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

 

[ipf]

Maybe some of our 'regs experts' should brush up a bit on their principles and theory.
I don't do 'dumbs'.....but my finger's itching.

 

[David Rann]

Crikey, it's a long time since I saw so much gibberish condensed into one thread. Reminds me of a nursery rhyme;
Up and down the single phase
In and out of the neutral
That's the way the current goes
Pop! goes the cable.

The neutral current will never exceed the highest line current provided the loads have either unity power factor (as in this case, with resistive heating in the showers), or a non-unity power factor caused by phase displacement only, similar on all phases. In practice, with a mixed load it is very unlikely that the neutral current will ever reach the circuit rated current.

The neutral current can easily exceed the actual line current with phase-angle controlled loads such as dimmed tungsten lighting. The waveform distortion introduces a third-harmonic component which adds in the neutral rather than cancelling. Also any assymmetry, such as half-wave rectification, can produce additive harmonics. If the harmonic distortion is severe, and can take place when the line currents are at maximum, then the neutral conductor may need to be larger than the lines.

Like in battery chargers?

 

[dmxtothemax]

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

How will this ever happen ?
Your forgetting it's a three phase supply
so all three loads are not drawing current at the same time,
Remember it's sequential,
Being sequential there is a small over lap
So in reality the current in the neutral will be no more than 1.7 x each individual load.
7000 x 1.7 = 11.9 KW
IT'S NOT 3 X
can never be

 

[marconi]

Re: Wilko's #40 - I think his is the neatest analysis.

Wilko starts from what we all know - three identical showers each connected to its own phase results in no residual current in the Neutral. One shower on causes a current of circa 30A to flow in the neutral. If two showers are on there is a residual current in the Neutral which becomes zero when the third is turned on. Therefore the residual current for the two shower case is equal in value (but anti-phase) to the current drawn by the third shower - the current drawn by the third shower is 30A - ergo the neutral current when two showers are on is 30A.

 

[Lucien Nunes]

So in reality the current in the neutral will be no more than 1.7 x each individual load.

Er, not quite! As per Marconi's post and Wilko's and mine.

 

[Pete999]

Phasor diagram anyone.

Sorry mine on the Stun setting

 

[markythesparky]

A shower passes a current of Iphase.

There are 3 phase currents I1, I2 and I3

Taking I1 as reference at 0deg, the I2 is =120 phase angle and I3 is 240deg (or -120deg)

Assume two showers on using I2 and I3, then In is I2 + I3 =

Iphase(cos120 +jsin120) + Iphase (cos-120 +jsin-120) =

Iphase (-0.5 + j0.58) + Iphase(-0.5 -j0.58)=

Ip (-0.5 + j0.58 -0.5 -j0.58) = -Iphase = -7000/240.

If you now add in a switched on I1 fed shower In = I1 + I2 + I3 =

Iphase-Iphase = 0

So, one shower magnitude of In = 7000/240A

Two showers magnitude of In = 7000/240A (but in anti-phase to the current the third off shower would pass).

Three showers In = 0A

Thus In is 7000/240 A for one or two showers or 0A fro three showers.

No need for N csa to be larger than line csa.

Knew there was a reason you are in the history books!

 

[bailes1992]

This thread is worrying. This is basic electrical theory.

They are single phase loads so no, the neutral current won't be zero.

Simplifying this as best as I can...

Each phase is AC at 50Hz and 120° from eachother. That means only one phase will be moving current down the neutral conductor at a time. If you put a clamp meter on the neutral it would read the same as one phase.