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Shower CB Ratings ...

John R.

I had intended to install an 8.5kW electric shower.  So RCBO for the shower is 40A, B curve and the cable is 10mm.sq.  

The chap installing the central heating reckons that a 9.5kW shower would be better ... 

At 230V, the 9.5kW shower would be c.42A, so over the 40A RCBO rating, but the I/In for that would only be 1.03.

From the RCBO curve (which is not super-detailed) it looks as if only a really long shower would trip the RCBO thermally! 
The shower instructions specify 40A or 45A MCB, so don’t really help!

Presuming that the cable rating is okay as installed (mostly clipped), I reckon that the 9.5kW shower would be fine - am I missing anything?

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  • The shower instructions specify 40A or 45A MCB,

    Which suggests that the shower power has been calculated for 240V ... 9500/240=39.58A. It's a common marketing trick .. bigger looking numbers are easier to sell so when the nominal voltage changed from 240 to 230V many a manufacturer stuck with the 240V figures (with the not completely illogical argument that most consumers' supplies remained at closer to 240V than 230V in practice). Presuming it's a simple resistive load, the current draw will be lower at 230V (and the heat output reduced accordingly) so all should be fine.

       - Andy.

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  • The shower instructions specify 40A or 45A MCB,

    Which suggests that the shower power has been calculated for 240V ... 9500/240=39.58A. It's a common marketing trick .. bigger looking numbers are easier to sell so when the nominal voltage changed from 240 to 230V many a manufacturer stuck with the 240V figures (with the not completely illogical argument that most consumers' supplies remained at closer to 240V than 230V in practice). Presuming it's a simple resistive load, the current draw will be lower at 230V (and the heat output reduced accordingly) so all should be fine.

       - Andy.

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