private secret private reason, would you then change the CPD to a 40A MCB?

If not, then why not?

After all, the 40A MCB would protect the cable.

 

[flameport]

The only possible problem is if the fusebox is a 3036 rewireable AND the cable was also installed in insulation or some other capacity reducing environment, the 30A fuse would not be suitable for 6mm twin and earth cable.

However we have no info on how the cable is installed, the length of the circuit or even if it is a rewireable fuse.
Unless we are to assume that every circuit in existence was installed wrongly from the start, it is likely this one was installed correctly.
Even if it wasn't, overloading the cable simply won't be happening as the fuses in the FCUs would blow long before the capacity of the cable was reached.

 

[Goldberg]

Imagine a radial circuit with a design load of 5A - let's make it easy and say the conditions make it acceptable to use a type B 6A MCB and 1.0mm² cable.

If I then chose to 6mm² cable, for my own secret personal private secret private reason, would you then change the CPD to a 40A MCB?

If not, then why not?

 

[SUNRAY]

I dont understand

Imagine a radial circuit with a design load of 5A - let's make it easy and say the conditions make it acceptable to use a type B 6A MCB and 1.0mm² cable.

If I then chose to 6mm² cable, for my own secret personal private secret private reason, would you then change the CPD to a 40A MCB?

If not, then why not?

After all, the 40A MCB would protect the cable.

Anyone can misquote
I was asking why is is neccessary to change the CPD when whats there is OK, you dont need to downrate just because its being used below capacity.

Would you replace the CPD for a 2.5mm ring main with a B6 MCB because its only running at 1.5KW, no I didnt think so.
The thread is about an existing circuit, not designing and providing a new one.

 

[Goldberg]

I wasn't intentionally misquoting; I was aiming to economise on words.

I was asking why is is neccessary to change the CPD when whats there is OK, you dont need to downrate just because its being used below capacity.

What's there isn't OK.

Would you replace the CPD for a 2.5mm ring main with a B6 MCB because its only running at 1.5KW, no I didnt think so.

I'll do a deal with you - if you answer the question about the 1.0mm² radial, then I'll wander into the otherwise questionably relevant arena of a 2.5mm² ring final.

The thread is about an existing circuit, not designing and providing a new one.

I fundamentally disagree. It's about an existing cable, not an existing circuit, and it's all about designing.

 

[Taylortwocities]

The 30amp fuse is there to protect the 6mm cable. So need to change.

I'm sincerely hoping that you don't work as an electrician with this approach to selecting circuit protective devices.

You might find the relevant Wiki topic illuminating:

//www.diynot.com/wiki/electrics:fuses_and_miniature_circuit_breakers:mcbtocable

My apologies. I missed out a vital word there.
The sentence should have read "So no need to change"

I have edited the post accordingly

 

[ban-all-sheds]

I fundamentally disagree. It's about an existing cable, not an existing circuit, and it's all about designing.

The rated current or current setting of the protective device (In) must not be less than the design current (Ib) of the circuit, and the rated current or current setting of the protective device (In) must not exceed the lowest of the current carrying capacities Iz) of any of the conductors of the circuit.

Ib = 6.5A
In = 41.4A
Iz = 47A (accepting flameport's reasoning).

So why must In be reduced?

 

[jpbunce]

The bungalow was built late 80s, the cable length is about 24 ft run in the loft not under insulation. It has a 4way cartridge type CU, but the circuit concerned is coming from its own 1 way CU fitted above the main CU also with a cartridge fuse.
Thanks PB
I could have the 1 way unit replaced with an RCD unit.

 

[ban-all-sheds]

a cartridge fuse.

Oh, OK.

I fundamentally disagree. It's about an existing cable, not an existing circuit, and it's all about designing.

The rated current or current setting of the protective device (In) must not be less than the design current (Ib) of the circuit, and the rated current or current setting of the protective device (In) must not exceed the lowest of the current carrying capacities Iz) of any of the conductors of the circuit.

Ib = 6.5A
In = 30A
Iz = 47A (accepting flameport's reasoning).

So why must In be reduced?