CALCULATING IMPEDANCE

When building a 3 phase bank, impedance matching ensures that the individual transformers will uniformly share the connected load.  

Transformer manufacturers recommend that banked or paralleled transformers not to exceed +/- 7%  of the nameplate impedance.

Here's how to calculate impedance:

XMFR nameplate impedance  X  .07 = 7% value

Nameplate impedance  +  7%  value  =  maximum impedance

Nameplate impedance   -  7% value  =   minimum impedance

Example:

                         1.75% (nameplate impedance)  X .07  =  .1225

                          1.75%  +  .1225  =  1.8725   (maximum recommended Impedance)

                          1.75%   -  .1225  =  1.6275   (minimum recommended Impedance)