3 X single phase showers

[David Rann]

I've been asked to price 3 X 7kw showers. There is a three phase board about 40 meters away. Can anyone see an issue with 40 amp three phase MCB with 10mm four core swa to a sub board then 3 X 32 amp rcbos for each individual shower.

Cheers

 

[sparks1234]

I'm guessing you can't get RCBO's for whatever board you have already installed, can't see any problem but what are you going to do with the spare ways? , might they get used for something else in the future? and take the total load up past the 40 amp MCB?

 

[chris gray]

Deleted comment as never saw replies

 

[David Rann]

That's on whoever installs after I presume. And I will put dedicated shower board. Someone suggested that I could have current issues down the neutral?

 

[Pete999]

That's on whoever installs after I presume. And I will put dedicated shower board. Someone suggested that I could have current issues down the neutral?

What do your cable calculations show?

 

[David Rann]

Volt drop fine.

 

[Pete999]

Volt drop fine.

Please show calcs for an answer

 

[Wilko]

Hi - if I understand correctly you plan to have one shower per phase to spread the load. This means you will need L and N for each shower (i.e. each 10mm L needs its own 10mm N).

 

[westward10]

You won't achieve correct discrimination between the 40 and 32A devices, with overcurrent yes but not fault current. Not the end of the world but not the best circuit design.

 

[westward10]

Hi - if I understand correctly you plan to have one shower per phase to spread the load. This means you will need L and N for each shower (i.e. each 10mm L needs its own 10mm N).

The triple-pole is a submain to another board for the showers.

 

[Andy78]

Will you be using shower priority contactors ?

 

[Deleted member 26818]

I've been asked to price 3 X 7kw showers. There is a three phase board about 40 meters away. Can anyone see an issue with 40 amp three phase MCB with 10mm four core swa to a sub board then 3 X 32 amp rcbos for each individual shower.

Cheers

Yes, I’m thinking 16mm2.

 

[David Rann]

Sorry I'm confused why would you need shower priority contactors? And are you saying you can't use a sub board?

 

[Wilko]

The triple-pole is a submain to another board for the showers.

Got it - in the case where the 4 conductors are the same ccc but fully loaded and all the loads are single phase only then N will be flogged surely (?).

 

[Andy78]

Sorry I'm confused why would you need shower priority contactors? And are you saying you can't use a sub board?

You might not need to use them. Was just asking as curious. I have no knowledge of the type of installation, type of use, existing capacity etc.

 

[David Rann]

Flogged?

 

[David Rann]

Flogged?

 

[Pete999]

Will you be using shower priority contactors ?

What is a "shower priority contactor"?

 

[Pete999]

Will you be using shower priority contactors ?

What is a "shower priority contactor"?

 

[Andy78]

What is a "shower priority contactor"?

Pre made units that can limit the amount of current drawn by only letting one shower be energised at a time.

For clarity, I'm not saying one needs to be used, but was asking if one was being used as I've never fitted one and would be interested if one was.

 

[David Rann]

I thought that in a three phase supply that if you have 32 amp running on each phase then they would be balanced and you would have nothing down the neutral?

 

[Pete999]

Pre made units that can limit the amount of current drawn by only letting one shower be energised at a time.

For clarity, I'm not saying one needs to be used, but was asking if one was being used as I've never fitted one and would be interested if one was.

As would I, as would I.

 

[Deleted member 26818]

I thought that in a three phase supply that if you have 32 amp running on each phase then they would be balanced and you would have nothing down the neutral?

They don’t balance out over the single phase part.
They only balance out over the 3 phase part, once the 3 neutrals are connected together.
Even then, they only actually balance if all the phases have the same loading at the same time.
As a rule of thumb, double the single phase load to determine the CSA of the neutral for the 3 phase part.
So if each phase will be feeding a 7kw shower, that makes 14kw.

 

[Pete999]

Flogged?

Urban Dictionary: Flogged - https://www.urbandictionary.com/define.php?term=Flogged

 

[westward10]

Phasor diagram anyone.

 

[David Rann]

They don’t balance out over the single phase part.
They only balance out over the 3 phase part, once the 3 neutrals are connected together.
Even then, they only actually balance if all the phases have the same loading at the same time.
As a rule of thumb, double the single phase load to determine the CSA of the neutral for the 3 phase part.
So if each phase will be feeding a 7kw shower, that makes 14kw.

So why on a 100amp supply do we have 25mm tails and neutral?

 

[Deleted member 26818]

Phasor diagram anyone.

Too complicated for me.
Have enough trouble just posting text.

 

[David Rann]

So a 100amp three phase board could have potential 300amp flowing down the neutral so why is it 25mm then?

 

[David Rann]

They don’t balance out over the single phase part.
They only balance out over the 3 phase part, once the 3 neutrals are connected together.
Even then, they only actually balance if all the phases have the same loading at the same time.
As a rule of thumb, double the single phase load to determine the CSA of the neutral for the 3 phase part.
So if each phase will be feeding a 7kw shower, that makes 14kw.

Yes so the neutrals will be connected at the sub board?

 

[Pete999]

Yes so the neutrals will be connected at the sub board?

If all the showers are the same loading, no need for a N Make them 4oo Volt showers job sorted, engage brains. Hypothetical scenario

 

[David Rann]

Haha i like it. So it okay to do or not

 

[Deleted member 26818]

If, you’ve stopped posting for a bit, I’ll try to explain.
25mm2 is over sized for 100A to allow for imbalances.
The outgoing circuits from the 3 phase board may all be single phase, 3phase or a mixture of both.
The conductors for each outgoing circuit should be sized appropriately.
When the neutral current from phase L1 gets back to the board, it can now use the L2 neutral conductor to flow to the L2 phase, and the neutral current from the L2 phase can use the L3 neutral conductor to flow to the L3 phase, etc.

So once the neutral currents get back to a single point, they can then start to balance themselves out.

In the OP’s situation, there is a very good chance that the 3 showers will not be used at the same time.
However there is a chance that 2 will be used at the same time.
If only 1 or 2 are used, the neutral currents will not be able to balance out.
If all 3 are used at the same time, they will balance out.
As such you need to size the neutral conductor to handle up to 2 lots of current.

 

[David Rann]

Thanks spindon. So if you have two showers then you got 64amp flowing down the neutral but if you put the third on then it cancels it out?

 

[Deleted member 26818]

Thanks spindon. So if you have two showers then you got 64amp flowing down the neutral but if you put the third on then it cancels it out?

Yes.

 

[David Rann]

Okay thanks so the cable needs to be able to handle 64 amp then.

 

[Deleted member 26818]

Okay thanks so the cable needs to be able to handle 64 amp then.

Hence why I suggested 16mm2.

 

[David Rann]

Got ya. Why wouldn't just two phases cancel each other out a bit?

 

[Risteard]

Got ya. Why wouldn't just two phases cancel each other out a bit?

They will. You will draw less than 32A but still nowhere near 0A.

 

[David Rann]

Okay two conflicting views here.....one says you double it to 64amp and one says it brings it doen

 

[Wilko]

Picking up the big hint from @westward10 - I had a go at a phasor diagram. If phase 1 shower is on, I get 30A (say) on the distribution circuit N. If phase 1,2 and 3 showers are all on then I get 0A on N, as they all cancel out. No big surprise as it's a balanced 3 phase load at that point. So what about when 2 showers are on? Hint - I was wrong before in #14 - it's my brain that's been flogged .