Transformers

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Overcurrent Protection

Fusing and circuit breaker protection. How to overcurrent protect 600 Volt class transformers and associated wiring per NEC 450-3(b) and NEC 240-3. not required. 1. Primary protection only is required if the transformer is single-phase and the secondary has only two wires. Overcurrent protection rating and location are shown in Diagram A. 2. If the branch circuit feeding the transformer has overcurrent protection to meet the individual protection requirements in Example 1, then individual transformer protection is

Primary CurrentOvercurrent Protection Rating Primary CurrentOvercurrent Protection Rating Less than 2 amps 300% maximum2 to 9 amps 167% maximum9 amps or more 125% of rated primary current (or next highest standard rating) Less than 2 amps300% maximum2 to 9 amps167% maximum9 amps or more125% of rated primary current (or next highest standard rating) Diagram A Diagram B

not required. Secondary OCP is required as shown below. 3. Primary and secondary protection is required if the transformer has more than two wires on the secondary circuit. 4. If the branch circuit feeding the transformer has overcurrent protection to meet the individual primary overcurrent protection requirements in Example 3, then individual primary protection is

Primary CurrentSecondary CurrentOvercurrentProtection Rating 250% primary currentLess than 9 amps 167% maximumNot more than 250%9 amps or more 125% (or next higher standard rating) Primary CurrentSecondary CurrentOvercurrentProtection Rating Diagram C 250% primary currentLess than 9 amps 167% maximumNot more than 250%9 amps or more 125% (or next higher standard rating)

Section 240.6 (a) of the 2008 National Electrical Code*

The standard ampere ratings for fuses and inverse time industrial circuit breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and 6000 amperes. Additional standard ratings for fuses shall be considered 1, 3, 6, 10 and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.

Diagram D * Reprinted with permission from NFPA 70-2005, National Electri-cal Code
® , ©2008, National Fire Protection As sociation, Quincy, MA 02269. This reprinted material is not the com plete and of cial position of the NFPA on the ref er enced subject which is represented only by the standard in its entirety.

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