RCBO query

[morqthana]

I would like to know if it's advisable to have the 32A RCBO fitted

How many amps does a 9.5kW shower draw?

 

[ebee]

How many amps does a 9.5KW shower draw , good question.
When our nominal voltage was stated as 240V that is what we used in the calculation and that is what the shower manufacturers stated as the power of the shower.

Our voltages did not actually change, it was all done with the stroke of a pen really.

did the shower manufacturers then call their showers power as to what power would be at the stated nominal voltage? No did they heck, it would be like a turkey voting for xmas.

a 9.5KW shower declared running at 240V would draw 39.58 amps therefore have an impedance (resistance) of 6.063 ohms would then become a 230V shower drawing 37.93 amps therefore a 8.725 KW. (a bog standard electric shower is merely a plain resistive element effectively) .

Would some manufacturers now describe their 9.5KW showers as a 8.7KW shower whilst others would keep theirs as 9.5KW? NO they would not.
If they were compelled to tell the truth they might get around this by stating their showers at both voltages (but which power rating do you think they would describe more prominently?
Yes, if they are allowed to do it their headline power would show as 9.5KW and no less.

If you happened to have a reasonable test meter to measure the voltage on you circuit and were confident it was accurate enough to display a realistic voltage (say a proper multifunction test meter) and say by test readings a real expectation that it maxed a certain figure at least 90% of the time and for the remaining 10% of the time then not much more and only for very short periods of time, for example, then would you have a reasonable expectation that your chosen breaker and cable was within limits?
Possibly, but who would do that? Nobody. I think.

PS - If you bought a batch of identical electrical showers and tested their resistance at room temperature they would all vary, if you tested them at running temperature they would also vary from that temperature too.

 

[diy_fun_uk]

How many amps does a 9.5kW shower draw?

Think for a sec why I maybe posted this topic in the first place. MMmmmm, could it be that I'm seeking the opinions of pros in this field? What I'm not getting into is answering loaded questions like yours.

 

[morqthana]

Nobody, no matter how much of a "pro" they are, can have an "opinion" which contradicts Ohm's Law.

And it's not a "loaded question".

 

[pete01]

Nobody, no matter how much of a "pro" they are, can have an "opinion" which contradicts Ohm's Law.

And it's not a "loaded question".

Mate...why dont you give him some advice instead of trying to be clever....(not being funny at all)

 

[SUNRAY]

Hi all

Thanks for further replies. Yes, he advised the tripping was due to poor installation of the isolator switch. Cables partially burnt out due to poor connections, not all of the live cores terminated.

The shower manufacturer doesn't recommend 6mm with 32A coupled to a 9.5kW model. Indeed, they state it's best to use a minimum of 10mm if the cable is likely to run in any situation that isn't ideal.

However they do state 'The current carrying capacity of the cable must be at least that of the shower circuit protection (see Table B).'

Essentially, assuming the issue has been fixed with the replacement and properly wired isolator switch, I would like to know if it's advisable to have the 32A RCBO fitted or just leave the 40A? This assumes 6mm cable left in-situ.

What that table is showing is the maximum current carrying capacity of a cable under various conditions, using Ebee's figures; basically any figure below 40A that on the chart should be avoided so that'll be the red, the amber may just be suitable for the heater if you have 230V but would not be correctly protected by a 40A MCB/RCBO and the green is fine.

This does not take account of voltage drop (cable losses) but at 5-6m it's not something you'd need to get involved with.

Although I'd need to check, I am doubtful any form of cable insulation has been used.

the reference to insulation is regarding thermal insulation, such as glass fibre, rockwool etc stopping the cable being cooled as it warms by the heavy current.

View attachment 390225

Don't get concerned about those who ask silly questions or don't offer advice.

 

[morqthana]

Mate...why dont you give him some advice instead of trying to be clever....(not being funny at all)

I actually think he would learn, and remember, more if he worked out the consumption of the shower himself.

 

[diy_fun_uk]

I actually think he would learn, and remember, more if he worked out the consumption of the shower himself.

This might surprise you, however we don't all want to learn about the subjects we post stuff on. Sometimes, believe it or not, folk are simply looking for an answer to their question.

Person A is thinking of buying Dunlop all weather tyres. Unsure, they post a question on a tyre forum.

Person A: Hey all, I'm thinking of buying Dunlop X21 All Weather tyres for my Audi Q5. Anyone got any experience of this setup?
Morqthana: Do you know the compound composition of the tyre?!? Do you know the kerb weight of your car?!?!?

 

[morqthana]

I consider that tyre "analogy" a very poor one compared to a simple bit of knowledge about how much current an XkW appliance draws.

Knowing that isn't really deep and involved effort to learn about electrical installation work. It's GCSE level science. It's one very simple calculation.

And think how much time it would have saved you if you could have answered this:

do you agree re dropping to 32A RCBO?

yourself.

 

[ericmark]

I have seen loads of showers with an MCB technically too small for the shower's rating, and they have not tripped in the time it takes to have a shower, over the years sometimes they do degrade, likely due to being overloaded so often, but it takes years.

The MCB/RCBO is there in the main to protect the cable, and the cable takes time to warm up, as does the MCB/RCBO, but we look at the worst case scenario, so the phone rings, someone steps out of the shower to answer it, and the news need fast action, so the running shower is the last thing they are thinking about, so shower is left running.

So with a 40 amp shower on a 32 amp MCB/RCBO the unit will trip, no harm done, but with a 40 amp MCB/RCBO on a 32 amp rated cable, the cable may overheat.

It is an unlikely scenario, but it points to the best option is to use underrated MCB/RCBO rather than chance overload the cable, in fact the shower tripping if used for an extended time, could be seen as a safety feature.

 

[pete01]

with a 40 amp MCB/RCBO on a 32 amp rated cable, the cable may overheat.

Well yes but thats not the case here as its 6mm cable

 

[ericmark]

Well yes but thats not the case here as its 6mm cable

I was not quoting size of cable, as no idea of the worst installation method for the cable.

 

[Harry Bloomfield]

Essentially, assuming the issue has been fixed with the replacement and properly wired isolator switch, I would like to know if it's advisable to have the 32A RCBO fitted or just leave the 40A?

Leave it as is. I have a 9.5Kw which has been running on a 6mm/32amp MCB, for many years and without any issues. The thing about a shower, is that they only run for a matter of minutes, rather than continuously loaded. It takes time for the cable to heat up. Even less chance of issues, if the shower is a temperature controlled version, which throttles the power down, to control the temperature. In the heat of summer, is when the ambient will be higher, and the current drawn least.

 

[pete01]

I was not quoting size of cable, as no idea of the worst installation method for the cable.

Fair dinkum

 

[Harry Bloomfield]

The MCB/RCBO is there in the main to protect the cable, and the cable takes time to warm up, as does the MCB/RCBO, but we look at the worst case scenario, so the phone rings, someone steps out of the shower to answer it, and the news need fast action, so the running shower is the last thing they are thinking about, so shower is left running.

Ignore the phone, if it is important, they will ring back!