Electricians Advice Needed - Shower Installation

Nomisf

Nomisf

Nomisf

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Hi All,

I am hoping we have some resident electricians here who can guide me,

My estate agent commissioned the replacement of the electric shower in my rented accommodation, however the new shower trips the MCB if used for more than 6-7 minutes.

I have done my own prodding and found out that the shower installed is 9.5kw (Triton Enrich) but is installed behind a 32A MCB (Wylex NSB32) on what "appears" to be 6mm2 core cable. Our incoming voltage is around 230v on the dot.

Looking at the datasheet for the NSB32, a trip time of around 10 minutes at 130% load (41A, I calculated) seems to be about right.

The agency are refusing to accept that the installation is incorrect, so I was wondering if anyone could point me at the part of the electrical code that covers this ?

Thanks!
 
6mm is within spec but should really be a 40amp MCB, as long as the cable is clipped not buried in anything.

I always though it best practice to just fit 10mm for showers.
 
A 9.5kW shower should be installed with 10mm2 cable with a fuse rating of 40-45A, NOT 6mm2 !
It should also be RCD or RCBO protected.

A >9kW shower supplied by 6mm2 cable on a 32A MCB (normally used for 5mm (2x2.5)) screams cowboy installer who doesn't know what he is doing.
 
I'm not an electrician but I have 6mm twin core which limited me to 8.5kw shower because the wiring is obviously hidden in the wall and under lots of insulation in the loft. I believe if it's left uncovered you can up it a little.

I'm not an electrician though but I'd consider changing to a less powerful shower. If they fitted it I'd question the installation as you have. Google it. As said 10mm is best practice.
 
robj20 said:
6mm is within spec but should really be a 40amp MCB, as long as the cable is clipped not buried in anything.

I always though it best practice to just fit 10mm for showers.
Click to expand...

In all fairness, I do need to find the offcut of the cable they left and check with with my calipers - as I can't tell 6vs10mm by eye.

C.R.A.Z.Y said:
A 9.5kW shower should be installed with 10mm2 cable with a fuse rating of 40-45A, NOT 6mm2 !
It should also be RCD or RCBO protected.

A >9kW shower supplied by 6mm2 cable on a 32A MCB (normally used for 5mm (2x2.5)) screams cowboy installer who doesn't know what he is doing.
Click to expand...

We have a split box, with the MCB for the shower and plugs running through an RCD - so we are at least safe in that respect.

The issue is, the contractor refuses to admit that the shower is wrong - and is trying to force the landlord to have the wiring in the walls checked.
 
The 'sparky' sounds very poor imho.
If the run is very short (<20m) and a specific curve type circuit breaker is used then YES you could get away with the setup you have in regards to it being functional.
However, although the installation is safe from current overload (32A breaker is VERY safe for 6mm2 CSA) I would question the earth loop impedance due to the fact CPC's are always <CSA of the phase & neutral.
i.e the earth/CPC cable might not have a high enough CSA to pass sufficient current quickly enough to isolate the circuit under fault conditions.

32A is too low to supply 9500W (9500 / 230 = 41.3A)
 
Another thing to consider is any short circuits i.e small leaks inside the shower unit

Also, most electric showers have a low water pressure switch, this will trip the fuse if the showers not getting enough water.

Personally id seek professional advice before changing MCBs as it will depend on your boards MAX amp load whether it can take a bigger one.
 
DO NOT replace the 32A MCB with a 40A one without upgrading the 6mm cable to 10mm, this is getting into an area of danger as 40A is slightly too high to sufficiently protect 6mm cable from current overload and poses a potential fire risk in certain situations and installs!!

Page 276, BS 7671, reference method B


For a 9.5kW load 10mm2 cable and a 45A fuse rating should have been used, anything lower is just too close to current carrying capacity's and ratings, although possible it would require lots of calculations with variables being taken into account before being deemed safe and electrically sound.

With electrics it's always best to go with the belt & braces approach, an over engineered and spec'd installation is the best option imho.
 
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It's taking 7-8 minutes to trip the 32amp MCB due to the Time/Current curves specified in Appendix 3 of the BS7671 regulation. It's not tripping due to any fault within the shower otherwise the RCD would trip the lot out.

A 9.5Kw shower will draw 41.3 Amps @ 230v

The only installation method where a 6mm cable will take over 41.3 amps is clipped direct throughout it's entire length where it will carry 47amps. If this is the case then you can upgrade the MCB to 45amps. Assuming all other factors are taken into account (grouping, ambient temps & volt drop calcs).

If the electrician installed the shower to an existing circuit then he is clearly liable. He hasn't done the most basic calculation to see if the existing circuit is suitable for the new load.

Off the top of my head a 6mm single cable will measure around 4.8mm using calipers and a 10mm will be around 5.4mm measuring the single cable outer insulation.
 
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Nomisf said:
The agency are refusing to accept that the installation is incorrect, so I was wondering if anyone could point me at the part of the electrical code that covers this ?
Click to expand...

BS7671 said:
433.1.1 The operating characteristics of a device protecting a conductor against overload shall satisfy the
following conditions:
(i) The rated current or current setting of the protective device (In) is not less than the design curent (I b) of the
circuit, and
(ii) the rated current or current setting of the protective device (In) does not exceed the lowest or the current-
carrying capacities ((z) of any of the conductors of the circuit, and
(iii) the current (12) causing effective operation of the protective device does not exceed 1.45 times the lowest or
the current-carrying capacities (lz) or any or the conductors of the circuit.
Click to expand...
 
Nomisf said:
The agency are refusing to accept that the installation is incorrect, so I was wondering if anyone could point me at the part of the electrical code that covers this ?
Click to expand...

Inform them in writing that the installation is dangerous and potentially life threatening, they will then be liable if some one is injured as they knew about the situation but chose to do nothing. Make that point clear in your letter and I suspect they will suddenly take a greater interest in the situation.

They don't care that your shower is being interrupted they will care if they might get sued!
 
a1ex2001 said:
Inform them in writing that the installation is dangerous and potentially life threatening, they will then be liable if some one is injured as they knew about the situation but chose to do nothing. Make that point clear in your letter and I suspect they will suddenly take a greater interest in the situation.

They don't care that your shower is being interrupted they will care if they might get sued!
Click to expand...

It is not life threatening, sensationalizing things will do nothing to forward your case. Life threatening would be a nail in a fuse holder or some such.

The selection of protective device vs cable size is appropriate, just the load on the end exceeds the rating. That does not make it dangerous but it is incorrect. Not that I test domestic but if I did that would get a code 2 Improvement required on the basis that the design current now exceeds the protective device. But to say again the protective device is doing its job.

Also to whoever mentioned breaker curves above its not that straightforward. Higher rated breakers (C or D for example) tolerate more inrush current and while it would take marginally longer for it to trip it still would, a shower doesn't have an inrush current as its a resistive load (assuming its not a pumped shower and even then the inrush on the pump motor starting would be insignificant), you also have issues with meeting the maximum ZS permissible for those curves in a domestic environment. I really cant thing of a reason to use anything other than a B curve in a domestic situation.
 
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All correct above from what I can see. It's likely you had a 7-8.5kw shower fitted before, should have never fitted a 9.5kw shower to your set up. As stated above, 6mm cable with typically have a solid cpc (earth) core.
 
Lumeycom said:
It is not life threatening, sensationalizing things will do nothing to forward your case. Life threatening would be a nail in a fuse holder or some such.

The selection of protective device vs cable size is appropriate, just the load on the end exceeds the rating. That does not make it dangerous but it is incorrect. Not that I test domestic but if I did that would get a code 2 Improvement required on the basis that the design current now exceeds the protective device. But to say again the protective device is doing its job.

Also to whoever mentioned breaker curves above its not that straightforward. Higher rated breakers (C or D for example) tolerate more inrush current and while it would take marginally longer for it to trip it still would, a shower doesn't have an inrush current as its a resistive load (assuming its not a pumped shower and even then the inrush on the pump motor starting would be insignificant), you also have issues with meeting the maximum ZS permissible for those curves in a domestic environment. I really cant thing of a reason to use anything other than a B curve in a domestic situation.
Click to expand...

Unless you've checked the cable run it is both potentially dangerous and life threatening as it is a fire hazard the cable will be heating during use and could well be buried in highly flammable insulation etc etc etc and a little over exaggeration and dramatisation never hurts when trying to get a property agent to do anything!
 
Then that has nothing to do with the new shower. The point is the breaker will trip on overload at a level that is acceptable for the cable and, we have to assume here as we have not seen it, the installation so the risk of fire due to cable overload is mitigated.
 
C.R.A.Z.Y said:
DO NOT replace the 32A MCB with a 40A one without upgrading the 6mm cable to 10mm, this is getting into an area of danger as 40A is slightly too high to sufficiently protect 6mm cable from current overload and poses a potential fire risk in certain situations and installs!!

Page 276, BS 7671, reference method B


For a 9.5kW load 10mm2 cable and a 45A fuse rating should have been used, anything lower is just too close to current carrying capacity's and ratings, although possible it would require lots of calculations with variables being taken into account before being deemed safe and electrically sound.

With electrics it's always best to go with the belt & braces approach, an over engineered and spec'd installation is the best option imho.
Click to expand...

Thank you :)


Also thanks :)

Burned_Alive said:
I have nothing to add other than that this thread is a great example of why I love these forums.
Click to expand...

Agreed :)


I have taken the face plate off the shower, and can confirm the CPC is a single solid core - as per below picture.

uDq8yFsm.jpg

Sorry its taken so long to reply - its turned into an all out war between the landlord and the letting agency, culminating in us changing agency mid term :rolleyes:
 
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