Isolation AND R.C.D Protection for Swimming pool Heater

[sparkyjohn1]

Hello fellow sparks.
Just a query about a job i've been asked to do recently. I've been planning it, regarding the cable sizing and method(s) of protection required. Client is thinking of having a Swimming Pool installed and the Heater is 20KW !! Roughly Equating to 80 Amps..! The Service Fuse for the House is 100 AMP,
I realise because it is a BS88 Cartridge Fuse it will not blow at 100, So due to this , and diversity factor 180 Total should be fine?!!? (Or possible get onto electricity board for an extra phase installed??!
If i did tap of Existing supply, using Henley Connecter, with 25mm cable into 80Amp Switch Fused Isolator, then 25mm Armoured cable buried across the garden into another CU close to Swimming pool.. The regs state you must provide R.C.D protection AND over-current protection. So was thinking about using 80 AMP R.C.D with 80 AMP Cartridge fuse. As i cannot find 80amp M.C.B. or 80 amp R.C.B.O anywhere..??! Ive used the correction factor with the BS88 Fuse, and Cable size still sufficient. I realise there must also be an Isolator Switch aswell,, Heavy Duty Rotary ON/OFF switch maybe.?
Does Anyone Have any ideas or come across similar situations in the past?? I'm trying to do it to the regs and apply correction factors where necerssary.. Major dilemma is whether or not to have an extra phase installed..?!​

 

[RogertheBodger]

20 kw. sounds like a lot, what type of Heater is it, Immersion or Heat Pump?

 

[sparkyjohn1]

Here is the heat pump the client has been adviced to buy,, Seems more like industrial use to me tho...

 

[RogertheBodger]

Looks normal for a domestic pool.
Check the specs, you will find that although it produces 20kw of heat, it will only pull 4-6 kw of Electric power.

 

[sparkyjohn1]

Heres the spec sheet.. reckons running current 16 Amp - 21 Amp? Surely this can't be right Compared to Heat Output of 20KW?!!

 

[RogertheBodger]

Seems a little high John yes.
May be it's just a thirsty model, what size of pool is it heating?

 

[sparkyjohn1]

Not sure mate, i just told client, that i couldnt give an exact quote, until i recieved the Wattage of the Heater, so i could work out the Current draw, then subsequent cable sizing,. And possible Isolation/ protection methods.
He's obviously chosen the most powerful and expensive one... (not being stereotypical, but there were 5 cars on the driveway.!..so not short of a few quid.).
I calculated it from the 20KW output, and worked out 25mm cable. (40 meter run) on an 80amp BS 88 Fuse.. Just wondering what you would do in this scenario..? Would you calculate from the Running Current??

 

[RogertheBodger]

Just looked at it again, the input power is 3.5 kw. so I'm guessing that what they are calling the running current is the inrush current, so you could calculate from that.

 

[sparkyjohn1]

Seems a bit odd tho. 3.5 Kw !!! So you treat it like a normal 16amp Radial circuit using 2.5mm cable?? Seems too low tho..for a major heating element,,(compared to normal electric shower for instance.)
Why do they throw figures like 20 KW onto the manual then??

 

[RogertheBodger]

Heat energy and Electrical power both rated in Kilowatts in this case, confusing!

 

[HandySparks]

Seems a bit odd tho. 3.5 Kw !!! So you treat it like a normal 16amp Radial circuit using 2.5mm cable?? Seems too low tho..for a major heating element,,(compared to normal electric shower for instance.)
Why do they throw figures like 20 KW onto the manual then??

You need to understand that a heat pump is not an electrical heating element. It's like a fridge in reverse. You get more heat energy out than the electrical energy you put in because you're pulling heat energy out of a suitable source such as the surrounding air, water or ground. Google it.

By the way, please don't ask the same question twice.

 

[sparkyjohn1]

So what cable and fuse would you use , in this instance?? thanks,

 

[HandySparks]

So what cable and fuse would you use , in this instance?? thanks,

If it's for the more powerful of the two units, then base your calcs on the quoted running current of 21A. So a 25 or 32A MCB (maybe type C to allow for start-up current of the motor) and cable to suit, taking into account voltage drop for the length you need.

By the way, the 21A will be because, being a motor not a resistive heater, the power factor won't be 1. So a simple calculation of Watts/Volts (4250/230=16.48A) won't be correct.

 

[sparkyjohn1]

thank you very much, really appreciate your patience, and detailed answer.