Cable size adn MCB size for Spa pool

[GC]

Guys

Posting from sunny New Zealand. Getting a spa pool delivered soon and just had wiring done.

Specs say it needs 32amp. It has 2 pumps rated at 2.25KW each. A blower at 700w and a circ pump at 0.25 kw. Reckon that should be everything if all switched on at same time...

Importer who sold it to me said his was running on a 40amp MCB and 6mm square cable....Sparkie who just did a homer used 4mm square cable instead saying it was ample. Two core and earth - PVC sheathed.

Cable run is 23 metres from fuse to outside switch to spa.... Will

4mm cable be sufficient? Suffice to say my experience of some of the building codes etc here is they stretch the specs somewhat more than when I lived in the UK.

Thanks for your input.

[GC]

Sorry.....Also has a 3 KW heater..

[stevelup]

Are you on 230V there?

Your total load is 8.45kW which is 36.74 amps. You would need 6mm cable to meet the minimum voltage drop requirement here in the UK.

But - having said all that, I don't think you have listed the specs correctly. Are you sure it really has two pumps rated at 2.25kW each? That seems excessive to say the least - are you sure it doesn't have two *heaters* rated at 2.25kW each? That seems more likely...

Assuming the extra 3kW you added on there at the end is a mistake, your total load would be 5.45kW which is 23.7A which would be fine with 4mm cable over 27M

Steve

Steve

[GC]

Hi Steve

Thanks for your reply...Yep..The supply is 230V / 50 Hz give or take... Rechecked the specs - Pump 1 - 2.25Kw, Pump 2 - 2.25Kw, Blower for Air Jets - 700w, Circulation Pump - 250 W and heater - 3Kw...

Now from further research I don't ' think' you run the 2 pumps AND the air jet pump at the same time...Its one or the other...

But yes, definintely 2 main pumps rated at 2.25Kw each...It seats upto

8 people...A big beast of a spa...

Cable run is 23 to 25 metres max..Into an IP56 rated switch and out into the Spa connection. The total length quoted includes the 2 metres on the spa side of the switch. So roughly 22 metres from fusebox to switch and a couple from switch to spa..

Am I correct in thinking that the 4mm is being pushed to its max...It is generally clipped to top of joists in loft space, and run down plastic trunking inside a cupboard before exiting the building into the switch.

[stevelup]

I don't understand the manufacturers rating then - they quote 32A supply required yet it is going to draw almost 37A. It looks like the advice your importer gave was correct rather than the manufacturers information.

It's outside of regulations here in the UK. The voltage drop would be too great over that length on a 40A MCB. The regulations may differ where you are and that may be why your electrician is saying 4mm is OK.

You could ask him for his calculations - ie. on what basis is he asserting that 4mm is adequate.

Steve

[js.b1]

4mm flat twin & earth if 1.5mm CPC also imposes EFLI limits. Type-B CPD 32A is 1.20ohm max EFLI, 40A is 0.96ohm. So as well as current limitations your house supply type (Ze) combined with 23m in 4mm (R1+R2) may push EFLI over spec.

Cable technically needs derating for the run in trunking (which is installation method 3 in the UK or about 12.5%). Some may not bother if the run is very short. I assume the cable is not

*surrounded* in insulation anywhere (which if >500mm needs a derating of 50% so your 4mm would be quite unsuitable).

Spark may have applied diversity calculations to the loading. So just query him on the calculations and actual design, he should be happy to provide as it justifies his skills/charge.

[Steve Walker]

That's the optional underpatio heating version.... :o)

[terry]

Voltage not mentioned but assuming it's 230 volts? Single phase? Using some North American design tables for the wire gauge closest to

4mm diam. it looks like voltage drop (with everything on) will be about 2% or roughly 5 volts. Wire length being twice 23 metres (roughly 140 feet). However a 40 amp breaker seems to be 'close to the wire' (pun intended!). Cos if you add up all the normal running wattages, ignoring start up surges of pump motors and also that heaters until warmed up draw more current one gets something of the order of 8.5 kilowatts? That's around 37 amps!!!! So if a 50 amp breaker then needed, to avoid trip outs, it may be necessary to comply with local wiring/insurance codes to use another (heavier gauge) of wire? Seems wise that you are checking. Not sure what's the legal or insurance liability outcome of knowingly installing inadequate wiring, to local code, would be, in your part of the world.

[stevelup]

Not sure I understand this bit. A 40A breaker will run a 40A load indefinitely without tripping.

I don't think there is an issue with the breaker rating but the cable would certainly under rated at 4mm according to UK regs.

[John Rumm]

Note we are assuming the OP means 4mm^2 cross section area and not 4mm diameter wire here. The latter would be in excess of our 6mm^2 cable.

Perhaps comparison with the outside dimensions listed here might help:

However a 40 amp breaker seems to be 'close to the wire' (pun

Using the type of breaker we traditionally do, this would not usually be considered a problem. It may be normal to specify a type C MCB to cope with the motor inrush currents however.