ECN Forum Forums Technical Discussion Electrical Theory and Applications Load Calculations

When calculating the load for a building, how do you spread the single and three phase loads over the incoming three phase lines? I belive the correct way for 120/208 Wye is that single phase 208 or 240 loads spread equally across two transformers (A&B, B&C, or C&A) whereas for 120/240 Delta, 208 or 240 single phase load is applied to only one transformer (A, B, or C)/phase (just as the 120 loads are only on the A & C phases). The three phase loads are spread equally across all three phases.

You seem to have it correct. The only thing that's really tricky is that, on a delta with a high leg (B phase), any load that has a neutral connection, i.e., 240/120 or just 120, must be connected only to the other two phases (A or C), as you noted.

Typically, the high-leg delta system has less load on B phase than the other two, because only line-to-line or 3-phase loads are served by this phase, whereas A nd C phases serve all of the line-to-neutral loads.

I agree with your thinking about spreading the loads as equally as you can, which would be balancing of the transformer.

Using a 208/120Y transformer you can use any of the phases for 120V loads, any 2 of the phases for single phase 208V loads and all three for 3 phase 208V loads. However you can't use any combination of these for 240V loads...

Using a 240/120 Ct transformer, you can use A or C phase for 120V loads, any 2 phases for single phase 240V loads, and all 3 for 3 phase 240V loads. You can't use any combination for 208V loads....

Mind you with this type setup and using the high leg, you must be concerned about using full voltage breakers where with the 208/120Y setup, the use of "slash" breakers could be used.


Dnk...



[This message has been edited by Dnkldorf (edited 03-20-2006).]