Shower kW Rating on 32A MCB??

[Stu1H]

Hi all,

I've moved into a new place and need to replace a leaky shower. The current one is 8.5kW (Triton T80i) and was about to order a 8.5kW replacement but had a quick look at the MCB and it's rated at 32A ... I was expecting it to be 40A!

Assume I should go with 7.5kW rather than 8.5kW??

Cheers!

 

[ericmark]

32 x 230 = 7.36 kW however:-

There is a curve time to current so at 50A you could run for 100 seconds, which is a bit short even for a shower, but 40A does not come into the limit of the chart at 10,000 seconds, so you can exceed the 32A for a short time and the cable plus MCB will not over heat, however the chart is not exact so at 9.5 kW you may not get full 10,000 seconds, and with age the MCB starts to trip quicker and quicker so in time it will start to trip. However likely you will get away with a 8.5 kW shower.

 

[Taylortwocities]

Take a practical view. If the old 8.5KW shower didn't trip the 32A breaker then there's no reason to expect the new one will.

 

[Stu1H]

Thanks. Trition T80z 8.5 ordered ... should be a straight swap!

 

[JohnW2]

32 x 230 = 7.36 kW however:- ............. There is a curve time to current so at 50A you could run for 100 seconds, which is a bit short even for a shower, but 40A does not come into the limit of the chart at 10,000 seconds, so you can exceed the 32A for a short time and the cable plus MCB will not over heat ....

Even without the graphs, we know that a Type B MCB is required to carry 1.13 times its rated current indefinitely without tripping - so a B32 should carry at least 36.16A indefinitely without tripping.

Even if the shower concerned is rated at 8,500W at 230V (unlikely), that's only about 37A (at 230V), but it's far more likely, the quoted 8,500W relates to 240V, that would represent only about 33.9A at 230V - in which case a properly functioning B32 should never trip with a supply voltage of 230V (which, for better of for worse, is the assumption we are expected to make when undertaking calculations!).

That supports TTC's pragmatic view and, as you have said, there is certainly no question of the cable being at risk. It should be said, however, that deliberately subjecting an MCB to currents above their rated value is not 'good practice' (and 'not compliant'), not the least because it may shorten the life of the MCB and/or cause it to become 'faulty'.

Kind Regards, John

 

[ban-all-sheds]

32 x 230 = 7.36 kW

Except that it is almost certain that the maker quotes his shower ratings at 240V.

you can exceed the 32A for a short time

You may not.

Ib ≤ In ≤ Iz​

You'll find nothing to support the use of diversity to adjust the Ib of an instant water heater, and I don't see how a domestic shower can reasonably be classed as a cyclic load within the context of 533.2.1.

Simply wanting to install a larger load is not enough.

 

[winston1]

in which case a properly functioning B32 should never trip with a supply voltage of 230V (which, for better of for worse, is the assumption we are expected to make when undertaking calculations!).

It is very much for the worse. At this point in time my supply voltage is 247.5 volts, yesterday it was touching 250v, and since this heatwave has not been below 245v. Even in "normal" weather it rarely falls below 239v.

Calculating at 230v is madness brought about by idiots.

Interesting why it rises in hot weather. I did wonder if it was all the solar panels, but it is still high after dark. Anyone?

 

[JohnW2]

It is very much for the worse. At this point in time my supply voltage is 247.5 volts, yesterday it was touching 250v, and since this heatwave has not been below 245v. Even in "normal" weather it rarely falls below 239v. .... Calculating at 230v is madness brought about by idiots.

I'm inclined to agree. Given that regulations generally cater for 'worst case' (often very unlikely) scenarios, I'm more than a little surprised that they don't require calculations to be undertaken at the extremes (minimum or maximum, as relevant) of permissible supply voltage.

It's not just at the high end. It took the regulations very many decades to recognise that fault protection needs to work right down to the minimum permissible supply voltage. They have now almost addressed that, requiring it to work down to 218.5V, albeit not yet right down to 216.2V (for some international/EU reason, I believe)

FWIW, my supply is similar to yours. Usually around 244-245V, and hardly ever below 240V.

Interesting why it rises in hot weather. I did wonder if it was all the solar panels, but it is still high after dark. Anyone?

Interesting question, and I have at least one additional suggestion. I've been carefully monitoring my electricity usage for some time and was intrigued to see that the night-time consumption on one of my phases had been appreciably and progressively falling for the last couple of months, despite nothing obvious having been changed (by me!) .....

On consideration/investigation, it seems that the reason was my water heating. It is heated by immersion during the night (cheap E7 hours) and used during the day. It appears that the cylinder could do with a bit more lagging (it already has a lot), since it seems that this phenomenon has been due to reduced heat loss from the cylinder (hence less heating required the next night) as the weather has warmed up. When I plotted this usage against Met Office figures for average daily temps in my area, there seems to be a pretty good correlation ...

... so that is perhaps one component of the explanation. Of course, fridges and freezers have to work a bit harder in hot weather, so I suppose that at least partially cancels out the water heating issue.

Kind Regards, John

 

[EFLImpudence]

I am beginning to agree with a Cmax figure being desirable.

Whilst it can be argued that CCC and associated things may allow for the higher range of voltage, MCBs are surely fixed at the stated rating

I.e. 32A is 32A and any more will be harmful to the device.
It cannot be otherwise, can it, without altering the characteristics?

 

[JohnW2]

I am beginning to agree with a Cmax figure being desirable.

As you will realise, that is very much my view. However, I think that all this "Cmin" and "Cmax" business is unnecessary confusion - all that is required is that we should be required to use the minimum or maximum permissible supply voltage, as appropriate, for calculations (i.e "Umin" for 'max Zs' calculations, "Umax for CCC ones etc.).

Indeed, as dear old westie used to often remind us, the requirement is only that the supply voltage remains within the 'permitted range' for some percentage (95%??) of the time - so maybe, if we wanted to be really safe/cautious, we should use figures a bit below 216.2V and a bit above 253V!

And I suppose that this is all the more important given that, as winston is already reminding us, the majority of installations in the UK usually have supply voltages considerably in excess of 230V.

Whilst it can be argued that CCC and associated things may allow for the higher range of voltage, MCBs are surely fixed at the stated rating ... I.e. 32A is 32A and any more will be harmful to the device. ... It cannot be otherwise, can it, without altering the characteristics?

Quite so.

Kind Regards, John

 

[ban-all-sheds]

I'm inclined to agree.

If you wish to agree that it was perfectly OK to use a nominal value which lay within the min-max range but now it is not OK to use a nominal value which lies within the min-max range, then fair enough.

But apart from Winston (and now you) everybody else here has a different view on where the madness and idiocy of that position lies.

 

[JohnW2]

If you wish to agree that it was perfectly OK to use a nominal value which lay within the min-max range but now it is not OK to use a nominal value which lies within the min-max range, then fair enough.

Not at all - my point is nothing to do with the changing in nominal supply voltage, nor anything to do with what the nominal supply voltage may happen to be - now, in the past or in the future.

Rather, my whole point is that one should not use, and never should have used, 'nominal values' (whatever they were/are) for safety-critical calculations - instead, one should use the minimum or maximum (as they case might be) envisaged possible supply voltages (which may or may not be the same as minimum and maximum 'permitted supply' voltages).

But apart from Winston (and now you) everybody else here has a different view on where the madness and idiocy of that position lies.

As above, nothing I've said has anything to do with value of the nominal voltage, or the change in nominal voltage, but relates to what, IMO, should, and always should have, been done.

What I would regard as appearing to have been "mad and idiotic" is that, for example, for many decades we had a situation in which a circuit which had a Zs just low enough to satisfy the regulations left a substantial proportion of installations (all those with a supply voltage below nominal) which would not actually meet the fault protection (disconnection time) requirements of those regulations.

Kind Regards, John