Current transformers are specified by size (VA rating), ratio and accuracy. The VA rating determines the maximum secondary impedance (lead+terminal+meter impedance) that can be driven at the stated accuracy. Metering CTs are specified for a 0.9 power factor at 60 Hz. Relaying CTs are specified at 0.5 PF. Current Transformer Application Note and Wire Length Chart  (pdf) Current Transformer Ratings for Various Motor Sizes (pdf)