Under Rated MCB?

[Spark123]

Yep, not that I can think of installing an 8kw shower recently. 8.5kw tends to be a common figure.

 

[ban-all-sheds]

Becuse a circuit was designed with a small overload and doesnt instantly meet every single requirement of the regs? There is more to designing circuits then sitting in an Ivory Tower with a regs booked ramed up your a**

I'll repeat the questions:

Do you really think that a 40A breaker for a 10.5kW load counts as good workmanship and proper materials, and do you really think that it's likely to comply with what the shower manufacturer specifies?

 

[Spark123]

Even with my grade C in A Level physics I can work out that a 10.5 KW shower pulls a tad more than 45 Amps.

The power rating for a shower is usually stated at 240v which equates to a tad under 44A for a 10.5kw, at 230v the power output is reduced to somewhere around 9.6kw.

 

[spongobongo]

The power rating for a shower is usually stated at 240v which equates to a tad under 44A for a 10.5kw, at 230v the power output is reduced to somewhere around 9.6kw.

Fair point. Just checked the manufacturer's spec and it's 43.8A at 240V. I worked it out using 230V as I know that the voltage here varies from 232 to 248V (measured by the Aqualisa installer).

Also, my memory fails me. I got a Grade D in A-Level Physics, not a C. One good reason why I never became an electrician.

 

[ban-all-sheds]

Fair point. Just checked the manufacturer's spec and it's 43.8A at 240V. I worked it out using 230V

You should have got an answer of 42A, not a tad more than 45....

Also, my memory fails me. I got a Grade D in A-Level Physics, not a C. One good reason why I never became an electrician.

Ohm's Law was O-Level physics, IIRC...

 

[spongobongo]

You should have got an answer of 42A, not a tad more than 45....

Okay, okay, I know I'm going to get hoots of laughter because of my naivety but where have I gone wrong?

If P=IV, then I=P/V

10500/230 = 45.65 Amps.

BAS, you did O-Levels. I had the misfortune to do GCSEs in the early 1990s, which aren't worth the paper they're written on. Hence, why I seemingly can't get the correct answer to a simple mathematical equation.

 

[flameport]

The power isn't constant. If the voltage is lower, so is the power, so it isn't 10500 any more.

 

[ebee]

If you know that the manufacture has stated a certain KW at a certain voltage then divide the KW by the V to get Amps.

From that you can work out the equivalent impedance (resistance) of the heater and substitute it in to your actual voltage to get the actual KW at that voltage.

This works pretty well because with a shower heater you are working on pretty much a purely resistive heating element.

Or to transpose

Square (the known voltage/quoted voltage) and multiply by the KW at the quoted voltage to arrive at the actual KW .

Example
a 10.5KW shower at 240V would give

(230/240) x (230/240) x 10.50 = 9.643KW if run at 230V.

Conversely
a 10.5 shower at 230V would give
(240/230) x (240/230) x 10.5 = 11.433KW if run at 240V

Now back to the OP.
Yes it is wrong to put an underated MCB on a circuit.
THE MCB must be equal or greater than your design current.
In practice you will often get away with this in a shower circuit but it is still actually wrong (you should not be running a breaker on its tripping curve).

The point BAN made about good workmanship & proper materials is probably the most overlooked but most useful reg there is.

 

[BSBS]

(you should not be running a breaker on its tripping curve).

You are right you should not run a breaker on its tripping curve. But since MCB tripping curves are 1.45* the current rating in this case it doesnt come any where near.

 

[ban-all-sheds]

Ib ≤ In ≤ Iz