MCB & Cable Size for 3 Phase Star (Wye) with Neutral

Firstly, I am NOT an qualified electrician, in fact I am just in week 4 of my training! But we did study three-phase systems last week.


1.  In a star-connected load, IP = IL so it should never be possible to get a different value, regardless of whether you use VP or VL ... I suspect user-error using the calculator. (Note: I tried using the calculator you linked and found IP = IL = 13A). To confirm from first principles:

VP = 230V

PP = 9,000W / 3 = 3,000W (resistive load only)

IP = IL = PP / VP = 3,000W / 230V = 13.04A


...and this is what the calculator gives.


2. AFAIK, the MCB is there to protect the cable, not the device, so the choice of MCB depends on the choice of cable, and the choice of cable depends on the reference installation e.g. in trunking, direct clipped etc. See 4. below.


3. According to BS7671:2018 554.1.2, "The supply to the electrode water heater or electrode boiler shall be controlled by a linked circuit-breaker arranged to disconnect the supply from all electrodes simultaneously and provided with an overcurrent protective device in each conductor feeding an electrode.". I would interpret this as needing a 3P breaker.


4. It is difficult to determine the correct cable without first understanding the cable type you intend to use or how it will be installed but assuming that you will be using 4-core PVC at 30℃ on its own in conduit then 1.5mm² cable is good for 15A (according to On Site Guide 7th Edition, Table F5(i)) and drops 25mV/A/m, or 1.95V/0.8% over 6m.
Note: You will need to consider derating this if the ambient temperature around the boiler is above 30℃ or other circuits are grouped in the cable run, tables F1 and F3 refer.


Please remember that I am very new to this, so please take my advice only as a starting point for your own research. I have only responded in an effort to help.

Questions like this will get answered faster in the wiring forum, and in future I strongly suggest you post there- but in any case the answer above is correct, but there are additional points and easier ways to think about it, no need to use a calculator, so  here goes.

In terms of the line current, which is what the MCBs will sense it matters not one jot if the load is connected delta or star so long as the load is balanced on all 3 phases - there will be no overall neutral current, and each line will carry 1/3 of the power.


If you wish you can faff about with 400V and factors of root 3, or you can realise that this is exactly the same as a single phase 3kW load 3 times.  So the current is the power divided by the single phase voltage 3000/230, so 13A. 

A 16 or 20A breaker would then be in order, as 13A is not a standard size. Depending on the cable length you may be limited by the cable current rating (and the nearest common size would be 2.5mm2 - the tables may suggest that for current handling, 1.5mm2 is OK and it is if you can avoid grouping it with other cables or embedding it in a wall on it's route, but nearly all real cable routes can not meet that requirement)*

On a longer run you may well be limited by voltage drop, and need something in larger cross section to keep the volt drop within limits..


Your choice of breaker should break all 3 phases, but breaking 3P and N is not incorrect, and is actually required if the supply is TT earthed

Best regards Mike.


* 6m will not have a volt drop issue, and if you can keep the cable cool, you can get away with 1.5mm2. Personally I'd suggest that 2.5mm2 is not a lot dearer, and remains the safe option in more situations - such as when someone adds more cables around it or boxes it in.


PS an immersion heater is not an example of an electrode heater - in the latter the water is live.

Why would this be the case?

I'm not sure what to say, Dan, I simply don't get those results when I use the calculator - see attached screenshots.

  

 

What is TT earthed?  

'TT' is an acronym for Terra-Terra where 'Terra' is latin for 'Earth' (or Terre-Terre in French). It means that supply and consumer are [only] connected by the general mass of the Earth i.e. the consumer has a grounding rod. Care needs to be taken to ensure the earth loop impedance is sufficient on TT earthing systems. This in contrast to TN-C systems (Terra-Neutral-Combined) where the consumer Earth is provided by the utility company so an earthing rod is [usually] not required. More info here.

Well, the slightly tongue in cheek answer is that  perhaps you should not trust software you have not written to do sums that you can verify with fingers (and maybe toes if you want a second decimal place), it is quite possible the software writer does not understand 3 phase, or has made a typo in the code. And any one who spells the word 'star' as w, y and e is probably not a native sparky, but then some of us say "ground" when we mean "earth".


Be aware that the  phrase "230V 3 phase" could  be taken to mean 230V between phases - so about130V to neutral on each phase, which would be a closer fit to the 20 odd amps. So the normal low voltage 3 phase in the UK street can be described both as 230V single phase and 400V 3 phase (to within the accuracy of assuming that 400/230 is root 3, which it is not quite.)

If in doubt say "XX volts phase to phase" or "YY volts phase to ground" to be utterly clear, or give both voltages (230/135 is not the same as 400/230).


In the same way the 11kV overhead lines (the three horizontal wires on wooden poles yomping over fields to supply transformers near small villages) is commonly called 11kV 3 phase (there is no HV neutral) even though each line is about 6.6kV to earth.

Yes, in the example given the line current will be about 13 amps. The exact calculated value will depend on whether 230 volts or 240 volts is assumed. The actual measured value will depend on mains voltage fluctuations and on manufacturing tolerances of the elements, what will be about 13 amps.


1.5mm cable would in theory be sufficient under ideal conditions, but in practice 2.5mm would be the usual choice to allow for high ambient temperature and for grouping with other conductors.  In practice 4mm might be needed not under the regulations, but for contractual reasons, a common clause in contracts for large installations is "no cables smaller than 4mm to be used for power circuits, and no smaller than 2.5mm for lighting circuits"


A 16 amp or a 20 amp MCB would be fine. I would prefer 20 amps because under worst case conditions the load current might be  about 15 amps if the "3000 watt" elements were actually 3,150 watts due to manufacturing tolerances, and if they are designed for 230 volts and if the actual voltage was 250 volts. 


A 3 pole MCB would be suitable in most circumstances, but nothing prohibits use of a 3 pole and neutral device or a 4 pole MCB.

I'm not sure what to say, Dan, I simply don't get those results when I use the calculator - see attached screenshots.

Glitch in the Matrix!... After re-starting my browser and revisiting the calculator it now gives the exact same results as your screenshots.  Confusion over!  Cheers.

3. According to BS7671:2018 554.1.2, "The supply to the electrode water heater or electrode boiler shall be controlled by a linked circuit-breaker arranged to disconnect the supply from all electrodes simultaneously and provided with an overcurrent protective device in each conductor feeding an electrode.". I would interpret this as needing a 3P breaker.

As an aside, electrode heaters are quite a different kind of beast - literally just a few rods exposed to the water - no metallic heating "element" as such - the current passes through the water and the resistance of the water produces the heat.


Normal immersion heaters don't fall into that category at all.


   - Andy.

And any one who spells the word 'star' as w, y and e is probably not a native sparky

I suspect an Americanism - I believe WYE is their usual term for star.

  - Andy.