We are asked to calculate size of transformers needed to service 3 phase 4 wire high leg delta based on the assumption that the system would need to continue running even if one of the non center tapped transformers failed and was removed.

I know that for a balanced 3 phase load you just divide the total load by .866 then divide it by two and that's your transformers to keep working even if one of the three is lost and/or removed. When you center tap one of the three and add more load to it via 120 volt circuits, how do you calculate the upsizing needed for the one transformer to compensate? I am taking into consideration the compensation for open delta while supporting the 3 phase and single phase loads. I am looking for the formula and not a particular example.

 

Goku... is there a sngle-phase, 208V load?

Regards,
Phil Corso

 

In an open delta arrangement, the center-tapped transformer is called the 'lighter', while the non-center-tapped is called the 'kicker'.

Here's the simple way to calculate...

Once you have calculated all three line currents (especially in the case of unbalanced loading of the windings), multiply the open-end currents by the nominal phase voltage and that will give you the minimum winding rating. Substantiation: all current on the open-end terminals pass through the connected winding. multiplying by voltage across winding equals minimum kVA.

 

Where the center-tapped winding has more load on one half than the other, compensate by virtually calculating both halves as the larger half's load.

For example one half is 100kVA of 120V loads and the other half is and 80kVA. Calculate line currents as if winding has 100kVA at 240V.

The preceding is for open delta. Closed delta is a bit more complex if using a bank of three 1-phase transformers of different ratings.

> ... When you center tap one of the three and add more load to it via 120 volt circuits, how do you
> calculate the upsizing needed for the one transformer to compensate? ...