__Can anyone please comment as to the reasoning for the conclusions reached in the solution, including the conclusion that the 50 amp breaker on the primary is the correct max size/rating for protection of the secondary conductor. ( why say that the sec/pri ratio that equals 0.5 means that the 50 amp breaker is right for protecting the 100 amp ampacity secondary conductor? I fail fully to follow/detect/understand the path of reasoning all the way through to the end conclusions. ).__

**Article/Section 240.4.**- - -

__Application Example__:

A single-phase transformer with a 2-wire secondary is provided with primary overcurrent protection rated at 50 amperes. The transformer is rated 480/240 volts. Conductors supplied by the secondary have an ampacity of 100 amperes. Is the 50-ampere overcurrent protection allowed to protect the conductors that are connected to the secondary? The secondary-to-primary voltage ratio in this example is: 240 480 = 0.5

Multiply the secondary conductor ampacity by this ratio: 100 amperes

Solution

0.5 = 50 amperes

Therefore, the maximum rating of the overcurrent device allowed on the primary of the transformer that will also provide overcurrent protection for the secondary conductors is 50 amperes.

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