Some electrical advice needed please

[SCW]

I would like to heat up my small 1800 litre plunge pool to jacuzzi type temperatures with a combination of six 3 Kw geyser elements in a home made heat exchanger. I have done the maths and reckon to increase the water temp from 18°C to 38°C will take just over 2 hours. This will cost an arm and a leg in electricity I know.

Normally the pool is heated by a donkey boiler, but this requires keeping the fire stoked for a whole day, which in turn requires forward planning. Normally, pool parties are a spur of the moment thing.

My new plan is to be able to remotely activate a Smart Plug, to energize the coil on a contactor, that will close and allow power through to the elements.
I will build in some safety with a theromostat and maybe a flow control switch.

What I would like to know is
1. What size wires would I need if I wired 2 elements per phase, over a distance of about 12 metres from the main DB?
2. What size contactor would I need to handle the load of 6 elements (3 kw each)?
3. Can I trust a Smart Plug to not activate randomly, or not turn off when I instruct it to?

[AndyD]

I would like to heat up my small 1800 litre plunge pool to jacuzzi type temperatures with a combination of six 3 Kw geyser elements in a home made heat exchanger. I have done the maths and reckon to increase the water temp from 18°C to 38°C will take just over 2 hours. This will cost an arm and a leg in electricity I know.

Normally the pool is heated by a donkey boiler, but this requires keeping the fire stoked for a whole day, which in turn requires forward planning. Normally, pool parties are a spur of the moment thing.

My new plan is to be able to remotely activate a Smart Plug, to energize the coil on a contactor, that will close and allow power through to the elements.
I will build in some safety with a theromostat and maybe a flow control switch.

What I would like to know is
1. What size wires would I need if I wired 2 elements per phase, over a distance of about 12 metres from the main DB? If the elements are 230v then the load will be around 26A per phase. I'd suggest a 6mm x 4-core cable but you might scrape through with a 4mm x 4-core depending on the installation method.
2. What size contactor would I need to handle the load of 6 elements (3 kw each)? Any contactor with an AC-1 rating in excess of 26A. If you're buying cheap unbranded products I'd suggest you go with a 30 Amp AC-1 rating or higher.
3. Can I trust a Smart Plug to not activate randomly, or not turn off when I instruct it to? Not sure why a smart controller should randomly activate or fail to turn off. It's only switching the contactor coil current so it should be reliable.

See comments in red.

By the way, using standard geyser elements with water that's got chlorine in it probably won't end well. Chlorine is highly corrosive so stainless steel elements would usually be used for this.

[SCW]

Thanks Andy.

[SCW]

@AndyD
If I want to make sure that no power gets to the elements in the event of the pool pump failing or straining due to a blocked inlet of filter, could I use a circuit breaker installed in the feed to the motor. Sorry, I am probably using all the wrong terminology here. If so, what amp rating should it be that it trips fairly easily - the pump is a 0.6 kw single phase.
I tried to sketch my wiring plan below.

[GCE]

I would install a " water flow switch" in the water line with a normal open contact - Fit the contact between the thermostat and coil in your wiring drawing.

When the pump comes on the water will flow and the switch will close allowing the heater contact to close

You get cheap ones that work fine with a pool pump - you don't need a high tech one costing in excess of 1K

[SCW]

Thanks GCE. I will shop around.