9.5kw shower on a 6mm cable in new build

[ShaunShep]

In a new build, we have a 9.5kw Mira shower connected to the consumer unit via a 40amp MCB and what looks like 6mm cable.
When we use the shower the MCB in the consumer unit makes a buzzing noise.

The builders have sent out an electrician to look at this three times now. The first time they tightened a loose cable, the second time they said the MCB would need replacing, and they third time they replaced the MCB. The buzzing still remains. On the final visit, the electrician said the shower must be faulty, and we needed a plumber.

The buzzing stops when you switch the shower onto it's lowest setting.

Today a plumber has been out and pointed out that he thinks the cable should be 10mm and not 6mm for a 9.5k shower.

Before I go back to the builder and ask them to arrange for a new lower power shower to be fitted, is the plumber correct?

 

[JohnW2]

The builders have sent out an electrician to look at this three times now. The first time they tightened a loose cable, the second time they said the MCB would need replacing, and they third time they replaced the MCB. The buzzing still remains. On the final visit, the electrician said the shower must be faulty, and we needed a plumber.

It is extremely hard to see how the shower could be responsible.

Today a plumber has been out and pointed out that he thinks the cable should be 10mm and not 6mm for a 9.5k shower.

It depends on how the cable is installed (i.e. it's 'route'). Even if the 9.5 kW relates to 230V (which is probably doesn't, it's probably 9.5 kW at 240V), that would only be about 41 A. If the 6mm² cable is installed by the common "Method C" (on surface, or buried in plaster wall etc.) then it can carry 47A, so would be fine for such a shower (and for the 40 A MCB). In other situations (e.g. if cable was covered by thermal insulation), larger than 6mm² would be required.

However, even if the cable were undersized, that, in itself, would not cause any buzzing.

Kind Regards, John

 

[ShaunShep]

Thanks John.

Do you know anything that would cause a buzzing coming from an MCB?

 

[JohnW2]

Thanks John. ... Do you know anything that would cause a buzzing coming from an MCB?

Only really those that have already been considered - loose connections or a faulty MCB.

Are you certain that the buzzing is coming from the MCB and not something else in the CU?

Kind Regards, John

 

[ShaunShep]

I'm sure it's the MCB, and the electrician thinks it's the MCB as well.

 

[JohnW2]

I'm sure it's the MCB, and the electrician thinks it's the MCB as well.

OK. I realise that it is 'grasping at straws', but does the electrician perhaps have 'yet another' MCB he could try?

Kind Regards, John

 

[oldbutnotdead]

You may do better identifying the source of the buzz with a screwdriver- tip on the suspect device, handle in your ear. Repeat til you get the loudest buzz, make sure the screwdriver is insulated and you have a friend on site (in case you c*ck the job right up and give yourself a shock).
Of course your electrician has already checked every connection before the MCB.....

 

[ShaunShep]

It took him three weeks to come back and fit a new MCB because they’re only allowed to order parts from their selected supplier and couldn’t use the screw fix around the corner that had one in stock.

I’ve already done the screwdriver trick myself, it’s definitely the mcb that buzzing.

I’ve asked the builders to send a different electrician and I’ll grab another mcb from screw fix as they’re only a few quid, just in case.

 

[Risteard]

It is extremely hard to see how the shower could be responsible.

It depends on how the cable is installed (i.e. it's 'route'). Even if the 9.5 kW relates to 230V (which is probably doesn't, it's probably 9.5 kW at 240V), that would only be about 41 A. If the 6mm² cable is installed by the common "Method C" (on surface, or buried in plaster wall etc.) then it can carry 47A, so would be fine for such a shower (and for the 40 A MCB). In other situations (e.g. if cable was covered by thermal insulation), larger than 6mm² would be required.

However, even if the cable were undersized, that, in itself, would not cause any buzzing.

Kind Regards, John

41A is not "fine for a 40A MCB".

 

[JohnW2]

41A is not "fine for a 40A MCB".

Yes, I suppose I should not have added that 'afterthought' comment without explanation. My response was in relation to cable size, hence my primary point was that 6mm² Method C cable would be 'fine' for 41A.

Whilst it is strictly true that a 40A MCB is not quite adequate for a 41A load, as I implied, the 9.5 kW almost certainly relates to 240V, so the conventional 'design current' (i.e. at 230V) would then be about 37.9 A - which would be OK with a 40 A MCB.

 

[Risteard]

Yes, I suppose I should not have added that 'afterthought' comment without explanation. My response was in relation to cable size, hence my primary point was that 6mm² Method C cable would be 'fine' for 41A.

Whilst it is strictly true that a 40A MCB is not quite adequate for a 41A load, as I implied, the 9.5 kW almost certainly relates to 240V, so the conventional 'design current' (i.e. at 230V) would then be about 37.9 A - which would be OK with a 40 A MCB.

Fair enough, although it's still best not to load a circuit breaker near to its rated current for long durations. (Admittedly, showers shouldn't generally be of excessive duration.) Although i make 9.5kW @240v to be in excess of 39.5A.

 

[JohnW2]

Fair enough, although it's still best not to load a circuit breaker near to its rated current for long durations. (Admittedly, showers shouldn't generally be of excessive duration.)

I agree, but also agree that showers are usually only very short duration loads In fact, I would imagine that the MCBs of shower circuits are nearly always running close to their 'limit' for the brief period of time the shower is drawing current.

Although i make 9.5kW @ 240V to be in excess of 39.5A.

9.5 kW at 240V is, indeed, slightly over 39.5 A (actually 39.58333333... ), i.e. a resistance of about 6.063 Ω. If ('conventionally') such a shower is powered with 230V, that would translate to a current of about 37.9 A.

 

[Risteard]

I agree, but also agree that showers are usually only very short duration loads In fact, I would imagine that the MCBs of shower circuits are nearly always running close to their 'limit' for the brief period of time the shower is drawing current.

9.5 kW at 240V is, indeed, slightly over 39.5 A (actually 39.58333333... ), i.e. a resistance of about 6.063 Ω. If ('conventionally') such a shower is powered with 230V, that would translate to a current of about 37.9 A.

Sorry i misread that you were stating it at 230V.

 

[JohnW2]

Sorry i misread that you were stating it at 230V.

I was. I said that a shower rated at 9.5 kW at 240V would draw about 37.9 A at 230V.

 

[SUNRAY]

I was. I said that a shower rated at 9.5 kW at 240V would draw about 37.9 A at 230V.

As long as the stated power is at 240V... I've been finding in recent times that 230V is being used and sometime even the hangover from 220V.

I mentioned a couple of drinks water boilers in another thread recently where one had failed and been replaced.