Electric shower sizing

neilmcl

I need to replace my electric shower which is currently on a dedicated 40A circuit. What's the max size shower can I safely install?

grumbler

By 'size' do you mean electric power?

40A*240V=9.6kW (at 240V). So, 9kW, possibly 9.5kW.

ETA: if 9.5kW is measured at 230V (see specifications), then the current will be 41A at 230V and even higher at 240-250V. In this it's 9kW max. for you.

tacpot12

You should be ok to install a 9.5KW shower on a 40A circuit. (The breaker shouldn't trip until the current is 13% above its rated value). 

Try to find one that is made in the UK and rated for 240v. It is likely that the installation instructions will be online for any shower you are considering, so yo can also check to see if the manufacturer confirms that nothing higher than a 40A breaker is required.  

neilmcl

I currently have a 9.5kw Triton which gave up the ghost this morning  I was surprised it was 9.5kw as most places I've seen only recommend 8.5kw for a 40A fused circuit but it's worked fine for me up until now.

Update: I've checked the installation guide for a replacement Triton T80Z and they do state that a 40A MCB will be ok for a 9.5kw, so I'm good to go.

nofoollikeold

Its not quite so simple as kW / voltage = amps.  This does give the correct answer provided the wire are short and not confined, but this is rarely realistic in a property,  The cable may be long, because of the distance involved or because of routeing. It may be run through conduit which will reduce the rate at which it can lose heat.  
The real answer needs to be calculated by an experienced electrician who can see the route and the size of cable in place, which also has a significant bearing.  

Personally I'd see 9.5 kW as just a bit too much for a 40A MCB and stick to 8.5.

grumbler

nofoollikeold said:

Its not quite so simple as kW / voltage = amps.  This does give the correct answer provided the wire are short and not confined, but this is rarely realistic in a property,  The cable may be long, because of the distance involved or because of routeing.

If it's long, the current can be only smaller because of the cable ohmic resistance. And if you mean the heat dissipation, it's per a meter of cable that matters, not total, i.e. it's not affected by the cable length,.

coffeehound

@neilmcl worth a quick look through this thread where we drilled down to the nitty-gritty.  The table shows the scenarios where the cable needs to be downrated

https://forums.moneysavingexpert.com/discussion/6270614

(Crikey it's even harder to find anything on this site now)