Why do we fit a 10mm cable for showers?

[HappyHippyDad]

I'm reading a thread at the moment about a 9.8kW shower that has been installed on a 6mm cable. My natural instinct was to think that's wrong, it should be on a 10mm. Most of us would fit a 10mm cable and believe that to be to the right choice but if the reference method is C then the 6mm has a CCC of 47A and is fine isn't it? I suppose you could argue that you should fit a 50A breaker but we all know a 40A breaker will be absolutely fine. You would have to have a constant shower for perhaps a year for a 40A MCB to trip with 42.6A running through it. In fact I doubt it would even trip then.

We don't seem to question a similar scenario, i.e a 4mm radial on a 32A MCB where virtually all reference methods bar C (in a domestic situation) give a CCC <32A.

 

[freddo]

Just a note but the 9.8Kw rating is almost certainly the rating for a supply of 240V - 40.8A (It certainly was for several manufacturers). If the supply voltage is 230V the current drawn would be about 39A

 

[MJPD29]

I'm reading a thread at the moment about a 9.8kW shower that has been installed on a 6mm cable. My natural instinct was to think that's wrong, it should be on a 10mm. Most of us would fit a 10mm cable and believe that to be to the right choice but if the reference method is C then the 6mm has a CCC of 47A and is fine isn't it? I suppose you could argue that you should fit a 50A breaker but we all know a 40A breaker will be absolutely fine. You would have to have a constant shower for perhaps a year for a 40A MCB to trip with 42.6A running through it. In fact I doubt it would even trip then.

We don't seem to question a similar scenario, i.e a 4mm radial on a 32A MCB where virtually all reference methods bar C (in a domestic situation) give a CCC <32A.

A little of topic, but youd be hard pushed for the shower to be pulling full current at any time. Installed a 9.5kw last week, left it on full for 5 mins and clamped it out of interest and it never passed 36.8a when I worked it out around 40

 

[HappyHippyDad]

Just a note but the 9.8Kw rating is almost certainly the rating for a supply of 240V - 40.8A (It certainly was for several manufacturers). If the supply voltage is 230V the current drawn would be about 39A

I new the 40A MCB would be fine ??

 

[Lucien Nunes]

I agree that in practice many resistive heating loads will run noticeably under nominal rating in typical installations. I would be hesitant to rely on that observation to show that Ib<In, unless the manufacturer's data explicitly stated it.

 

[ipf]

Just a note but the 9.8Kw rating is almost certainly the rating for a supply of 240V - 40.8A (It certainly was for several manufacturers). If the supply voltage is 230V the current drawn would be about 39A

But remember, we only work to 230v, most supplies I come across are still nearer 240...…...I'll bet them Brexit burgers aren't too bothered about our small discrepancies, though.

 

[Bellendian]

But remember, we only work to 230v, most supplies I come across are still nearer 240...…...I'll bet them Brexit burgers aren't too bothered about our small discrepancies, though.

Indeed, but 240 is still within the permitted voltage range of a 230V nominal supply, so you'd like to think that equipment designed to operate on a 230V supply, with design calculations using the 230V nominal figures, would be perfectly capable of safe operation at the worst case scenario within the permitted voltage range.
The actual supply voltage should not invalidate calcs carried out based on 230V provided it is within the range permitted by ESQR.

 

[FatAlan]

Perhaps 10mm Is the safer option to cover future installs with bigger loads ... or indicative of a previous heavier rated shower?

 

[ipf]

Indeed, but 240 is still within the permitted voltage range of a 230V nominal supply, so you'd like to think that equipment designed to operate on a 230V supply, with design calculations using the 230V nominal figures, would be perfectly capable of safe operation at the worst case scenario within the permitted voltage range.
The actual supply voltage should not invalidate calcs carried out based on 230V provided it is within the range permitted by ESQR.

Nowt to do with it, mate. Just a point in replying to #16.