Emerson TRE Ex e Transformers User Manual

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Transformers
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197
Overcurrent Protection
Fusing and circuit breaker protection. How to overcurrent protect 600 Volt class transformers and associated wiring
per NEC 450.3 (B), NEC 240.3 and NEC 240.6 (A).
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.
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 not required.
Diagram B
3. Primary and secondary protection is required if the
transformer has more than two wires on the secondary
circuit.
Diagram C
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 not required. Secondary
OCP is required as shown below.
Diagram D
Primary Current Overcurrent Protection Rating
Less than 2 amps 300% maximum
2 to 9 amps 167% maximum
9 amps or more
125% of rated primary current (or
next highest standard rating)
Primary Current Overcurrent Protection Rating
Less than 2 amps 300% maximum
2 to 9 amps 167% maximum
9 amps or more
125% of rated primary current (or
next highest standard rating)
Primary
Current
Secondary
Current
Overcurrent
Protection Rating
250% primary current Less than 9 amps 167% maximum
Not more than 250% 9 amps or more
125% (or next higher
standard rating)
Primary Current Secondary Current
Overcurrent
Protection Rating
250% primary current Less than 9 amps 167% maximum
Not more than 250% 9 amps or more
125% (or next higher
standard rating)