OK, just one more hint: Starting to think I have said too much?
The ratings or settings of the overcurrent protective device obtained from Table 450.3(B) are based on the transformer-rated current and whether or not secondary protection is provided. According to Table 450.3(B), the maximum ratings or settings of overcurrent protective devices for transformers rated 600 volts and less are separated into two categories: primary only protection and primary and secondary protection.
According to Table 450.3(B), transformers with currents of 9 amperes or more must be protected by either of two methods. Method 1 requires primary protection only and is set at not more than 125 percent of the primary side rating. Method 1 does not require secondary side overcurrent protection. Method 2 requires secondary side overcurrent protection to be set at not more than 125 percent, provided the primary side overcurrent protection is set at not more than 250 percent of the primary side rating. Although not required, following either protection method will free the user from any further protection requirements of Table 450.3(B). According to this table, smaller transformers have protection requirements that are less restrictive. For overcurrent protection of motor control circuit transformers, see 430.72.
450.5 is for autotransformers...
[This message has been edited by e57 (edited 06-05-2006).]
