Amps per phase for 3 phase?

[DaveWalsh]

One for the sparkies just out of college.
If I have a 3 phase supply rated at 12kW and the volts are 230 what Amps per phase can I draw?
Is the formula to calculate (V x A = W) the same for 3 phase and single phases? Do I just divide the total amps by three?

 

[tim west]

Is it 12KW per phase or total?

Is 30 years just out of college ok?

 

[DaveWalsh]

30 years sounds great - you obviously haven't made any fatal wiring errors so far!
The rating is 12kW total

 

[JohnD]

That's a very weedy 3phase supply.

What's it for? A machine in a shed?

 

[DaveWalsh]

Yes I know its a weedy supply - this is the problem! I am trying to get the supply uprated as I want to run a 6kW single phase oven.
Do you know the answer to my original question OR question number 2:

What 3 phase power do I need to be able to run this 6kW single phase oven; I calculate the oven will draw 25A?

(I would like to keep the 3 phase to run lathe/mill in adjacent barn)[/b]

 

[JohnD]

You have not said if this supply is for a house or an outbuilding or what.

12kW at 230V is 52A

If as you say it is 12kW total over the three phases that is (only) 17A per phase, but this is enough for any single, and some double, domestic ovens. It is not enough for an electric hob, but you do not mention one, so I presume your cooker has a gas hob.

If you have a 6kW oven I presume it is some sort of catering appliance as it is at least twice the power of domestic ovens used over here. You can't run a single-phase 6kW oven if, as you say, your supply is only 12kW total over three phases. You might consider gas ovens if you have a catering business.

However, in some countries (but not common in the UK) 3-phase cookers are available, to cope with the problem that you have got.

 

[plugwash]

What 3 phase power do I need to be able to run this 6kW single phase oven;

Obviously you need a rating of at least 6KW per phase which works out to 18KW total. If you want to use the three phase tool at the same time then you will need a bit more to supply that as well.

 

[harrry]

Well the first thing is that Volts X Amps does not give watts in alternating current
Watts = volts X amps x cos phi
(phi is a greek letter) and = cosine of phase angle between volts and current
The amps and volts are not synchronised in AC and therefore cannot be simply multiplied together
If the electrcal load is resistive (ie heating)cos phi = 1 (phase angle is zero degrees)
So in this case volts x amps = Watts
However if you have an inductive load phi = 0 (phase angle is 90 degrees)
So you can have as many volts and amps as you like an there will be no Watts at all!

In practice. the phase angle lies between these two extremes.
An example would be an electric motor where cos phi might typically be 0.8
This data is usually on the nameplate on the motor either as cos phi or the POWER FACTOR.
So you always need a bigger wire than you might think for motors.
If you have say a 6kW load motor), that's 2kW per phase, divide by 240 equals about 8 amps, divide by your power factor (cos phi) [say 0.8] equals 10 amps. (in each of the three wires)

If it was a heating load, cos phi is 1 so the answer would be 8 amps.
See how it works?
Remember phi is greek letter phi, (O with vertical slash down the middle)
As an aside inductive loads , current lags the voltage. Capacitive loads, current leads the voltage. But don't worry about that!

 

[JohnD]

some stuff...
If the electrcal load is resistive (ie heating)cos phi = 1 (phase angle is zero degrees)
So in this case volts x amps = Watts...

...a 6kW single phase oven...

Ok by me

 

[harrry]

Further to above. To say your supply is rated at 12Kw is meaningless.
The limitation on any supply is the current that can be drawn (or the voltage x that current.) NOT KW but KVA.
So if you had a heating load, KVA is the same as KW. But if you have a motor you will only be able to draw less KW than the KVA figure.

So if you had a 12 KVA supply you could run 12 KW of heating.
But if you had a motor you might only be able to run 9 KW. And this neglects the start up current which would have to be considered. (Which might bring the max motor down to 7 or 8 Kw) This would depend on the nature of the load it was driving)
Matters can be improved by POWER FACTOR CORRECTION, that is capacitors connected in parallel with the motor. (Size would need to be calculated)

 

[GaryMo]

The limitation on any supply is the current that can be drawn (or the voltage x that current.) NOT KW but KVA.

Very good point.

kVA = kW/pf

To convert kVA to Amps (3phase):

kVA x 1000 / (400 x root3)

 

[DESL]

Thanks Harrry

That was a good bit of revision for me it's xx yrs since I did my Electrical Eng degree.

 

[DaveWalsh]

Thanks for the above; I now understand a little more.
The 6kW oven is actually a "night storage" electric domestic AGA, (to be installed in a house, some 15m from the supply pole) so the load is wholly resistive/heating. It only heats at night when any other small 3 phase machinery (lathe, drill) will not be running.
The reason I described the supply as 12 kW is that this is how it's described on my electricity bill; although it's also described as 12kVA on the supply companies website. I think, thanks Harry, that I now understand the difference between the two.
The British built AGA will be installed in France but since I assume volts, amps and watts are the same across Europe I've been cheating by asking the question here on the "Electrics UK" forum. I did ask on the "Electrics abroad" but no-one answered! Many thanks for all the help.
The EDF website describes my supply as:-
Power 12kVA; disjoncteur 60A