kVA calc on Panel Schedule

[emc2]

Sorry for a basic question, but I am confused ...

I am looking at a set of plans that show a panel schedule with kVA calculations.

It shows a 100 amp, 3 phase, 3W, 120/240V circuit with the following calculation PER POLE ...

100 amps x 120 v = 12 kVA per pole

Is this correct or should it be (100 amps x 240v x 1.73) /3 = 14.3 kVA?

... or what ?

Thanks for the help.

 

[SooryaShrestha]

I think it is not three phase, but two phase with center tapped. This means the line to line voltage is 240 V and line to center tapped point is 120 V. Each 120 V from center tap is 120 V and 180 degree out of phase.Thus no question of root 3 as done in your calculation.

 

[mustafagerz]

first sorry for my english;

i think 240 V is a diffrent system like european. that means if the voltage doubled then the current will be half becouse of the power calculation.

then;

S=50 amp.*240= 12 kVA it is same.

in three phase systems you have to calculate line to line current too. line to line curent is I/1,73 then result doesnt change for three phase calculation.

 

[bru]

If this is all in the same panel, the most likely configuration is that the panel is supplied from a delta connected transformer (secondary) where one leg is tapped in the center. In this case, the phase current is still 100 A for the 240V connection. That would mean that you have a single phase supply at 120V, another at 120 V and a third at 240V plus 240V 3 phase as well.

The calculation for the phase current inside THIS delta is
100/SQRT(3) or 57.7 amps. That is the maximum current available at the 120V tap.

Soooo....

If you think about it this makes sense as the available power in this panel is 100A *240V * SQRT(3) =41.6kVA.

Now if you were to supply a single phase load with 240V, your available power would still include a root3 calculation as the rest of the 3 phase load is still present. Bear in mind that the phase current inside this delta is 58amps. 58A*240V=13.86kVA. (if you multiply that by 3, you get 41.6kVA, how 'bout that)

At the 120V supply, you can still only get 58 amps due to the limitations of the delta.

58A*120V=7kVA.

I am guessing at what you have here but that configuration is not uncommon.

WATCH YOUR LOAD BALANCING!!!

Hope this helps
Bru

 

[bru]

Sorry
One more thing...
The 6.9 or 7kVA that you get on the single phase 120V supply is one half only, the other half, (from the other leg) will also supply the same. If the loads are balanced on either side of the 120V tap, you can still have your 13.8kVA total load on that leg of the transformer.

you will again have to be aware that you should subtract any 3phase load that you have in total on this panel from your available power, as the TOTAL power on a leg cannot exceed your allowable kVA rating.

Regards, Bru

 

[jbartos]

Suggestion: Usually, the set of plans that you have at hand is supposed to have one line diagram with the proper transformer identification, connections, winding voltages, phases, etc. If you are missing it, you may have a legitimate claim to obtain it. Unless, you have your drawings supported by additional independent source/document, the answer to your question may be incorrect since some of the data, you mentioned in the original posting, are usually more specific, e.g. winding connections seems to be missing.