32 Amp or 25 Amp RDBO


Postby ml09616 » Wed Mar 25, 2009 11:27 pm

Hi all....

hope someone can help me here....

I am installing a Spa, which has the following electrical details:
2 X 3 HP pumps (total 4.475 kW, 18.6 Amp)
1 X 3kW heater (total 3 kW, 12.5 Amp)

I can put a 25 Amp circuit in and load shed the heater when the pumps come on (this facility is available via the Spa controller), or I can install a 32 Amp circuit to fully load it.

If I go the 32Amp, I will need a 32Amp RCBO, no problem there, otherwise I will need a 25Amp RCBO.

I have 2 questions....
1. What would be the better option..... and
2. If I go the 25 Amp option, is a 25 Amp RCBO too big considering that the load will be either 9.3 Amp (1 pump), 18.6 Amp (2 pumps going), or 12.5 amp (heater only going).

Any thoughts on this???
regards
Mike
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Postby kbrownie » Thu Mar 26, 2009 10:19 am

ml09616,
the fact that you are asking this question, suggests that you don't understand electrical princibles and this is beyond your scope.
Don't want to sound harsh call a suitable qualified electrician, it,s a dangerous quantity and should be treated so.
Loads of reasons why;
you and your family/friends safety
safety of your properety,
legal requirments,
insurances.
KB
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Postby BLAKEY1963 » Thu Mar 26, 2009 2:08 pm

MIKE
U need to assume full load conditions. if pumps 18.6 AMP plus heater , 12.5 AMPS ( TOTAL OF 30 . 1 AMPS ) , assuming both pumps could operate when heater comes in . that would make choice of protective device a 32 amp . NEED 2 also take account of start up current
from pumps , could influence type of rcbo , b , c , d ect.
before putting the circuit into service u probaly would have already performed inspection and testing to 17th ed.
check also that u have adequately earthed this installation , so your RCBO
PASSES rcd tests when the installation is inspected and tested.
check if your project involves part p notification . could come under as a special location , an area of increased shock risk . i am sure u would be aware of this



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Postby ml09616 » Thu Mar 26, 2009 10:24 pm

[quote="kbrownie"]ml09616,
the fact that you are asking this question, suggests that you don't understand electrical princibles and this is beyond your scope.
....... snip ..............
KB[/quote]

Thanks for your reply KB, your many replies to various questions have been great! keep up the good work! ...... but I do understand the principles well.

Although I may not have spelled it out in my note, the Spa controller is shunted to automatically shed the heater when the pumps come on.
If only the heater is on, and one pump starts, the heater will go off.
If either of the pumps are on, the heater will not come on at all , so the max load in this case is less than 25 amps, so 25 Amps is plenty.

However, I [i]can [/i] remove the shunt and fully load it (on a 32 amp circuit).

My question was really, what would you folks think? Is it worth going 32 Amp just to have the heater on demand? My thought is that the time people spend in the spa probably doesn't justify the extra cable cost. Heat loss for the extra hour or two when people are actually using it would be minimal.
It was just a "whats your opinion" sort of question.

regards
Mike
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Postby kbrownie » Fri Mar 27, 2009 7:42 am

Well Mike,
The point I was getting at, I may have been a bit aggressive putting over is as follows;
That this work will come under approved document part p of the building regulation, have a look in projects: http://www.diydoctor.org.uk/projects.htm for more info on part p. what I can tell you this is a legal requirement.
The term scope is used to determine the knowledge/skill of a tradesman at any particular level.
In electrical terms full scope in this instance is applied to someone who has the skill, knowledge, experience to complete all electrical work within the house and outside areas.
Defined scope is a tradesman for example, a gas engineeer who has the knowledge, skill, experience of his particular work and can apply the electrical installation of a boiler say but can't put a cu in or a re-wire because that is beyond his scope.
The questions that you asked, I could only assume that you did not have the scope to do this safely. Safety is always my first and an important concern when working on jobs and giving advise. Electricity kills people, plastering a wall or fitting a laminate floor unlikely too!
Won't knock anybody for having a go at DIY but lets do it safely, it is something that should be enjoyed as well as saving a few quid (with some luck).
I think Blakey1963 has pointed you in the right direction, with some suttle hints!
I'll add some to consider, cable size needs to be calculated, volt drop, the route the cable is taking is it safe?, method of fixing and mechanical protection of cable. Means of isolating and switching circuit, IP ratings.
Remember be SAFE!
KB
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Postby sparx » Fri Mar 27, 2009 10:18 pm

Hi Mike,
just to answer your main question, we faced same thoughts a couple of years ago when we installed our spa. Originally it was going under draughty 'covered way' so thought it would need heat all the time it's in use.
However it finally got installed in one end of a large conservatory (which is only single glazed) & we chanced not going for the heavy supply option, I can say we don't regret it as the time/temp. drop ratio is good, if you notice the drop you've been in too long (& crinkly) LOL!
regards SPARX
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Postby ml09616 » Sat Mar 28, 2009 7:29 am

Sparx, KBrownie and BLAKEY1963....

many thanks folks for your replies.... keeps the spirit of a forum like this alive.

KB.... no worries about being too aggressive..... I agree with you, you cant be TOO forceful when it comes to safety, and I agree with everything you say.

Sparkx.... yes.... my real question was, should I have the heater on demand... thus the 32 amp option. In reality I think I will go with the 25 amp option and load shed the heater when I am actually using the spa. If it gets too cold, it will be a sign from God that I have been in it too long!! LOL javascript:emoticon(':D')

Looking forward to more questions and answers..... its a pleasure to be on a forum such as this where we can bounce ideas around.....

regards to all.....

Mike :D :D
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