Statement: A transformer rated 25 kVA or larger must have a nameplate that includes the impedance.

Statement: A transformer rated 25 kVA or larger must have a nameplate that includes the impedance.

• A. True
• B. False
• C. Only If Requested
• D. Not Specified

Answer: (A)

This tests why impedance is listed on the nameplate of larger transformers and how that data is used in protection and fault calculations. The impedance on the nameplate, expressed as a percent, tells you how much voltage is required to push the transformer’s rated current through a shorted secondary. That value is essential for predicting short-circuit current at the transformer terminals and for coordinating protective devices. For a practical sense, suppose a transformer is rated at 25 kVA with a rated voltage of 480 V and has an impedance of about 6%. If the secondary is shorted, the short-circuit current available at the terminals is roughly I_sc ≈ V_rated / (Z%/100) = 480 V / 0.06 ≈ 8,000 A. This information lets you size fuses and breakers correctly and ensures the protection system will operate reliably without excessive nuisance trips or under-protection. Because short-circuit and coordination calculations become more critical as transformer size increases, the NEC requires showing impedance on the nameplate for transformers 25 kVA and larger. Smaller transformers may not always include it, but the code requirement applies at or above this size.

This tests why impedance is listed on the nameplate of larger transformers and how that data is used in protection and fault calculations. The impedance on the nameplate, expressed as a percent, tells you how much voltage is required to push the transformer’s rated current through a shorted secondary. That value is essential for predicting short-circuit current at the transformer terminals and for coordinating protective devices.

For a practical sense, suppose a transformer is rated at 25 kVA with a rated voltage of 480 V and has an impedance of about 6%. If the secondary is shorted, the short-circuit current available at the terminals is roughly I_sc ≈ V_rated / (Z%/100) = 480 V / 0.06 ≈ 8,000 A. This information lets you size fuses and breakers correctly and ensures the protection system will operate reliably without excessive nuisance trips or under-protection.

Because short-circuit and coordination calculations become more critical as transformer size increases, the NEC requires showing impedance on the nameplate for transformers 25 kVA and larger. Smaller transformers may not always include it, but the code requirement applies at or above this size.