Shower and load on board

It's-me · Jun 11, 2023

Went back it a house after they upgraded shower to a 9.8kw, pulling 40a constant.

We had wired it with 10sq and 40a RCBO.

10sq Neutral cable looping from neutral bar to RCDs and on to shower RCBO melted at neutral bar.

That's fair enough, must have been pulling a fair whack through it.

So I put in a new 10sq neutral going straight from neutral bar to shower RCBO.

They say the smell is back again.

I think the installation is pulling way over 63amps, when shower, cooker, dryer etc is on.

It's fed through a 100a double pole isolator, should I swap this out for a 63 amp MCB to cover my arse

littlespark · Jun 11, 2023

It's-me said:
Went back it a house after they upgraded shower to a 9.8kw, pulling 40a constant.

We had wired it with 10sq and 40a RCBO.

10sq Neutral cable looping from neutral bar to RCDs and on to shower RCBO melted at neutral bar.

That's fair enough, must have been pulling a fair whack through it.

So I put in a new 10sq neutral going straight from neutral bar to shower RCBO.

They say the smell is back again.

I think the installation is pulling way over 63amps, when shower, cooker, dryer etc is on.

It's fed through a 100a double pole isolator, should I swap this out for a 63 amp MCB to cover my arse

63A MCB will not trip off at exactly 63A... can take a bit more, for some considerable time. Same with the 40A RCBO.

Do you have the measured voltage? As 9.8kW / 230 is 42.6A... over your 40A

Depending on installation method, its on the limit of 10mm cable now.

If the link cable at the neutral bar was melting, then more likely a loose connection than undersized cable.
Did you use a different terminal hole? The one that had the melted cable originally in it may have carbon deposits, which is now causing the smell, and still possibly heating up.

DPG · Jun 11, 2023

As above, if it's overheating at the joint the it's most likely the joint. With the short length of time a shower is on for I can't see the 10mm cable itself overheating.

timhoward · Jun 11, 2023

I've seen a 10kw shower survive many years on 2.5 sq mm cable which is obviously not compliant but does illustrate that this issue is unlikely to be the cable size (and the 10 sq mm might be compliant, as per above).

Did you change the B40 RCBO after this happened or splice onto the Neutral fly lead of the old one?
Also as above, I'd take a good look at all connections.

It's-me said:
It's fed through a 100a double pole isolator, should I swap this out for a 63 amp MCB to cover my arse

A B63 MCB won't trip at all until about 90 amps is flowing and then it would take over 2 hours before it trips. I really can't see that idea helping this situation at all.

It's-me · Jun 12, 2023

I swapped neutral bar out completely, it was well shagged.

I also brought a 10sq neutral straight from shower RCBO to neutral bar.

I'll attach some pictures.

Second one is the load on the shower neutral after I replaced it and ran it direct to the shower RCBO

Obviously couldn't tell if cable was loose at that stage but tightened up everything so should be ok now.