Three phase to single phase

[Jpflex]

When going from a 3 phase delta delta 3,800 volt primary to 480 volt secondary for a 300 KVA transformer

which then feeds another step down transformer for a black smith shop

The blacksmith shop’s transformer is 25 KVA 480 volt primary to 240/120 secondary

Then, if feeding from the 3 phase 300 kva delta transformer secondary to a 25 kva single phase transformer in the blacksmith shop, do you treat the equation as a single phase system or 3 phase system including the square root of three to find input and output amperes at single phase transformer? (Just asking because source is 3 phase)

KVA/ secondary voltage

Or

KVA / secondary voltage x 1.732

I believe we don’t use 1.732 even if coming in with a three phase transformer to feed a single phase transforner

Also does it matter if using a, b or b, c or a, c phases from 3 phase system to feed single phase panel?

 

[tortuga]

do you treat the equation as a single phase system or 3 phase system including the square root of three to find input and output amperes at single phase transformer? (Just asking because source is 3 phase)

When going from a 3 phase delta delta 3,800 volt primary to 480 volt secondary for a 300 KVA transformer
..../snip/...

Also does it matter if using a, b or b, c or a, c phases from 3 phase system to feed single phase panel?

Is the 300KVA transformer delta/ wye?

 

[ScottWJ]

Do you want to calculate currents in the single phase 25 KVA or in the 300KVA delta-delta? The formulas will be different for each.
For the 25KVA you do not use the 1.73 factor.
For the 3 phase 300KVA, the single phase transformer load creates an unbalance, which will be a more complicated calculation than just using 1.73

does it matter if using a, b or b, c or a, c phases from 3 phase system to feed single phase panel?

No it will not matter which phase pair you connect the single phase transformer to.

 

[Jpflex]

Do you want to calculate currents in the single phase 25 KVA or in the 300KVA delta-delta? The formulas will be different for each.
For the 25KVA you do not use the 1.73 factor.
For the 3 phase 300KVA, the single phase transformer load creates an unbalance, which will be a more complicated calculation than just using 1.73

No it will not matter which phase pair you connect the single phase transformer to.

I do know that three phase use 1.732 but what is confusing is that the single phase transformer is coming from a three phase panel, and fed from two of three phases from a 3 phase system so I just wanted to confirm that I am doing the calculation correct by not factoring in 1.732 for the single phase primary and secondary current amperes

 

[Jpflex]

Is the 300KVA transformer delta/ wye?

Delta delta

 

[Jpflex]

Correction the 3 phase transformer is a delta delta 4800 volt primary 480 volt secondary but not 3800p 480s

I am also considering placing the system bonding jumper at the single phase main panel disconnect/ power distribution rather than inside the single phase transformer via the typical panel supplied ground strap from ground neutral bus to EGC bus?

 

[ScottWJ]

I do know that three phase use 1.732 but what is confusing is that the single phase transformer is coming from a three phase panel, and fed from two of three phases from a 3 phase system so I just wanted to confirm that I am doing the calculation correct by not factoring in 1.732 for the single phase primary and secondary current amperes

The single phase transformer does not know or care that it is fed from a 3-phase system, so 1.73 is not a factor in the calculations.