Uprating shower cabling and MCB

[cal_rourke]

Hello,

I have a 9.5kW electric shower connected as follows:

1) 6mm2 cable from consumer unit to isolator switch (pre-existing)
2) 10mm2 cable from isolator switch to shower (I fitted this larger cable)
3) protected with 30amp MCB (pre-existing)
4) No RCD

This arrangement is tripping the MCB after about 3-7 mins due to (I am fairly sure) the 6mm2 cable getting hot. The shower needs a bigger MCB and bigger cable.

What I want to do is uprate the 6mm2 cable to 10mm2 and also uprate the MCB to 45amps.

My main question is:
1) is this safe and straightforward to do or are there other considerations?
a) Does the MCB solely protect the cable or does it have any other purpose?
b) What do I need to consider at the consumer unit side for such a replacement?
c) Do I need an RCD?

For additional info, my consumer unit actually seems to be 2 units (installed roughly 21 years ago):

The first larger box says "Total load not to exceed 60amps" - in this are 4 MCB's rated at 5, 30, 30 and 30amps each.
The second smaller box says "Total load not to exceed 45amps" - in this are 2 MCB's rated at 15 and 30amps each.

Thanks very much in advance for any help.

Cal

 

[Adam_151]

Hello,

I have a 9.5kW electric shower connected as follows:

1) 6mm2 cable from consumer unit to isolator switch (pre-existing)
2) 10mm2 cable from isolator switch to shower (I fitted this larger cable)
3) protected with 30amp MCB (pre-existing)
4) No RCD

Not good, 9500/230 = 41A

This arrangement is tripping the MCB after about 3-7 mins due to (I am fairly sure) the 6mm2 cable getting hot. The shower needs a bigger MCB and bigger cable.

The thermal element of the MCB is opperanting because its passing more than its rated load and is shutting down the circuit to prevent a sustained overload

What I want to do is uprate the 6mm2 cable to 10mm2 and also uprate the MCB to 45amps.

Sounds sensible

My main question is:
1) is this safe and straightforward to do or are there other considerations?

Most likely, pretty much so, and a few

a) Does the MCB solely protect the cable or does it have any other purpose?

It protects the cable from overload, and short circuit, it must also disconnect the circuit in 0.4 seconds in the event of an earth fault

b) What do I need to consider at the consumer unit side for such a replacement?

That the device you want to fit is appropiate for the circuit, and the CU will take such a device

c) Do I need an RCD?[/quotes]
Regs say no, good practice says yes, and I wouldn't want to fit a shower without one

For additional info, my consumer unit actually seems to be 2 units (installed roughly 21 years ago):

The first larger box says "Total load not to exceed 60amps" - in this are 4 MCB's rated at 5, 30, 30 and 30amps each.
The second smaller box says "Total load not to exceed 45amps" - in this are 2 MCB's rated at 15 and 30amps each.

Any chance of a picture? are these old wylex standard consumer units, if so you might have a problem, I'm not sure the smaller ones (having 60 and 45A main switches) will take devices of over 30A, you'd probably be best fitting a dedicated shower cu

Thanks very much in advance for any help.

Cal

You're welcome

 

[plugwash]

got some photos? we need to determine exactly what you have before we can give much advice

putting a 45A breaker in a unit thats only got a 60A incomer and lots of other stuff on top is pushing it a bit. also it sounds like you may have a wylex standard and in that range its not (meant to be) possible to fit a breaker over 32A in a unit with an incomer rated below 100A (the base can be forced in but it won't sit straight and this is NOT reccomended)

 

[HDRW]

I have a 9.5kW electric shower connected as follows:

1) 6mm2 cable from consumer unit to isolator switch (pre-existing)
2) 10mm2 cable from isolator switch to shower (I fitted this larger cable)
3) protected with 30amp MCB (pre-existing)
4) No RCD

This arrangement is tripping the MCB after about 3-7 mins due to (I am fairly sure) the 6mm2 cable getting hot. The shower needs a bigger MCB and bigger cable.

No, the 30A MCB is tripping because you're drawing 9500/230 = 41.3A, so it's doing its job! Any thermal contribution from the cable shouldn't make much difference unless the cable is practically glowing...

What I want to do is uprate the 6mm2 cable to 10mm2 and also uprate the MCB to 45amps.

Good! The 10mm² cable makes no sense when you have 6mm² in there - a chain is only as strong as its weakest link, so replacing that is a Good Thing...

My main question is:
1) is this safe and straightforward to do or are there other considerations?

I'm not quite clear when you say the 6mm cable goes from the CU to an isolator - do you mean it goes from one of the 30A MCBs in the CU? Replacing this cable 1-for-1 with a 10mm cable isn't difficult if you take care and remember that parts of the inside of the CU will be live, even with the Big Switch turned off (but do turn it off!) Replacing the MCB isn't usually difficult but at that point Part P raises its head (just changing the cable because it's damaged (it is damaged, isn't it? ) doesn't need need Builfing Control involvement, but I suspect that changing an MCB does).

a) Does the MCB solely protect the cable or does it have any other purpose?

It protects anything connected to it which doesn't have its own fuse or circuit breaker - in most circuits that's just the cable. It's also a switch which disconnects a circuit when you want to do that.

b) What do I need to consider at the consumer unit side for such a replacement?
c) Do I need an RCD?

That's up to you, but it is a good idea and most people would say that you should fit an RCD for added safety. Can you replace the isolator switch you mention with an RCD in a small modular box?

Cheers,

Howard

 

[cal_rourke]

Thank you for your replies.

In response:

Any chance of a picture? are these old wylex standard consumer units, if so you might have a problem, I'm not sure the smaller ones (having 60 and 45A main switches) will take devices of over 30A, you'd probably be best fitting a dedicated shower cu

Indeed, I do have Wylex units - pictures are here:
http://www.manwomanandmyth.com/images/60amp_CU.JPG
http://www.manwomanandmyth.com/images/60amp_CU_label.JPG
http://www.manwomanandmyth.com/images/Both_CU.JPG

The MCB's are GE brand - when you say that my Wylex units may not take devices over 30A, does this mean the 45amp MCB is physically larger than a 30amp?

I'm not quite clear when you say the 6mm cable goes from the CU to an isolator - do you mean it goes from one of the 30A MCBs in the CU?

Yes, that's the case.

That's up to you, but it is a good idea and most people would say that you should fit an RCD for added safety. Can you replace the isolator switch you mention with an RCD in a small modular box?

So the RCD can be at the isolator end, in the bathroom? If so, would Screwfix have this combined isolater and RCD unit? Any particular model?


If I had to go for a seperate CU for the shower, can anyone indicate the typical charge (London) by an electrician to do this considering that I would do some of it by laying the cable to connect the isolator switch to where the new CU would go?

And lastly, do you think that it would be more sensible to replace both of these Wylex CU's with a new one? If so hopefully, the GE MCB's could still be used - cost me £10 each as I recall...

Thank you,
Cal[/quote]

 

[plugwash]

how are theese CUs fed? thats not clear in the pictures (a photo with the lids off would be nice but take great care when removing the lids those things are cramped inside and the covers on the incoming wiring may be missing). and what are the 6 cuircuits.

replacing with a single new cu would be an option but the only CUs that fit those MCBs would be pretty much identical to the ones you've got already (including thier inability to accomodate RCDs)

personally i'd be tempted to replace one of the CUs with a small (say 4 way) RCD only CU and leave the other one for cuircuits you don't wan't on RCD but it really depends on how everything is fed.

 

[Adam_151]

Indeed, I do have Wylex units - pictures are here:
http://www.manwomanandmyth.com/images/60amp_CU.JPG
http://www.manwomanandmyth.com/images/60amp_CU_label.JPG
http://www.manwomanandmyth.com/images/Both_CU.JPG

Yes they are wylex standard units, and I'm 95% certain that only the ones with 100A main isolaters can take anything over 30A

The MCB's are GE brand - when you say that my Wylex units may not take devices over 30A, does this mean the 45amp MCB is physically larger than a 30amp?

Not really (the 'legs' on them do vary with size tho), the problem is that its not designed to take more than 30A through each fuse way (and so will be in danger of over heating if you try), there will be some kind of protection against putting an oversized mcb in, but its nor fool proof and they will go in, and sit at an angle (so I've heard)

So the RCD can be at the isolator end, in the bathroom? If so, would Screwfix have this combined isolater and RCD unit? Any particular model?

could do, would seem more sensible to fit it near the cu though

If I had to go for a seperate CU for the shower, can anyone indicate the typical charge (London) by an electrician to do this considering that I would do some of it by laying the cable to connect the isolator switch to where the new CU would go?

No idea sorry, and not sure he would be happy using a cable installed by some one else, though I suppose its no different really to doing a CU change, not entirely sure about the situation here tbh

And lastly, do you think that it would be more sensible to replace both of these Wylex CU's with a new one? If so hopefully, the GE MCB's could still be used - cost me £10 each as I recall...

yes, and sadly not, you could try and sell them on ebay though

Where are the CU's fed from, a henley block? Is there an isolator before it?

 

[cal_rourke]

Hello,

Thank you for the replies thus far - very helpful in making my mind up.

I have decided to go for a min-CU with an RCD on it solely for the shower. I will lay the 10mm2 cable between the mini-CU location and the isolator switch (an awkward loft arrangement would significantly increase time and therefore labour charge), but I will leave the electrician to terminate both ends.

Remaining questions, if you can help, are:

1) Can any inform me what the approximate labour cost for this sort of job would be (London)? If not cost then how long would it likely take assuming no unexpected issues?

2) As I will be renting our the property in question, is it straightforward to get an NICEIC certificate of safety from an electrician registered with that organisation?

If so, what does such an inspection involve and what am I looking at cost-wise considering it is a studio flat?

Thanks very much again,
Cal.

 

[Cork]

Just as a passing piece of information. If this work were being done in Ireland an RCD with an operating current not greater than 30mA would be a requirement and not optional when installing a shower.

Different jurisdiction, Different regs. So just FYI for those interested.