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.

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.