Correct MCB rating?

[flinty]

I came across an interesting one the other day.

It was a faulty Triton shower rated at 9.5KW.

However the MCB for it in the consumer unit was rated at 32amps. (Definitely the shower one as when switched off the power to the shower was off.)

Now if you take 9500watts and divide by 240 volts you would get 39.58 amps (of course the rating should be even higher if you work on 230Volts)

So how was the shower working on a 32 amp MCB?

 

[redpis]

Check out the graphs for time/current relating to the specific type of mcb installed, you will see the curve for the thermal side of the mcb and the straight line cut-off for the electro-mechanical overload trip.

As the graphs are non linear, a type B 32A would carry 40A for over 16 mins (approximatly).

Hope this helps

 

[flinty]

Interesting..

The shower had been installed by a qualified sparks so I would have thought that the correct MCB would have been a 45amp for a 9.5KW rating as it would draw 42 amps.

 

[Spark123]

If the shower takes roughly 40A @ 240v it will take a little bit less at 230v - think of the element as a fixed resistance (not wanting to go into the temperature coefficient of the element material).

 

[Softus]

The rating of 9.5kW is the maximum consumption. If you run shower on the 'medium' setting then only one heating element is used and the power consumption is roughly halved.

And before you even think about uprating the MCB, check that the cable can handle the design current.

 

[GaryMo]

The rating of 9.5kW is the maximum consumption. If you run shower on the 'medium' setting then only one heating element is used and the power consumption is roughly halved.

So what are you suggesting?

The cable and its protective device should be capable of supplying full load.

 

[Spark123]

Think it may just have been a suggestion as to why it didn't trip, but you are correct in the supply should be adequate for the load.

 

[GaryMo]

A misunderstanding on my part then. Should have known really as Softus' advice is usually spot-on.

 

[Softus]

The rating of 9.5kW is the maximum consumption. If you run shower on the 'medium' setting then only one heating element is used and the power consumption is roughly halved.

So what are you suggesting?

I don't believe that I was suggesting anything.

The cable and its protective device should be capable of supplying full load.

We know that, but we also know that it wasn't capable.

I must be missing your point somewhere; you'll have to be less subtle.

 

[GaryMo]

As the graphs are non linear, a type B 32A would carry 40A for over 16 mins (approximatly).

Well over 16mins, it's off the scale which finishes at 166 minutes!

 

[GaryMo]

The rating of 9.5kW is the maximum consumption. If you run shower on the 'medium' setting then only one heating element is used and the power consumption is roughly halved.

So what are you suggesting?

I don't believe that I was suggesting anything.

The cable and its protective device should be capable of supplying full load.

We know that, but we also know that it wasn't capable.

I must be missing your point somewhere; you'll have to be less subtle.

Maybe but I did try to apologise in the post above

If I took your post the wrong way then I'm sure it's possible for others to do the same.
I didn't want anybody to think it's OK to fit a larger shower and run it on medium setting.
At least we have clarification which is the main thing.

 

[Softus]

Sorry GaryMo - I see what happened now. - I hadn't noticed Spark123's explanation, so yes that's what I was driving at.

Our posts are crossing, so I'll shut up now.

 

[GaryMo]

Sorry GaryMo - I see what happened now. - I hadn't noticed Spark123's explanation, so yes that's what I was driving at.

Our posts are crossing, so I'll shut up now.

No worries. We're all here to help

 

[flinty]

Actually I down rated the replacement shower as the original 9.5KW was on 6mm cable so it was right on the limit of the cables capacity.

I put an 8.5kw in to be on the safe side for the cable, but was trying to work out the original sparks logic of putting 9.5kw on a 32 amp MCB.

 

[dingbat]

Actually I down rated the replacement shower as the original 9.5KW was on 6mm cable so it was right on the limit of the cables capacity.

I put an 8.5kw in to be on the safe side for the cable, but was trying to work out the original sparks logic of putting 9.5kw on a 32 amp MCB.

I reckon he did the reverse of what you just did... i.e. he put a bigger shower in as a replacement.