9.5 kw shower on 6mm cable - electrician says it's ok but tripping

[MattB83]

Hi,

We had a shower blowing the fuse regularly in our old fuse box with the cartridge fuses. We ended up, on the advice of a highly recommended electrician, changing to a new distribution board. I asked about the shower on the old cable and he said although he would use a larger cable if he was installing from scratch this one would be fine - he checked the shower pull cord which he said was installed badly so he sorted this out - this still stuck a lot so we leave it on - i.e. the red light is on - all the time.

The other relevant detail is the shower occasionally doesn't switch off at all - i.e. we switch it off and it is stuck on - not just takes a few seconds but won't switch off at all and we have to toggle, medium, full, medium, full, off - until it decides to go off.

However it's still tripping the power occasionally - typically when my wife has has a long and (very) hot shower - has never tripped during my quick warm showers. So my thoughts are that this cable isn't adequate and we either need to:

- have a 10mm cable installed
- switch to a lower powered shower
- get rid of electric shower and run off combi-boiler

Have I understood the situation correctly? seems like 2 is probably the quickest and cheapest option?

In case it's relevant:

We had another circuit and the RCD tripping a lot last week - we narrowed this down to a faulty washing machine and have replaced it and (touch wood) no tripping of this circuit at the moment. Fairly sure this is unrelated as the shower is on its own circuit.

 

[securespark]

Is the shower tripping the RCD or its MCB?

What size is the MCB?

 

[JohnW2]

However it's still tripping the power occasionally - typically when my wife has has a long and (very) hot shower - has never tripped during my quick warm showers. So my thoughts are that this cable isn't adequate and we either need to:...

Even if the cable were 'inadequate', that would not result in anything tripping.

I presume that it's an MCB which is tripping (since you mention an RCD elsewhere, in a different context)? What is its rating (i.e. what is written on it)?

Kind Regards, John

 

[MattB83]

My understanding is the MCB is the individual one with 'Shower' written under it - as long as that's correct it's the MCB that's tripping.

40A I think - pic to clarify:

https://photos.app.goo.gl/rgiEn1t6MjBPNden7

 

[JohnW2]

My understanding is the MCB is the individual one with 'Shower' written under it - as long as that's correct it's the MCB that's tripping. 40A I think - pic to clarify:

Thanks. Yep, 40A.

The "9.5 kW" quoted for your shower probably relates to 240V, in which case it would draw about 39.6A at 240V - which certainly should not trip a 40A MCB. In theory it should take at least 45.2A (40 plus 13%) to trip a 40A MCB and such a current ought not to be possible unless your supply voltage were at least 273V, which is neither credible nor allowed.

Although it should not trip a 40A MCB, you'd be more certain if you had the 40A MCB upgraded to a 45A one (if one is available for your CU) - and, unless the cable goes through insulation or conduit (your electrician could advise), that should be possible with the existing 6mm² cable.

It could, of course, be that your 40A MCB is a bit too 'trigger happy', and hence is tripping at a lower current then it should, so one might try replacing it with another 40A one. However, if it's to be replaced then it would be silly not to upgrade to 45A, if one is available for the CU and the cable can support it (see above).

Kind Regards, John

 

[MattB83]

Thanks. Yep, 40A.

The "9.5 kW" quoted for your shower probably relates to 240V, in which case it would draw about 39.6A at 240V - which certainly should not trip a 40A MCB. In theory it should take at least 45.2A (40 plus 13%) to trip a 40A MCB and such a current ought not to be possible unless your supply voltage were at least 273V, which is neither credible nor allowed.

Although it should not trip a 40A MCB, you'd be more certain if you had the 40A MCB upgraded to a 45A one (if one is available for your CU) - and, unless the cable goes through insulation or conduit (your electrician could advise), that should be possible with the existing 6mm² cable.

It could, of course, be that your 40A MCB is a bit too 'trigger happy', and hence is tripping at a lower current then it should, so one might try replacing it with another 40A one. However, if it's to be replaced then it would be silly not to upgrade to 45A, if one is available for the CU and the cable can support it (see above).

Kind Regards, John

Thanks very much for your detailed answer.

Does that still leave the shower itself potentially causing the issue?

Or could the pull cord be causing problems? In the past it's got stuck when pulling it and this has caused the fuse (old DB) to blow even without switching the shower on.

I'm just trying to work out who I need to call - plumber or electrician (or, if it's like anything else in this house, an exorcist)!

 

[JohnW2]

Thanks very much for your detailed answer. Does that still leave the shower itself potentially causing the issue?

Not really - it would require an incredibly improbable fault in a shower to result in it drawing an excessive current, particularly given that the shower is still 'working'.

Or could the pull cord be causing problems?

No. The switch merely switches the supply on/off. There is no way that it could increase the current.

I'm just trying to work out who I need to call - plumber or electrician (or, if it's like anything else in this house, an exorcist)!

Definitely an electrician.

Kind Regards, John

 

[EFLImpudence]

As the RCD doesn't trip, it can only be 'overload' which is not likely with a shower, or 'short circuit' - Live wire coming into contact with Neutral wire.

 

[terryplumb]

When did the electrician work on the pull cord switch ? As it does not work properly get it replaced. As the shower does not switch off at its on/ off switch ,then there is also an issue with the shower internally.

 

[JohnW2]

When did the electrician work on the pull cord switch ? As it does not work properly get it replaced. As the shower does not switch off at its on/ off switch ,then there is also an issue with the shower internally.

All true, but none of that could explain the MCB tripping.

Kind Regards, John

 

[terryplumb]

All true, but none of that could explain the MCB tripping.

Kind Regards, John

Short within the switch ? Tracking within the switch ?

 

[endecotp]

Are you certain it”s a 9.5 kW shower, not more?

 

[JohnW2]

Short within the switch ? Tracking within the switch ?

That would produce a bang and an immediate MCB trip. I can't see how it could cause ..

... However it's still tripping the power occasionally - typically when my wife has has a long and (very) hot shower - has never tripped during my quick warm showers.

Kind Regards, John

 

[MattB83]

When did the electrician work on the pull cord switch ? As it does not work properly get it replaced. As the shower does not switch off at its on/ off switch ,then there is also an issue with the shower internally.

The first electrician replaced the switch - badly the second electrician told me but I don't know what that means. The second electrician just reinstalled the same one - so only a week old.

So you reckon new shower + new switch?

One of the electricians commented that the old switch (which had stopped pulling) contained a lot of melted wiring which had fused together so maybe something has been going on with this for a long time.

 

[terryplumb]

The switch was refitted one week ago ,and it doesn't work properly / switch on and off ? Then replace it ,as said already. The shower on/ off may be a simple repair ,if it is not then fit a new one.
Seems odd that you have had two switches fail ,but in what time period ?