If a transformer is rated at 250 VA with 250 volts primary and 125 volts secondary, what size fuse should be used on the primary side?

To determine the appropriate fuse size for the primary side of the transformer, it is necessary to understand the relationship between voltage, amperage, and the transformer’s rating in volt-amperes (VA). The VA rating indicates the maximum amount of power the transformer can handle safely.

Given the transformer's rating of 250 VA and its primary voltage of 250 volts, you can calculate the primary current using the formula:

[ \text{Current (I)} = \frac{\text{Power (VA)}}{\text{Voltage (V)}} ]

Substituting the values into the formula:

[ I = \frac{250 \text{ VA}}{250 \text{ V}} = 1 \text{ A} ]

This calculation shows that the primary side could draw up to 1 amp of current when operating at full load. For safety and to protect the transformer, the fuse should be selected to handle normal operating conditions without blowing immediately under standard load.

The choice of a 1 amp fuse aligns perfectly with this calculated current, allowing for safe operation while providing adequate protection against overcurrent situations. Fuses are typically rated slightly above the expected operating current to avoid nuisance blowing; however, using a fuse rated much higher than the