Control Transformer Sizing & Fusing
It is common that Industrial Control Engineer’s are designing a control system that includes a control power transformer (CPT).
It is not always necessary to provide overcurrent protection for both the Primary and Secondary of the CPT, and selecting when you need both will depend greatly on the individual application the CPT is intended for.
In order to calculate the required protection, you must first calculate the Full-Load Current (FLA) of the CPT. For a single-phase transformer, the following formula is used:
Amps = Power/Voltage—Where power is in VA, Watts, or VAR
If both primary and secondary protection are to be used, then this calculation must be done twice, using the voltage of the respective winding.
Once the FLA has been calculated, use the following table to calculate the required protection for Primary and Secondary.
| Maximum Rating of Overcurrent Protection for Transformers Less than 600 Volts | |||||
| Protection Method | Primary Protection | Secondary Protection | |||
| More than 9A | 2A to 9A | Less than 2A | More than 9A | Less than 9A | |
| Primary only protection | 125% or NH | 167% | 300% | Not required | Not required |
| Primary and secondary protection | 250% | 250% | 250% | 125% or NH | 167% |
| NH: Next Higher Standard Size, if calculated size does not correspond to a standard fuse or circuit breaker rating. | |||||
Reference: overcurrent-protection-transformer, NEC 450.3, UL508A-42.1, 50.6