Calculating Delta Loads

[mbrooke]

I am adding single phase loads to a Delta service. I might also end up changing a few step down transformers in the process.


Any good links or personal knowledge on calculating phase-phase loading and Delta transformer size selction?

Thanks!

 

[mivey]

Anything in particular you are concerned about? One thing to consider is that the loads will not be balanced. Find the phase that has the largest single-phase load total, triple that, and that is the load to use for transformer sizing. That keeps you from overloading a single winding which can happen if you use average loads for the three phases.

 

[kwired]

If you have an open delta supply you need to remember there is less capacity than there is for a full delta with all same sized transformers. Some open deltas do use a larger pot for the lighting transformer and you will have more capacity for that part of the source but the stinger pot (high leg) may be smaller. This is usually done when it is not expected to have as much load on the third phase.

If you have three equal sized supply transformers than it is not a lot different than loading a wye other than if it has a neutral you will have a wild leg and will not be able to connect line to neutral loads to that leg, so you have to consider that if you have significant line to neutral loads.

 

[mbrooke]

If you have an open delta supply you need to remember there is less capacity than there is for a full delta with all same sized transformers. Some open deltas do use a larger pot for the lighting transformer and you will have more capacity for that part of the source but the stinger pot (high leg) may be smaller. This is usually done when it is not expected to have as much load on the third phase.

If you have three equal sized supply transformers than it is not a lot different than loading a wye other than if it has a neutral you will have a wild leg and will not be able to connect line to neutral loads to that leg, so you have to consider that if you have significant line to neutral loads.

Its a closed Delta bank with all equal kva units. Its a high leg system, one of the legs are center taped for a few 120 volt loads. Most of the loads are 3 phase though.

More 120 volt loads are going to be added along with a few heaters. The heaters 240 and are a small part of the service but will be balanced across the legs. My main concern are the 120 volt loads, in that Im worried about overloading the center bank or causing a voltage imbalance.

 

[Smart $]

...

More 120 volt loads are going to be added along with a few heaters. The heaters 240 and are a small part of the service but will be balanced across the legs. My main concern are the 120 volt loads, in that Im worried about overloading the center bank or causing a voltage imbalance.

If that is your concern, don't balance the heaters among the phases. Include the 120V loads in the balancing. Say you had 6 heaters 10kVA each and 30kVA of 120V loads, you have to connect the 120V loads A-N-C... so you put 3 heaters A-B, and the other 3 heaters B_C. That balances out to 30kVA on each phase.

 

[mbrooke]

If that is your concern, don't balance the heaters among the phases. Include the 120V loads in the balancing. Say you had 6 heaters 10kVA each and 30kVA of 120V loads, you have to connect the 120V loads A-N-C... so you put 3 heaters A-B, and the other 3 heaters B_C. That balances out to 30kVA on each phase.

The heaters are only a small portion of the load. The 120 volt loads will be quite large as the building is having offices added to it and the single phase 120/240 sub panels are going to be extended.

Im thinking about setting down a 240 to 120/208v transformer and putting the new 120 volt loads on that, but i would rather avoid it as much as I have to. Another thought would be to put some 208/240 rated loads on the high leg, but I have no clue how that effects the center can or if it will do anything at all.





Does any body have any current/loading equations for calculating the amps per line and the transformer loading?

 

[Smart $]

If that is your concern, don't balance the heaters among the phases. Include the 120V loads in the balancing. Say you had 6 heaters 10kVA each (or other 240V 1? loads amounting to 60kVA) and 30kVA of 120V loads, you have to connect the 120V loads A-N-C... so you put 3 heaters A-B (or other 240V 1? loads amounting to 30kVA), and the other 3 heaters B-C (or other 240V 1? loads amounting to 30kVA). That balances out to 30kVA on each phase.

The heaters are only a small portion of the load. The 120 volt loads will be quite large as the building is having offices added to it and the single phase 120/240 sub panels are going to be extended.

Im thinking about setting down a 240 to 120/208v transformer and putting the new 120 volt loads on that, but i would rather avoid it as much as I have to. Another thought would be to put some 208/240 rated loads on the high leg, but I have no clue how that effects the center can or if it will do anything at all.

Does any body have any current/loading equations for calculating the amps per line and the transformer loading?

Apparently you missed the gist of my post (to which I added to the quote above so as to elaborate a little).

As for calculating, just use a typical 3? panel schedule. For 240V 1? loads, put one half the load's kVA in each of the connected line columns. For example, if you have a 240V 10kVA load connected A-B, you put 5kVA in column ?A, and 5kVA in column ?B. This is the conventional method used by electrical engineers. Doing this actually produces some error in the column total converted to line current, but the error is typically to the conservative side of a full vector analysis result.

IMO, you should only add a transformer if the total 120V only load exceeds the rating of the A-C can.