What is the effect of starting at 480V on the amperage capacity of the supply cable before the transformer?

• A. It increases significantly
• B. It decreases
• C. It stays the same
• D. It is unpredictable

Answer: (A)

The key idea is that the amount of current a circuit can deliver in a fault (short circuit) grows with the system voltage. If you have the same electrical impedance feeding the transformer, raising the supply voltage from a lower level to 480 V increases the fault current roughly in proportion to the voltage. In other words, the higher the voltage, the higher the potential short-circuit current that the supply cable and protective devices must be able to handle.

Note that under normal operation, the primary current for a given transformer rating actually drops as voltage increases (I = VA/V_primary). The question, however, focuses on the amperage capacity during a fault, where higher voltage means higher fault current. That’s why starting at 480 V implies a significantly higher amperage capability to withstand or interrupt a fault.