Hot tub temporary power supply.

Hellrazer

Hi there.
Before I go on - I have a registered person calling out to run 2.5mm swa from fuse board to outside socket in the next few weeks. However I need a temporary solution in case we get a few days of good weather
Im not holding my breath though.

Anyway Im fitting a new hot tub this weekend. It comes with an RCD on the cable but the cable is only 5 metres long. I do have power in my shed - all wired correctly with swa that was installed about 12 years ago but thats around 7 - 8 metres away from where I need to be.

It says you cant use extension leads because of a voltage drop because that can damage the motors but Ive read on some hot tub forums thats because the usual cheap extension cords are way too thin and will cause voltage to drop but a "heavy duty" extension lead of 2.5mm or more will be ok.

So my question is this and Ive read online that this will work as a temporary solution until the permanent socket is installed.

Ive purchased some arctic blue 2.5mm x10m - rated for 25 amps which Ive attached correctly using the provided gland to an ip66 rated outside socket that will be installed permanently in the next few weeks.

Ive put a 3 pin household plug on the other end of the arctic arctic blue.

Ive mounted it on the wall the recommended 2 metres from the tub and ran the cable to the shed around the garden wall where its plugged in.

Will this be safe for a few weeks?

When the lad comes out to fit it permanently he`ll only have to run the swa cable to the socket and remove my temporary cable.

2011

The fact that your requirements are "temporary" is neither here nor there
If the solution is fit for purpose in the short term it will be fit for purpose in the long term.

What you are trying to do here is limit the volt drop. Volt drop is a function of 2 things, resistance and current. The current drawn depends on the power rating of the hot tub. What is it rated at? (in kW). Or perhaps the current (in amps) is stated in some of the documentation you have. Once we know this we are 1/2 way there.

Next you need to know the resistance. This will (mainly) be made up of:

1) Resistance of cable from ESB cabinet to distribution board. Normally this will be negligible as it should be at least 16 mm sq. and is generally a short run.

2) Resistance of cable from distribution board to shed. What size cable was used and how long is the cable run? What other loads are connected to this cable? These other loads will effect the volt drop too.

3) Resistance of cable from shed board to shed socket. What size cable(s) used and how long is the cable run? What other loads are connected to this cable? These other loads will effect the volt drop too.

4) Resistance of 2.5 mm sq. copper cable is around 0.0069 ohms per m according to Google. As your lead is 10m long and we have to consider the phase as well as the neutral the resistance of this is around 0.138 ohms. So there will be a volt drop of 0.138 volts (volt drop = resistance x current) for ever amp on this cable alone.

Note:
Just because your shed was "wired properly" does not mean that the wiring suitable to power a hot tub! It all depends on the power rating of the hot tub in question.

According to the documentation with the hot tub what is the volt drop to be limited to?

Although the 2.5 mm sq. extension lead is rated for 25 amps this is no guarantee that the volt drop will not be excessive.

I hope this helps

Hellrazer

Hot tub is rated for 2.3kw @ max 13 amps. From reading online the most they usually draw is between 10 - 11 amps.

1. I dont know the exact size but it was replaced 2 years ago by the ESB.Its as thick as your thumb Id say.

2 and 3. Cable from fuse board to shed is swa feeding a double socket directly - theres no distribution board in the shed - If im not mistaken its 3.5mm swa. There is no load on it other than the shed double socket which runs a few led outside lights and a lawnmower when cutting the grass. Theres a clothes dryer in there aswell @ 2.5kw but that wont be used at the same time as the tub.

Maximum safe allowed voltage drop is 5% according to the manual of the tub.

So by your reckoning I should loose only 1.56 volts over the length of the cable if it draws 13 amps and lower if it draws only 10-11?? 13amps x 0.128 volts

2011

Hellrazer wrote: »

Hot tub is rated for 2.3kw @ max 13 amps. From reading online the most they usually draw is between 10 - 11 amps.

Let's go worst case and say 11 amps.

That will be a volt drop of 1.5 volts on the extension lead (0.138 x 11).

1. I dont know the exact size but it was replaced 2 years ago by the ESB.Its as thick as your thumb Id say.

The ESB generally do not do work on the customer side of the meter. I would imagine they upgraded the supply to the meter.

2 and 3. Cable from fuse board to shed is swa feeding a double socket directly - theres no distribution board in the shed - If im not mistaken its 3.5mm swa.

There is no such cable. I would guess it is a 2.5 mm sq. cable.

There is no load on it other than the shed double socket which runs a few led outside lights and a lawnmower when cutting the grass. Theres a clothes dryer in there aswell @ 2.5kw but that wont be used at the same time as the tub.

You can not make the assumption that the dryer will never be used at the same time. That is not how electrical design works. I have seen "temporary" supplies outlive the owners! You can not definitively state that the kids / wife / next owner of the home will never, ever plug into the same socket.

You need to know the length of run of this SWA and the power rating of loads fed from it.

Maximum safe allowed voltage drop is 5% according to the manual of the tub.

That sounds about right.

So by your reckoning I should loose only 1.56 volts over the length of the cable if it draws 13 amps and lower if it draws only 10-11?? 13amps x 0.128 volts

Over the length of the extension lead you will loose about 1.5 volts, but that is only part of the overall volt drop. We also need to consider the volt drop on the SWA to the shed and the volt drop that occurs before the distribution board in your house.

Lets say the SWA to the shed is 20 long. It could have the dryer load on it (10 amps) and the hot tub (11 amps), and a few other small loads (+1 amp). Total load on this cable is in the region of 22 amps. If the SWA resistance is 0.276 ohms (using above approx value from Google) the volt drop on this is about 6.1 volts. We need to add this to the extension lead volt drop, new total volt drop is 7.6 volts.

5% of nominal voltage (230V) is = 11.5 volts (this is the maximum acceptable volt drop value). This means that you need the volt drop to the distribution board to be less than 11.5 - 7.6 = 3.9 volts.

Please note the above is not a proper calculation, it is only approximate. It is just intended to give you an idea of what is involved.

Hellrazer

2011 wrote: »

Let's go worst case and say 11 amps.

That will be a volt drop of 1.5 volts on the extension lead (0.138 x 11).

Ok thats within my limits

The ESB generally do not do work on the customer side of the meter. I would imagine they upgraded the supply to the meter.

House was built in the late 70s - it still had a fuse box with the screw in fuses in it. Beside the row of fuses was a black plastic square unit - one night it started smoking and the wires going into it were melting.Called the emergency number and they came out and removed this unit , replaced the wiring to the road and advised to get a new fuse box (well the one with the trip switched in it) which I did.

There is no such cable. I would guess it is a 2.5 mm sq. cable.

It is 2.5 mm

You can not make the assumption that the dryer will never be used at the same time. That is not how electrical design works. I have seen "temporary" supplies outlive the owners! You can not definitively state that the kids / wife / next owner of the home will never, ever plug into the same socket.

Not if I take the plug off the dryer until the permanent job is finished

You need to know the length of run of this SWA and the power rating of loads fed from it.

Its only around 5m from fuse box to shed.

Over the length of the extension lead you will loose about 1.5 volts, but that is only part of the overall volt drop. We also need to consider the volt drop on the SWA to the shed and the volt drop that occurs before the distribution board in your house.

Lets say the SWA to the shed is 20 long. It could have the dryer load on it (10 amps) and the hot tub (11 amps), and a few other small loads (+1 amp). Total load on this cable is in the region of 22 amps. If the SWA resistance is 0.276 ohms (using above approx value from Google) the volt drop on this is about 6.1 volts. We need to add this to the extension lead volt drop, new total volt drop is 7.6 volts.

5% of nominal voltage (230V) is = 11.5 volts (this is the maximum acceptable volt drop value). This means that you need the volt drop to the distribution board to be less than 11.5 - 7.6 = 3.9 volts.

Please note the above is not a proper calculation, it is only approximate. It is just intended to give you an idea of what is involved.

Thanks for that. It looks like Ill be ok if I remove the dryer from the equation until the permanent install is done. That was my biggest concern as the dryer is a bit on the power hungry side!!
All thats on that would be a string of led lights @ 3 watts total.

2011

From what you have described it looks like it will be ok but it is impossible to say for certain without a proper look at it.

If it were me I would measure the voltage at the hot tub (while it is running) with a suitable test instrument. That way I would be able to verify what the voltage is actually is at the hot tub. Please note this will only verify that one parameter is up to scratch. Other important tests need to be carried out so as to be sure that the hot tub is safe. Many people disregard this but water and electricity have been known to be a lethal combination. The voltage measurement should be taken with maximum loading in the entire installation occurring as this will provide close to a worst case volt drop. Needless to say this work is best done by a qualified professional.

Hellrazer

I agree which is why I have someone booked - hes running a dedicated length of cable from fuse box to outside socket with nothing else on it to a (I might be wrong with the name of it) mcb with rcd protection - does that make sense?
Didnt even think such a thing existed - I always though they were 2 separate devices.

2011

Hellrazer wrote: »

I agree which is why I have someone booked -

That’s all very well but it is the “temporary” solution that would concern me. Even a very temporary install must be safe and compliant with the regulations. It would be unwise to take chances with something like this even in the short term.

Didnt even think such a thing existed - I always though they were 2 separate devices.

Yes, they are generally referred to as RCBO’s.

Nothing lasts longer than a temporary fix that works!
Does it heat the water ?

Hellrazer

Have I any other options then short term?
How about a heavy duty extension lead?

If not Ill just wait until the jobs done correctly.
I take it that I should get a cert for this work?

Hellrazer

BrownFinger wrote: »

Nothing lasts longer than a temporary fix that works!
Does it heat the water ?

Yep -The heater is 1.8kw - bubble blower is 0.2kw and the filtration system is 0.2kw.
Jets are .2kw or something. I might be 0.1 kw out there on some of that but the total with everything on is the 2.3kw.

But you cant run everything together - for example you cant run the bubbles with the jets. You cant run he filtration with the bubbles. The heater cant run with both on but can run with either.

Most they`ll draw is a around 2kw

2011

Hellrazer wrote: »

Have I any other options then short term?

Not really.

How about a heavy duty extension lead?

That may work but you really need to get a professional to assess that properly.

If not Ill just wait until the jobs done correctly.
I take it that I should get a cert for this work?

Sometimes it’s better to wait. Do it right and pay once.

Yes, a cert would be required as it is a new circuit.