Hot tub trips MCB

[Trev2020]

Hi guys, I’m after some advice. I have just run a supply for a hot tub for a family friend as follows:

Everything tested ok and the hot tub powered up fine when I was there although I didn’t run it for any period as there was no water in there. The following day the owner was running it and it was fine for 30 mins then the 20A MCB tripped.
They rang me and I told them to switch everything off and turn on the MCBs/RCD as follows
1. 20A MCB
2. RCD
3. 16A MCB
Now every time they switch on the 16A MCB the 20A MCB trips. If there was a fault with the hot tub surely the 16A MCB or RCD would trip first and not the 20A MCB? I’ve used type B MCBs, would the pump starting current cause these to trip?
I’m newly qualified and would just like to get some advice as to why this is happening before I go and have a look. Any advice would be greatly appreciated.
Trev

 

[mechelec]

What KW rating is the hot tub? if this is within the MCB rating you need to check the current being drawn using a clamp meter. This will tell you if it is a problem with the hot tub or the MCB

 

[Paul.M]

What else have you got on the 20a mcb side of the board? Besides I've got a spa bath running from 6a mcb and 30ma rcd, what specs is this hot tub. Another thing tell your friend that 30 mins in a spa bath is not healthy. First time I was in for about 30 mins and came out looking like a lobster and sweating like a pig for a couple of hours afterwards (something to do with blood circulation). Be in 15 mins tops.

 

[davelerave]

no other load on the 2.5 sub-main?

are these a large pump using stored hot water?

you would expect the b16 to trip if it was startup

 

[Trev2020]

There was no rating on the hot tub instructions, though the installation instructions option were either;

1 x 32A feed into line 1 with internal link to line 2,
2 x 16A feed into each line,
1 x 16A RCD protected feed into line 1 with internal link to line 2 (with this option an internal jumper on the control box pcb needed to be moved)

Theres a heater rated at 2kw and a pump and LED light.

I used the last option as I believe RCD protection is required, and as the hot tub is Canadian then I thought the other two options may be for countries where perhaps RCD protection was not required?

At the house side the main CU side was full so I split the tails after the meter and installed a separate 2 way metal clad CU so the hot tub circuit is completely independent of anything else.

Thanks for the replies.

 

[bugsy]

Are you sure it's only rated at 2kw, ?
The heater would be rated at @ 8.9a and the pump @ max 5a , why the 32a supply?

 

[Guest123]

Overload or short circuit will cause an MCB to operate as we know. For the 20A breaker to operate before the 16A would suggest a L-N short circuit perhaps on the sub-main to the shed, but as it only trips when the 16A MCB is energised suggests it isn't on the sub-main but further down. Could the 16A MCB be faulty perhaps???

I'd rule out earth fault as the RCD is not tripping so as the lads have said loading may be an issue or short circuit.

 

[davelerave]

not familiar with hot tubs

trying to figure the 16/32a arrangement

it would make sense in canada if elements were 120v and supply was 120/240
prob missing the obvious

 

[ian.settle1]

Hi guys, I’m after some advice. I have just run a supply for a hot tub for a family friend as follows:
View attachment 2738
Everything tested ok and the hot tub powered up fine when I was there although I didn’t run it for any period as there was no water in there. The following day the owner was running it and it was fine for 30 mins then the 20A MCB tripped.
They rang me and I told them to switch everything off and turn on the MCBs/RCD as follows
1. 20A MCB
2. RCD
3. 16A MCB
Now every time they switch on the 16A MCB the 20A MCB trips. If there was a fault with the hot tub surely the 16A MCB or RCD would trip first and not the 20A MCB? I’ve used type B MCBs, would the pump starting current cause these to trip?
I’m newly qualified and would just like to get some advice as to why this is happening before I go and have a look. Any advice would be greatly appreciated.
Trev

As they are both B types there is not much discrimination between the 16A and 20A MCBs don't forget they can trip anywhere between 3-5 times their current rating thus 16 could be between 54-80 Amps and the 20 is 60-100 Amps.

 

[davelerave]

i don't series the breakers like that-can't see the point

suppose if they were the same brand -you could expect them to discriminate for SC?

 

[ukpablo]

i install alot of hot tubs for a company.
always asked to run in a 32A type c mcb, meaning the SWA is usually 6mm due to the lower ZS on the type c breaker.
Also, the manufacturers instructions can be misleading, i.e it may say it is for example 3KW, but looking at small print, does not include lighting, built in radio etc?
I also run it into a 32A rotary isolator before going to the hot tub.
They always send in their own engineer to comission hot tub so im not familiar with the connections after the rotary isolator?
give them a call and ask for some guidance?
hope this helps.
ideally you should run from main CU on its own cct.
The reason that they req type c mcb is that they have had alot of trouble in the past of tripping probs...............sometimes weeks after! This is done as a standard for them now regardless.

 

[Marvo]

The fault you describe doesn't make sense as an overload caused by the hot tub. It points to either;

1. A cable fault (L-N low insulation) between the house and the shed.
2. A defective 20A breaker at the house.

I would recheck for obvious problems, insulation and loop impedance test the cable then if none are found replace the 20Amp MCB as a first step to localising the problem by deduction.

At the risk of asking a stupid question, why did you install a 16Amp breaker at the shed to supply a cable that's rated at 20Amps supplying the hot tub?

 

[nickblake]

Right ho , the hot tubs i have installed all require a 32 / 40amp supply ,the heater uses 16 amps of this , then there is 4 to 10 amps for the pumps ,but you realy need a C type MCB as the surge when the pumps switch on can trip the MCB so the problem could just be that or as its protected by a 20 amp mcb could be over load , biggest problem you have is the cable you have installed will be to small for the supply it should be a minimum of 6mm depending on installation methods

 

[ste mac]

The hot tub I have installed at home is running through a separte CU - 40A 30mA rcd and 32A 'B' curve MCB & 15m 4mm2 3C SWA. My hot tub is also canadian (bought from JTF Wholesale) and the power reqirements in the manual are rather confusing but state (for smaller models) 230v 16A 3 wires & (for the larger models) 230v 32A 3 wires. Also in bold text "hot tub must be connected to a dedicated circuit breaker that is not shared with any other equipment" & "A new GFCI must be used for your hot tub installation, do not use an existing GFCI as its condition is unknown."

I would agrre with Nick, but for the fact that a "b" curve breaker works fine for me and therefore only 4mm SWA needed.


Nb: GFCI = RCD

 

[Wiredspark]

Out of precaution I would tend to carry out insulation resistance testing. And as others have said before me I would consider a type C MCB (as long as you can up your supply to a 6mm csa) I cant see the pump in the hot tub being massive so the initial load although will be higher than normal operationg load shouldn't be that high.