Connection to Delta Primary Transformer

[funseebee]

I have a main switchboard at 120/208V, 3-phase, 4-wire. There is a 400A/3-pole circuit breaker in tha main swbd and connecting it to a delta primary transformer, 208V,3-phase.
The question is "How can a 120/208V, 3-phase, 4-wire (wye) be connected to a delta primary transformer 208V, 3-phase? How do I explain that this is a legitimate connection? Please provide layman's language explanation and please not too technical. Thanks.:roll:

 

[augie47]

same way you can use your 208Y/120 service to operate a 3 phase motor.
The transforemr does not need the neurtral to "operate". The neutral "tap" is there to provide a 120v when the trnasformer is used to SUPPLY 208/120. The XO tap is not used when connection to the 3 coils in a "delta" manner.

 

[LarryFine]

How can a 120/208V, 3-phase, 4-wire (wye) be connected to a delta primary transformer 208V, 3-phase?

Easily, just connect the three lines.

Even if the transformer primary had an H-0 terminal, you should leave it floating. Not bonded, not grounded, nothing, no connection to anything.

In other words, regardless of the primary's source, whether 3-wire or 4-wire, a Delta primary is always correct.

 

[winnie]

How about " A 208/120V 4 wire service is designed to supply loads which require any subset of the 4 available terminals. It can line to neutral 120V loads, line to line single phase 208V loads, three phase 208V loads, or 208/120V loads using all three lines and the neutral. A delta primary transformer is simply a three phase load that does not require the neutral; it is not harmed by the presence of the neutral elsewhere in the supply system."

-Jon