What is the maximum fuse size for a transformer with a six-ampere full load current rating using primary only protection?

To determine the maximum fuse size for a transformer with a full load current rating of six amperes using primary only protection, you can refer to the Overcurrent Protection Device (OCPD) sizing guidelines typically found in electrical codes such as the National Electrical Code (NEC).

For transformers, the proper calculation for determining the maximum size of the OCPD on the primary side is based on the full load current (FLA). According to standard practices, the OCPD should not exceed 125% of the full load current when using primary only protection. This is calculated as follows:

OCPD ≤ FLA × 1.25

However, when given the option of using values higher than 125% for sizing the fuse, the code allows for sizing up to 167% in specific applications depending on the transformer type and use case. This leads to the conclusion that, in this scenario, the correct approach is to apply the 167% multiplier guideline for primary protection, thus yielding:

OCPD ≤ FLA × 1.67

By using this maximum fuse size, the transformer protection can accommodate inrush currents and other temporary overloads, while still safeguarding against sustained overloads and faults.

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