If possible, having some way to test with load imbalance would be helpful.

@TonyStewart.EEsince'75 No, the load will most likely not be balanced. Can you elaborate on the Y-Delta step up and why it is not advised? I think the people who are working on this currently have issues with unbalanced loads. I am just interested in this topic and would to know more about this if possible please.

I fully agree! i just wasn't sure if your reasoning was for safety alone or i had messed up my calcs somewhere! Thanks for your help!

I know its been a while since this discussion but i eventually got around to revisiting this & calculated this all out with the 28.8K ohm resistors + the 25% margin which came out at 36K and then looked at the 47k resistors as you suggested. Capacitance =9.67uF. When i calculated the time to discharge using 47K ohm resistor it came out at (T=RC) T=0.45s. Time to fully discharge =5T=2.3s. My question is what was your reasoning for needing 5 resistors in parallel for L to Gnd and 5 for N to Gnd? From the calcs i did, it seemed like 1 resistor in each application would work?

@ThePhoton i am looking for the theory behind how to calculate the correct size for a diode. I am not asking anybody to calculate it for me. thanks. 12V power supply with max current of 1.25A from the power supply. Does that mean it is as simple as say 15V 2A diode or increase to 50V 3A Diode what the reasoning behind what diode to choose?

@Mark im not actually ORing two separate power supplies i am actually ORing two control signals, each of those control signals being a 12V supply from the same power supply. ie I have two relays and the are controlled by other devices, the relays both have a NO contact and when the relays activate the 12V is passed through each of the respective relays. These control signals can be independent or on together and they control a contactor. So either 12v A or 12v B can operate the contactor. I want to have all the "control" signals there are 5 of them OR together with 1 O/P to contactor.

At Sparky,256, how much of an issue is it if one diode dominates? and what does that mean long term? ie will one power supply be more likely to burn up before the other? thanks.

AT EUGENE : Well, i do actually understand that the diodes need to be sized in relation to voltage and current at the extremes, the question is "How do you size them" and what is a good safety factor? You answer provided zero help. I am looking for a step by step guide which details the "how". If anybody can explain it, it would be appreciated. Professor responses like the above are not helpful at all.