You can consider a balanced 3-phase load to be three single-phase loads of 1/3 the power. In the case of electric heating that is often a true representation of what is inside.
So 15+11=26kVA consumes
26000 / 3 / 230 = 38A
You still have 42A available or 126A for the single-phase stuff. Actually is the boiler only 15kVA?
For convenience, one can still calculate this way even for loads that don't have a neutral and operate at 400V connected between the lines. In this case, the calculation ought to be
kVA / sqrt(3) / 400 but the answer for the line current is the same. This is intuitively logical because the line current depends on the power consumed, not how the individual coils are connected within the machine.
The reason for the equivalence is that the loads or coils, when delta-connected at 400 volts between pairs of lines, pass a current of kVA / 3 / 400 i.e. the load can still be thought of as divided into three equal ones. But because of the delta cinnection, the line current is now the vector sum of a pair of coil currents which is higher by a factor of sqrt (3).
In normal 3-phase working, one does not routinely say or think 'per phase'. Circuit current always represents that in one conductor, regardless of the number of conductors. TBH it grates when people unaccustomed to 3-phase systems say 'I've got a pump that takes 5A per phase' or similar, or ask whether that makes 15A total. After all, one does not declare the line and neutral currents separately in a 2-pole device, a 100A DP main switch is not rated at 200A on account of both the line and neutral being good for 100.
But there is nothing sneaky about looking under the hood of a 3-phase device and finding it's just three 230V devices in one box.
Dear Lucien,
Many thanks again!
Ah, it
was OK (I later got cold feet, and decided that I had been wrong!).
It's OK only if one knows the kVA of an appliance, or if it is obvious that this must be the same as, or very close to, the kWh rating.
Given that the appliances in question (even the charger?) are predominantly non-inductive, the kVA
should be either identical, or very close, to the kWh. The boiler's element, like that of an electric kettle or immersion heater etc. wound using an alloy such as nichrome, with a highly positive coefficient of resistance, will draw a starting current much higher than the operating current. This will very quickly drop as the winding heats up. so the appliance ends up running at a kVA more or less (the winding form, especially if spiral, may well be inductive) identical to the quoted kWh.
So the current (A) calculation is:-
USING kWh AS kVA IF ONLY kW IS GIVEN, AND IF THIS IS OBVIOUSLY VERY CLOSE TO kVA
kVA (of appliance)/3 (phases)/230 (V per phase)
So the two appliances together (15,000VA+11,000VA) work out, as you say. at :-
26,000/3/230 = 37.68A,
38A rounded.
The same calculation
can't be done with the 3-P phase to phase line voltage at 398.36V (230 x 1.732) because this does not use (one phase V x 3), so gives a very different result (65.27A!!).
The correct calculation leaves, as you say, (42A x 3 single phases) 128A total for the S-P installation.
The 42 could sensibly be rounded to 45, leaving (35A x 3: 105A total) for the S-P circuits
There can be give on one side and take on the other, or vice-versa.
BOILER: IS 15,000 W SUFFICIENT (Your implied question)?
Our 1997 non-condensing gas boiler generates, in Wh. about (28,000 x 0.76): 21,280 of heat energy. This includes DHW heating. This boiler is probably a bit over-sized, though not to the monstrous extent of some of the combi boilers installed in more in recent years.
So, with DHW being heated directly at 3,000 Wh by one of two S-P immersion heaters rather than by a boiler circuit ,15,000 Wh may be enough for this house, which is much better insulated now than it was in 1997. Coincidentally (and importantly!), it appears that any electrical boiler with an output higher than 15 kW has to be floor-standing. We can't accommodate this - only a wall-hung one.
The heat requirement would have to be worked out on the basis of a proper energy survey of our house.
We could accommodate, an 18,000 Wh boiler:
3-P: (18,000 + 11,000)\ 230\3 = 42A. Say 45A.
That leaves 3 x 35A (105A) for S-P. Should be fine. Given the circuit/splitting/diversity problems it would be sensible not to reduce S-P total below 100A.
T
HE AMPS-FROM-kWH CALCULATOR THAT I FOUND VIA GOOGLE
This is at
Kilowatts (kW) to amps (A) conversion calculator.
www.rapidtables.com
fUsing 26 kWh, this calculates (I) 21.77 Amps with voltage entered as 3-phase line to line 398V, and (ii) 37.68 A for 3-phase 230V line to neutral.
No wonder I became unsure!
This calculator
looks serious, though calculation methods are not displayed.
What do you make of the current levels that it calculates? They're clearly not right - at least for UK 3-phase. (ii) is closer than (i), which is way out.
Many thanks again, and best wishes,
Carl