8.5kw shower on a 32amp mcb

DirtyDog368 · Dec 18, 2018

Hello All
Can any one provide non sarcastic suggestions as to why an 8.5kw shower has not tripped the 32amp mcb protecting that circuit?

Found this install in a domestic property. It seems to have been there for years, without out issue.

Thanks
DD

Doomed · Dec 18, 2018

Look at the time to trip on overload characteristics of a B?32 MCB.
They don't have teenage daughters then.

suffolkspark · Dec 18, 2018

Its only a few amps over so will last a while, youve surely come across 9.5kw showers exactly the same?

Zdb · Dec 18, 2018

Also was it on the highest possible heat setting? I doubt it

HappyHippyDad · Dec 18, 2018

Im just dealing with something similar.
8.5kW on a 30A breaker. This one is actually tripping it, but only if the temperature is high and they spend too long in there.

As in the first post, have a look at appendix 3, page 370 of the new edition of BS7671 and you will see that it is highly unlikely a 32A breaker will trip with a 8.5kW shower (approx 37A-no calculator to hand).

hasel5 · Dec 18, 2018

Mines 8.5 KW shower is on a 30amp Cartridge fuse and been running spot on for 3 years since i moved in do have a spare Fuse sat in cupboard

Yozza19 · Dec 18, 2018

Slight variation in input voltages also play apart
Put your clamp metre on it usually about 28-31 a for a 8.5 on the 7-8 heat setting

Deleted account · Dec 18, 2018

If it's a B type it would need to get to about 46amps before it started to think about tripping.
8.5 kW @230v is about 37A.
See page 370. BBB

Remember a B type breaker only needs to disconnect within .4 seconds at 5 times it's rated current

Taylortwocities · Dec 18, 2018

As Yozza indicates. Check your supply voltage. Current varies with voltage for a fixed resistive load (even my mum knows that......)

PEG · Dec 18, 2018

Not all showers function with a fixed resistive load...or so me mum reckons

buzzlightyear · Dec 18, 2018

hasel5 said:
Mines 8.5 KW shower is on a 30amp Cartridge fuse and been running spot on for 3 years since i moved in do have a spare Fuse sat in cupboard

that needs up grading to 40amp any way .

buzzlightyear · Dec 18, 2018

some manufactures are now making 7.5 kw showers .

Rpa07 · Dec 18, 2018

buzzlightyear said:
that needs up grading to 40amp any way .

Might be on 4mm T+E Buzz - never assume and you know why!

DirtyDog368 · Dec 21, 2018

Gentlemen and electricians
Thank you all for you valuable input.

Merry Chrimbo and be careful with the Christmas tree lights.

wackyracer · Dec 22, 2018

The 8.5kw is usually at 240v, at 230v the same shower is normally rated at around 7.8kw, so would only be over the MCB rating by a couple of amps.

oracle · Dec 22, 2018

Doomed said:
Look at the time to trip on overload characteristics of a B?32 MCB.
They don't have teenage daughters then.

My teenage daughter spent so much time in the bathroom I installed PIR switching. Occasional screaming, but fuel costs went down a bit.

coolsparx · Dec 23, 2018

A few years ago I upgraded a shower cable for a customer, it was an 8.5kW shower on a B32 breaker. It wasn’t tripping & had been in for over 4 years, however there was some discolouring on the mini trunking above the shower. The 2.5mm cable was working beyond its limits!

Darkwood · Dec 23, 2018

Like has been expressed, the trip curve characteristics explains why the 32amp doesn't trip but relative to design of the circuit we have a regulation that states small overloads for long periods should not be possible so anyone designing/altering this circuit should never have done that in the first place.