Shower 9.5 kW on a 40 A fuse

[pat]

My original 8.5 kW nominal shower stopped working due to hard water. I replaced it and upgraded to 9.5 kW nominal. That gave me a much better shower. Now that one has stopped working for the same reason. I installed a water softener but still need a new shower. I want to replace the shower with another 9.5 kW nominal unit. The house is only about 7 years old. The shower circuit has its own 40 A fuse on the board.

My simple arithmetic tells me:

9500 W / 230 V = 41.3 A 9500 W / 240 V = 39.6 A

But some people say 40 amps means 8.5 kW.

1. Which voltage should I use in the calculation: 230 or 240 V?
2. Which voltage do UK shower specifications use when describing it as

9.5 kW?

1. Anything else I should consider?

Thanks in advance

[Phil L]

Use a 45A fuse, although if the house is only 7 yrs old, you will need a breaker (mcb)

[Nigel Molesworth]

Fuse?

[David Hansen]

On 15 Mar 2006 10:05:00 -0800 someone who may be "pat" wrote this:-

The voltage the shower is rated at, which will be marked on the shower but is very likely to be 230V.

What size of cable runs to the shower? How is it installed?

What sort of fuses are they? You will probably be able to re-use the carrier as 40 and 45A cartridge fuses are the same size.

[pat]

I don't understand. Are you saying that an mcb would not be required on older houses?

If I should have mcb's for everything rather than fuses, I could replace them all.

I don't know how to answer that. What types are they? They are just larger versions of the types you get in a fused plug.

If I upgrade the fuse from 40 to 45 amps, isn't that putting the cable at risk? Note that I have been running a 9.5 kW shower on a 40 A fuse for the last two years. Surely that means it is fine?

The consumer unit is made by Wylex.

The shower and socket circuits are covered by an RCD: Wylex WRS80/2 rated at 80 A, 30 mA. The other circuits (e.g. lights) are not covered by RCD. The whole board has a chunky red on/off switch.

I can't see any part of the cable and I do not know its size or its route. The consumer unit is at the front door and the shower is in the centre upstairs. The cable is probably 5 to 10 metres long.

[David Hansen]

On 15 Mar 2006 16:09:52 -0800 someone who may be "pat" wrote this:-

Then they are cartridge fuses to BS 1361. This will be written on the fuse itself.

That is why I asked you the questions I did.

What it means is that the fuse may eventually get tired of being abused and operate.

There is no easy answer to this that doesn't involve a little bit of effort on your part. Nobody else can see your installation. You should be able to see the cable at the consumer unit or shower ends. How big is it?

[Nigel Molesworth]

Explains a lot. Replace the 40A fuse with a 40A MCB, all will be well.

[Phil L]

I'm saying that he won't have re-wireable fuses in and they will probably be MCB's....you also might have MCB's if your house has been rewired in the last 15-ish years.

[John Rumm]

You seem to be changing stuff for the sake of it. Without more information on what is actually installed it is not possible to say accurately what changes (if any) are required.

There is nothing intrinsically wrong with a cartridge fuse so there may be no need to replace it with a MCB. In fact in some cases a fuse will be preferable since it will have a higher current braking capacity.

Regarding the load, A 40A fuse or a 40A MCB will run with a 41A load indefinitely without tripping or blowing. The real issues here are the cable - i.e. given the length and the way it is installed will it have the current carrying capacity, and will it meet the voltage drop requirements.

Also I see no mention of a RCD on the shower feed - this may also be OK since it is not absolutely required so long as you can meet the required disconnection times in the event of a fault etc. Again we come back to cable size and earth fault loop impedance.

[Ed Sirett]

The problem is that the 9.5kW (is in big letters on the packaging box). The real rating is less and specified at 230V and is the one for the calculations.

[David Hansen]

On Thu, 16 Mar 2006 19:55:43 +0000 someone who may be John Rumm wrote this:-

Indeed. The OP appears to want someone to say that their proposed installation is fine. Someone may do so eventually, but one may think that anyone who says so, on the information given, is a fool or a liar.

[David Hansen]

On Thu, 16 Mar 2006 12:27:19 GMT someone who may be [e-mail] (Windmill) wrote this:-

I would quibble with the value of the currents a little, but at that particular point on the curves the rewirable fuse may operate more quickly. However, that is a carefully chosen point, as a glance at the time/current curves shows. That the curves overlap in places is not news.

Not in the least. These things have to be looked at in the round. The cartridge fuse does not age like a rewirable fuse, has a higher breaking capacity, is easier to replace and operates rapidly enough to limit high fault currents.

[pat]

Thanks for the interesting replies. I did indeed want somebody to say it is within spec. Now I know it is more complicated. I have decided to get an electrician.

For info, the cable is 6 mm2. I will convert to mcb's for convenience. I will make any other changes to the consumer unit that are necessary. Cost is not the top issue.