A motor control circuit utilizes a Class 2 power-limited transformer for its control power. The primary side of the transformer is connected to a 480V circuit, and the secondary side provides 24V AC. According to NEC 430.72(A), which statement accurately describes the overcurrent protection requirements for the primary side of this control transformer?
- AThe primary side does not require separate overcurrent protection if the branch-circuit overcurrent device is rated 20A or less.
- BThe primary side must be protected at not more than 125% of the transformer's rated primary current.
- CThe primary side must be protected at not more than 200% of the transformer's rated primary current.
- DThe primary side overcurrent device must not be rated over 20A for a 480V primary.
Show answer & explanationAnswer & explanation
Correct answer: A. The primary side does not require separate overcurrent protection if the branch-circuit overcurrent device is rated 20A or less.
NEC 430.72(A) Exception No. 2 specifies that if the control circuit transformer is a Class 2 power-limited transformer, primary overcurrent protection is not required where the control circuit conductors are protected by the branch-circuit overcurrent protective device. This exception typically applies when the branch-circuit overcurrent device is rated 20A or less, as defined in other articles (e.g., 725.121(A)(1) for Class 2 circuits). The question implies this condition by stating 'Class 2 power-limited transformer'. If the branch circuit OCPD protects the primary, no separate primary protection is needed.
Why the other options are wrong
- B. This is a general rule for transformers, but specific exceptions apply for Class 2 transformers in motor control (430.72(A) Exception No. 2).
- C. 200% is not the correct maximum for primary protection of control transformers; typically it's 125% or specific values based on transformer rating (NEC 430.72(C)).
- D. This is incorrect. The 20A limit is usually for the branch circuit OCPD that *protects* the primary, not a separate primary OCPD rating for a 480V primary.
Control Transformer Overcurrent Protection (NEC 430.72(A))
NEC 430.72(A) outlines the requirements for overcurrent protection of control circuit transformers, including specific exceptions for Class 2 power-limited transformers where protection can be provided by the branch-circuit overcurrent device.
- Primary protection is generally required at not more than 125% of the rated primary current.
- Exception No. 2 allows omission of primary protection for Class 2 power-limited transformers if the branch-circuit OCPD provides adequate protection.
- Secondary protection is also required, typically based on conductor ampacity.
Memory trick: Primary protection: 125% or branch circuit for Class 2, don't let it burn out!