MCB Selection

[johndevin]

Having a discussion at work ..

We are installing a 9.8 KW Shower.

If my calculations are correct this will draw just above 40A

The distribution board we are using does not fit a 45A breaker.

I think it would be safer to use a 40A than jump to a 50A.

I highly doubt the current would ever reach anywhere near 40A.

Whats your thoughts ?

P.S I know i could change the board but trying to avoid this ....

 

[Darkwood]

Hi John Devin - Welcome to the Forum

May I ask why you think a fixed load like a shower will not pull the calculated value you arrived at?
If your load calculates above the rating of the mcb then no its not safer to create a small overload for a long duration as the reg's stipulate must be avoided. So 50amp it will be.

Are you worrying about the size of the cable here then and having to upgrade it?

PS Don't you mean 'Doesn't fit a 45amp mcb'

 

[johndevin]

i meant does not fit a 45 A breaker **

Because it will never be on maximum temp or else they might scold themselves.

I was thinking try a 40A first if it blows use an alternative method.

 

[telectrix]

as darkwood. bung a 50A in.

 

[johndevin]

as darkwood. bung a 50A in.

Surely a 40A will be safer to try first than a 50A ? If the current is only ever say for example ever going to reach 38.5 A

 

[Darkwood]

Surely a 40A will be safer to try first than a 50A ? If the current is only ever say for example ever going to reach 38.5 A

You gave us the 9.8Kw shower this does not equate to 38.5amps its as you stated a little bit above 40amps thus you have to fit the next size mcb.

Where do you get 38.5 from?

 

[telectrix]

no, because it's a fixed load and yo don't need to provide overload protection, just short-circuit fault protection. 432.3 and 433.3.1 (ii). (green BS7671)

 

[davesparks]

Surely a 40A will be safer to try first than a 50A ? If the current is only ever say for example ever going to reach 38.5 A

No a 40A will not be safer than a 50A if the load is over 40A, the 40A mcb will overheat and become a fire hazard.

 

[davesparks]

i
Because it will never be on maximum temp or else they might scold themselves.

I .

It doesn't matter what temperature they set it to, it will use 9.8kW at the rated voltage.
If it has a half power setting it will draw 4.9kW at it's rated voltage

 

[DuaneMHunt1976]

Are we trying to set a max temperature to prevent scolding (ie for old or handy capped persons)

Also what size is the cable your are using or is installed

 

[Darkwood]

i meant does not fit a 45 A breaker **

Because it will never be on maximum temp or else they might scold themselves.

I was thinking try a 40A first if it blows use an alternative method.

You should be using and following regulations to design and wire your circuits its this that ensures the safety of the users and of the installation, I would read and listen to the advice given, TBH it's truely is disturbing reading this comment from a practising Electrician, you have to design the circuit for the manufactered stated ratings, I would also do some research on how showers actually work, most reduce flow rates to increase temperature and don't regulate the voltage to the elements so at full power it will draw the 9.8Kw and you increase the flow rate to reduce the water temp', those with 2 power setting may have 2 elements and one ised in low power.

I would also revise you time/current tripping curves and you will find that the small overload in this scenario would never trip the mcb putting you in contravention of the regulations regarding ensuring by design you do not create a small overload of the OCPD for any length of time.

 

[Deleted member 9648]

What CSA cable is supplying this shower and what is the installation method?

 

[davesparks]

What CSA cable is supplying this shower and what is the installation method?

He can't have calculated that yet as he still hasn't selected In, unless of course he's used Ib

 

[uksparks]

Just fit the appropriate sized MCB to protect the cable and make sure its not maxed out such as the 40A as like others say its a potential fire hazard.

 

[uksparks]

no, because it's a fixed load and yo don't need to provide overload protection, just short-circuit fault protection. 432.3 and 433.3.1 (ii). (green BS7671)

I know you are correct and wouldn't question it, I have just read those regs you have stated and can you just clarify, if its overload protection are we saying hypothetically protection of the cable / circuit from being overloaded so putting an induction hob on a 1mm light circuit for example.

And short circuit fault protection, as I read it, I may be wrong but would like you to explain, are you saying you could put a 63A or even greater so long as it falls under that reg up to PFC etc, but put an MCB on a 6mm circuit with a shower on the end that only draws 40A as its a fixed load falling with that reg as it will not exceed the specified load, so you do not need to provide protection for the cable but protection for fault only as in L-N short for example.

Is that correct?