Enter the transformer primary current (or use the kVA tab to calculate it) and select the applicable multiplier to estimate the maximum primary overcurrent protection (fuse) rating. Always follow the transformer manufacturer instructions and the applicable electrical code for your installation.
- All Physics Calculators
- All Electrical Calculators
- Transformer Calculator
- Transformer Impedance Calculator
- Circuit Breaker Size Calculator
- Fusing Current Calculator
- CT Ratio to Current Calculator
- Transformer Efficiency Calculator
Fusing Transformer Formula
The following formula is used to estimate the maximum primary overcurrent protection (fuse) rating based on the transformer primary current and an overcurrent-protection multiplier. A common set of multipliers for control-circuit transformers is given in NEC 430.72(C); other transformer types and other protective devices can have different requirements.
FS = TA * OF
- Where FS is the maximum primary fuse rating (amps)
- TA is the transformer primary current (amps)
- OF is the overcurrent protection factor
- = 125% for 9 amps or more (OF = 1.25)
- = 167% for 2–8.99 amps (OF = 1.67)
- = 300% for less than 2 amps (OF = 3.00)
To calculate the maximum primary fuse rating, multiply the transformer primary current by the overcurrent protection factor.
Definition
What is transformer fusing?
Transformer fusing is selecting and installing fuses (or other overcurrent protective devices) on a transformer’s primary and/or secondary to protect the transformer and connected conductors from overcurrent and short-circuit conditions. Proper fusing helps limit damage and fire risk during faults, but it does not increase the transformer’s kVA rating or its normal current-carrying capability.
Example Problem
How do you calculate the maximum primary fuse rating for a transformer?
First, determine the transformer primary current. In this example, the primary current is 7 amps.
Next, reference the factor list above to determine which overcurrent protection factor applies. In this case, it is 167% (OF = 1.67).
Finally, calculate the maximum fuse rating required:
FS = TA * OF
FS = 7 * 1.67
FS = 11.69 amps
