Hot tub wiring.

[Dozy Danum]

Hi, I've just had a cable put in for a hot tub. The electrician used 4mm armoured cable approx 20m from consumer unit to 32amp switch outside. The hot tub company states they require a 32 amp isolator switch with a (6mm) swa cable to go to the hot tub but the electrician says 4mm will be ok. From what I have crudely worked out the maximum the tub will be drawing is 26/27 amps. Does this mean 4mm is ok. I have contacted hot tub company and they say 4mm is ok if the electrician says it is. So why would they stipulate on their website. Just need to someone to put my mind at ease thanks.

 

[EFLImpudence]

4mm² is satisfactory.

Don't know why people put silly things on their websites - it is quite often the case - suspect incompetent staff.
Never assume those in charge know what they are doing.

 

[Dozy Danum]

Thanks (again) EFL.

 

[Dozy Danum]

Sorry EFL, or any other electrician. I am a born worrier. Could you explain ( in simple terms) why 4mm2 cable will be ok, as I have looked on various sites ( including this one) and all seem to state 6mm2 minimum for a 32amp hot tub. Thanks.

 

[EFLImpudence]

4mm² has a maximum Current Carrying Capacity of 37A (more than the presumed 32A of the MCB and 27A of the appliance) when installed in such a manner that it will not overheat - clipped direct to surface or buried in the ground or masonry.

Running through conduit or thermal insulation will decrease this CCC so a larger cable would be required.

 

[JohnW2]

Sorry EFL, or any other electrician. I am a born worrier. Could you explain ( in simple terms) why 4mm2 cable will be ok ....

If I'm not mistaken, buried 4mm² SWA is rated at 37A.

... as I have looked on various sites ( including this one) and all seem to state 6mm2 minimum for a 32amp hot tub.

You would have to ask them

Kind Regards, John
Dammit - too slow again!

 

[333rocky333]

All my career 4mm has never really been a popular size, im not even sure wholesalers stocked it years ago, was it even in the early regs tables.
Most older electricians I have met tended to have a rule of thumb quide
1.5 mm lighting
2.5 mm power
6 mm cookers and showers
Back in the day, in most cases this would have beeen over the top and save actually doing any calculations, which unfortunately was often the case.
Even crimps and lugs, never really covered 1mm or 4mm cables exclusively
Obviously cookers and shower loads have increased over the years but were only about 6kw initially

 

[DaveHerns]

That's good news . I've got 4mm SWA buried running to my garage and fused at 30A. Did it myself in 1992 following my DIY book before I knew any better!

 

[JohnW2]

That's good news . I've got 4mm SWA buried running to my garage and fused at 30A. Did it myself in 1992 following my DIY book before I knew any better!

I would suggest that you can sleep very soundly, without concerns!

Kind Regards, John

 

[Dozy Danum]

Can someone tell me if I've calculated the amps the hot tub will be drawing at 26/27.
Hot tub has 1 x 0.5 hp circulation pump, 2 x 2 hp jet pumps and 1 x 3 kw heater. But I forgot to add two speakers and a few tiny led lights.
The electrician has given me paperwork signing it off so I suppose he wouldn't do something that would be wrong. I even feel guilty coming on here asking these questions to be honest.

 

[Screech20]

Sorry EFL, or any other electrician. I am a born worrier. Could you explain ( in simple terms) why 4mm2 cable will be ok, as I have looked on various sites ( including this one) and all seem to state 6mm2 minimum for a 32amp hot tub. Thanks.

Simple terms......
Because you employed an electrician who has installed it as per the regs and given you certication for it....

 

[EFLImpudence]

Hot tub has 1 x 0.5 hp circulation pump,

1.6A but more when starting.

2 x 2 hp jet pumps

Don't know what they would be without looking up.

and 1 x 3 kw heater.

12.5A

But I forgot to add two speakers and a few tiny led lights.

Negligible.

The electrician has given me paperwork signing it off so I suppose he wouldn't do something that would be wrong. I even feel guilty coming on here asking these questions to be honest.

What rating MCB or RCBO did he fit for the circuit?

Looks like you overestimated - no matter - forgotten anything?

 

[SimonH2]

Most older electricians I have met tended to have a rule of thumb quide
1.5 mm lighting
2.5 mm power
6 mm cookers and showers
Back in the day, in most cases this would have beeen over the top and save actually doing any calculations ...

From memory, back when the fuse board actually had fuses (mostly re-wireable), a 30A fused circuit would need 6mm² cable. And the RFC was specifically invented to allow use of 2.5mm² (or rather 7/029? in pre-metric days). 1.5mm² does sound excessive for lighting though.
As for avoiding calculations, the impression I've had is that very few installations actually involve calculations - most are done by rule of thumb aren't they ?

 

[EFLImpudence]

All my career 4mm has never really been a popular size, im not even sure wholesalers stocked it years ago, was it even in the early regs tables.

I don't think you are very old but it wasn't much needed with 3036 fuses. 30A required a CCC of 41A.

Most older electricians I have met tended to have a rule of thumb quide
1.5 mm lighting
2.5 mm power
6 mm cookers and showers

Is rule of thumb "use next size up anyway?"

Back in the day, in most cases this would have beeen over the top and save actually doing any calculations, which unfortunately was often the case.

It's getting worse.

 

[Dozy Danum]

EFL, the RCBO fitted is B32 MA30.