# Appox. the Impedance of Single Phase Pole Transformers using only Nameplate Ratings?

I know the following of one pole transformer (don't have access to the nameplate):

333KVA, Z = 5.75 % 25kV/12kV

Given another transformer, I don't know Z, but know it's rating and voltage

250KVA, Z = ?, 25kV/12kV

How can I find Z roughly? Construction of transformer is roughly the same except for the higher rating. Higher rating would require thicker conductor for the coil and/or a bigger area for the coil on the high and low side? Are these good assumptions? Could I use these assumptions to make these very very rough conclusions:

1. Resistance would decrease but lets neglect R for now.

Since

Z ~ L ~ A/l

1. Self-inductance and mutual inductance of two coils would increase because we are increasing the area of the coil that the magnetic field can pentrate.

I don't know that much about construction of transformers to make the correct assumptions about what would change by increasing the ratings. I would like to be able to make some linear assumptions and use the ratio of 333/250 to scale the impedance.

• Commented Nov 12, 2015 at 8:18
• Short answer: Probably about 4% impedance per-unit. Commented Nov 12, 2015 at 8:19

## 1 Answer

A transformer designer has some flexibility to control the impedance. Increasing the impedance reduces the prospective short circuit current. Reducing the impedance provides better voltage regulation. On a percentage basis pole transformers are likely to have close to the same impedance regardless of the kVA and voltage rating.