Protection for transformers rated 600 volts or less is determined by selecting one of three levels of full load current (FLC) found in Table 450.3(B) of the National Electrical Code (NEC). Levels are based on either primary protection only or by providing both primary and secondary protection. The appropriate percentage needed to calculate and size of the overcurrent protection device (OCPD) is selected from the table and is strictly based on one of these current levels.

9 amps or more

When a transformer has a primary current of 9 amps or more, Table 450.3(B) requires this value to be multiplied by 125 percent to obtain the size of the primary OCPD. For example, the OCPD for a 480-volt, three-phase, 50-kilovolt-ampere transformer having a FLC of 60 amps per Table 450.3(B) must be increased by multiplying this value by 125 percent (60 × 125 percent = 75A). A 75 amp standard-size OCPD is not listed in 240.6 (A). Therefore, the rounding-up rule of Note 1 to the table should be exercised. Based on the application of this rule, the size of the OCPD selected for the primary side is 80 amps.

The secondary side has an output of 139 amps, and if used for continuous duty, Section 215.2(A)(1) requires an increase by 125 percent that derives a sum of 174 amps. This calculated load requires 2/0 THWN copper conductors with an ampacity of 175 amps each, which is adequate to carry the load. A 225 amp lighting and appliance branch-circuit panelboard is installed and equipped with a 175 amp, three-pole main to which the 2/0 terminates. The transformer is considered protected only because the primary scheme of protection is applied. Note 1 to 450.3 refers the user to 240.4 and 240.21 for the protection of conductors. The 175 amp main is capable of protecting three major items: the conductors per 240.4, the panelboard per 408.36(A) and 408.34(A), and the output of the transformer.

Less than 9 amps, more than 2 

Transformers with a primary current of less than 9 amps but more than 2 amps are allowed to have their OCPDs increased by multiplying the determined amps by 125 percent as described in Table 450.3(B). Consider an input of 5.5 amps, and when multiplied by 125 percent, the OCPD can be sized at 6.8 amps. By rounding down to the next size OCPD, a 6 amp fuse per 240.6(A) can be selected. A 6.5 amp fuse that is listed and approved for the purpose also can be used, even though when 240.6(A) is viewed, a 6.5 amp size is not shown.

Less than 2 amps

OCPDs protecting transformers with a primary current of less than 2 amps per Table 450.3(B) can be sized by applying a 300 percent multiplier. For example, using the table, a transformer with 1.5 amps must be multiplied by 300 percent (1.5 × 300 percent = 4.5), which equals 4.5 amps. A fuse rating of 3 amps from 240.6(A) or a listed 4.5 amp can be selected. A 500 percent rule outlined in 430.72(C)(4) can be used to size a fuse to protect the primary of a control-
circuit transformer. The size of the fuse is determined by multiplying the 1.5 amps by 500 percent (1.5A × 500 percent = 7.5A), which equals 7.5 amps. Therefore, as permitted by 240.21(C)(1) and 430.72(C)(4), a 6 or 7.5 amp fuse can protect both the primary and secondary conductors. The windings of the control transformer also are protected.

Transformer secondary conductors

Primary protection on the supply side of the transformer protects only the transformer and not necessarily the secondary conductors. Note 1 to 450.3 recommends, when protecting conductors, to use 240.4 or 240.21. I will use 240.21(C)(2) to describe the procedures for protection of conductors in the above example of 9 amps or more. Sections 408.34(A) and 408.36(A) permit secondary conductors to be terminated to the 175 amp OCPD in the lighting and appliance panelboard. Note, both the secondary conductors as well as the output of the transformer are considered protected.

Now, consider a transformer with a primary OCPD of 15 amps, and the secondary conductors are 10 AWG copper with an ampacity of 35 amps each. If the primary voltage is 480 and the secondary is 240 volts, then the ratio is 2-to-1 (480V/240V = 2). By multiplying the 15 amp OCPD by 2, the secondary conductors can be loaded to 30 amps. Divide the 35 amp on the secondary by 2, and 17.5 amps is derived. The 15 amp OCPD on the primary provides the protection permitted by 240.21(C)(1). Remember, when using the Code, find the article and section that covers the subject, and calculate the load and size of the component.   EC

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.