MCB Rating for hot tub

[Mitchet]

Hi, Just looking for some help if you don't mind? If I have a hot tub which is rated at 20A, then + 25% would mean a 25A MCB... but since I can't seem to find 25A MCB's anywhere... what do I do? Surely if I go for a 32A, then I'm risking damage to the hot tub should a fault occur?

Interestingly... the hot tub has 16A stamped on the plate where the mains connection is. Though the total power rating is 4.7kW (hence 20A)???

Thanks

 

[FingRinal]

Why + 25%?

 

[Mitchet]

(I believe), to accomodate the initial surge of current when the pumps are first started up?

 

[Taylortwocities]

But the fuse is there to protect the cable.

You first must determine the load Ib (16A you say)
Then select the fuse rating In

Ib is always < In so the protective device will be 20A

Then you need to determine any correction factors and that will give you a figure that you will use to look in the relavent tables in BS7671(eg) to determine what size cable you need (you didnt yet mention cable size).

Then you'll need to look at the length and work out the voltage drop and maybe review the cable size.

Now you have done the design and before starting anything you'll need to notify the local authority as this is notifiable work.

Dont forget you'll need to install an RCD and an isolator switch.

 

[securespark]

It is a fixed load. It will not go overcurrent.

4.7kw+ 25% = 25.54A

So you would use a 32A breaker.

Just make sure that your cable size is correctly calculated with respect to this design current, the circuit length & any other correction factors.

If this is gobbledegook, you need a spark!

 

[securespark]

?????

 

[Mitchet]

It all makes sense... sorry, forgot to mention the cables specs! I'm using 3 core 4mm SWA XLPE with a run of 20m (Voltage drop = 6.5v / 2.6% if my calcs are correct?)

The garage has a 40A 30mA RCD and I've fitted a 32A isolator approx 5m from the hot tub on the outside garage wall.

 

[Adam_151]

What is the submain to the garage fused at, I have a nasty feeling there won't be much spare capacity...

 

[Mitchet]

The garage is off a 30A fuse on the house CU... so as you say.. not much spare capacity!

This is what prompted me to check on using a 32A MCB rather than an unobtainable 25A one. If I use a 32A MCB in the garage for the hot tub, then it'll likely never trip due to the 30A fuse blowing in the house CU first.

This was also the reason for using 4mm cable... if the hot tub needs to be wired to the main CU, the total cable run would be 35m (which is still within limits).

 

[Adam_151]

I'd definatly run it from the main board in the house* rather than stick a heavy load on a submain which wasn't designed with it in mind

4mm² would do, but incase you ever upgrade the heater unit (not sure if thats possible? don't know anything about hot tubs!), you could install 6mm as you only want to dig that trench once!...

Where abouts are you in Lincolnshire? if in Lincoln itself then let me know how you get on with building notice submission (initial investigations suggest that they are willing to accept an EIC from anyone who can convince them they are competant to do the work)

*whats the main board in the house like, if its a wylex with 60A main switch, might need upgrading...

 

[stargazer]

And don’t forget Mitchet, when you have completed your hot tub installation, IEE wiring regulation 26-69-26 requires you to install one of these in it

 

[Mitchet]

As long as she is suitably rated.

I have a suspicion the main CU will be 60A. The fact that it's the old 'fusebox' style CU doesn't give me much hope. I believe the hot tub is one those models that turns off the heater when the pumps are running, so it's unlikely to be upgraded.

I'm in south Lincs (Crowland, nr Peterborough)... interesting to know that about LCC though!

 

[ban-all-sheds]

As long as she is suitably rated.

XXX?