looked at a little job today for a customer. he wants to remove his imersion tank located in the roof and fit 2 local instant heaters under sinks in cupboards in bathroom and kitchen. The imersion supply is switched in kitchen and goes via a spur outside the bathroom that used to feed a whirlpool bath (so both on same radial via a B16 breaker). he is happy to fit both heaters onto same radial, ive explained that he will only be able to use 1 at a time, and in reality thats all that will happen.
I was innitially looking at a couple of 3.5kw models(perfect 35), but after reading reviews etc would prefer the 5kw models. obviously this is pushing it a bit with the cable and the breaker. I could upgrade the breaker to a 20amp (2.5 cable) but still a bit tight.
On the other hand its an instant heater and will only run for a few seconds at a time so not really a problem even with the B16.

An opinions/advice would be appreciated.

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese

Is the cable likely to suffer from overheating by it's install conditions if you were to put in a 20 or even 25A breaker (and the load running times) ?

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Empty barrels make the most noise.

I don't think there will be a problem on the 16amp to be honest if i'm reading the curves right. It needs over 20amps before it even touches the 16amp curve, that with the fact that they will only run for a minute maximum so i see no problem with cable overheating. there is no insulation in the floors.

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese

I think i'd be happy with it Gary, after all the breaker will be protecting the cable, the worst that could happen is the breaker tripping, I think i'd go for a 20 amp breaker though,

Dave.

Hi gary, the 5 kw instantaneous unit might be ok for hand washing use in the bathroom but I would seriously consider recommending a heater with a bit of storage for the kitchen for washing up etc, the instantaneous 5kw would take 5 minutes to fill a 10L bowl @50C

http://www.ascot-hygiene.co.uk/technical.html

Thanks guys. cheers for the link weirdbeard, thats really helpful.

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese

Dependent on the installation method the cable rating may be 18.5A so uprating the fuse may not be an option (cable type ?)
Also is VD an issue? 5kw on a 2.5mm may not be a long run before there are issues (30M ish)

Forgive my ignorance- since I moved to Canada, has there been a change in the regs- I believe it used to be a max of 3kW fixed load on 2.5 - hence immersion ccts.

its standard pvc/pvc . the run is 10m max .They had some building works done 15 years or so ago and the ceilings were down so he has told me and there is not any insulation in the ceiling. running in a new supply would cause a lot of disruption because of the directions of the joists and polished up old floorboards and a chimney breast. thats before navigating the kitchen

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese

As the design current is greater than 20A you would have to rate the breaker at (433.1.1) 25A
If we are saying the pvc/pvc is clipped direct with no insulation it could be rated at 27A but personally having not installed the cable etc etc I would install a new cable, to tight for my liking!!
I tend to draw a bit of a line at 3kW for 2.5mm

you are the designer, how good is your indemnity insurance!. Ask yourself, if this was in your gaff, would you be happy with it?. In theory, pretty close. Practice? In reality, will only one be used? Will someone competent be there who knows the set up?

After re-read
2no 5kW heaters on one circuit? unless they cant be operated together you would have to asume they could (would) be operated together.
I would run two new circuits in and atleast you can sleep at night!!

Although I tend to agree with you GB, yes in an ideal world another circuit would be preferred or maybe replace the existing cable with 4.00mm or 6.00mm and fit a 32 amp breaker but in the real world it's not always practicable to do things the way we would like. At the end of the day the existing cable will be well protected by the 16a or 20a mcb, the worst that could happen is occasional tripping of the mcb, (also bear in mind that they are instantaneous heaters so will only be on for a few minutes at a time)

Dave.

If it were a perfect world I would not be in it....thats for sure!!>>>>
Although it is over to the designer you would have to say it would be very poor design and will only come back bite somebodies backside,
I would also tend to say the problem is not the sparks but the clients............ until you undertake the work.
Fuse would have to be sized at 25A to comply and not sure I would fancy that!!

In practice it should be fine, 2.5mm cable on a 20 amp MCB, provided that the conditions do not require that the cable be de-rated to less than 20 amps.

Not strictly complliant with the regs though, since the design current is greater than the OCPD.

If both water heaters are used at the same time then the MCB should trip before any harm is done.
No worse than someone plugging in 10KW worth of portable heaters on a 20 amp radial circuit.

Not for Me!!

Understand the thinking but not sure it justifies the possible downsides, sometimes the client just has to cough up so we can sleep tight. fuses should be for emergency operation not to cover flaws in the design, if you put two taps on the fuse will "pop" aint a good way forward.

Isn't it permissible to design for thermally equivalent current rather than instantaneous current (a bit like diversity). I'm sure there is (or was) something in one of the GNs about it (sorry, I don't have it to hand at the moment).
- Andy.

Think I read something some where about that but thought it was to do with grouping factors?
May see if I can find the piece in a bit.