High leg delta

[hhsting]

The following transformer 480V primary is delta and secondary is 240/120V high leg delta with center tap

https://storage.googleapis.com/greensidebucket/product-spec-sheet/43/12-kva-3ph-doe-transformer-460v-delta-primary-240v120-delta-secondary-n3r-ventilated-60-hz-FhdTp.pdf

Question:

1. Would secondary be three phase or singe phase?

2. How would one size secondary overcurrent protection?

 

[winnie]

1) The secondary is a 3 phase secondary with a twist. The center tap on one of the secondary coils means that you can get 120/240V single phase from that coil. In general transformers of this type have a limit on the amount of 120V unbalanced load they can supply, but I don't see this limit on the spec sheet you posted.

2) Size as for any other 3 phase transformer. The primary is rated 14.5A per phase, The secondary is rated for 12000/240/sqrt(3) = 29 amps per leg terminal, select OCPD based on that.

However: I expect that the individual coils are rated about 17A each, and that if you were to supply a single L-N load greater than 17A that the center tapped coil would be overloaded. This transformer is really intended to supply mostly 3 phase 240V loads with a little bit of 120/240V single phase load on top of that, say for control circuits.

-Jon