Wiring - Transformer Overcurrent Protection for Primary and Secondary Windings Using Fuses or Circuit Breakers

Power transformers and control transformers both require primary circuit protection and secondary circuit protection is either required or recommended also.  Here is how to calculate the overcurrent protections amps.

transformerthumb-burned.pngUL 508A section 35 covers power transformers and section 42 covers control transformers which is the same requirements set out in the National Electric Code (NEC) Article 450.

See chart at the bottom of the page for quick access to transformer fuse recommendations.

Primary protection goes on the incoming side of the transformer or the primary winding.  Secondary protection goes on the outgoing side of the transformer or the secondary winding.

Primary and Secondary Fusing Calculation

Why use secondary transformer protection in addition to primary transformer protection? The advantage of having primary and secondary protection is it allows you to up-size the primary protection to prevent nuisance trips. 

Transformer Primary Transformer Secondary
Current (amps) Maximum overcurrent protection % Current (amps) Maximum overcurrent protection %
9 amps or more 250% 9 amps or more 125%
2-8.99 amps 250% Less than 9 amps 167%
Less than 2 amps 300%    

Example.  You have a 2KVA (2000 VA) transformer and your primary voltage is 460VAC and your secondary voltage is 120VAC.  

Primary Amps = VA/Primary VAC = 2000/460 = 4.35 amps.  According to the chart above, 4.35 amps is in the range of the middle row, 2-8.99 amps, so the overcurrent protection should be 250% of the primary full load current.  4.35*250% = 10.88 amps.  You can then round to the next fuse size which would be 15 Amps.

Secondary Amps = VA/Secondary VAC = 2000/120 = 16.67 amps.  According to the chart above, 16.67 amps is in the range of the top row, 9 amps or more, so the overcurrent protection should be 125% of the primary full load current.  16.67*125% = 20.84 amps.  You can then round to the next fuse size which would be 25 Amps.

Primary Fusing Size Calculation When Not Fusing Secondary

The primary fuses are sized as a % over the primary current in amps according to the chart below when secondary fusing is not used.

Transformer primary current (amps) Maximum overcurrent protection %
9 amps or more 125%
2-8.99 amps 167%
Less than 2 amps 500%

Example.  You have a 2KVA (2000 VA) transformer and your primary voltage is 460VAC and your secondary voltage is 120VAC. 

Primary Amps = VA/VAC = 2000/460 = 4.35 amps. According to the chart above, 4.35 amps is in the range of the middle row, 2-8.99 amps, so the overcurrent protection should be 167% of the primary full load current.  4.35*167% = 7.26 amps.  You can then round to the next fuse size which would be 10 Amps.

See primary fuse chart at the bottom of this article to quickly determine your fuse size.

Primary and Secondary Fuse Chart

KVA
  50.0 75.0 100.0 150.0 250.0 350.0 500.0 750.0 1000.0 1500.0 2000.0 3000.0 5000.0 10000.0 15000.0 20000.0
Primary                                
440 460 480 0.3 0.5 0.7 1 2 2 3 5 5 8 11 16 27 54 82 109
220 230 240 0.6 0.9 1.3 2 3 4 5 8 10 16 21 31 52 104 156 208
110 115 120 1.3 2 3 4 5 8 11 16 22 33 43 65 109 217 326 435
                                 
Secondary                                
220 230 240 0.4 0.5 0.7 1 2 3 4 5 7 11 11 16 27 54 82 109
110 115 120 0.7 1.1 1.5 2 4 5 7 11 11 16 22 33 54 109 163 217
24 3 5 7 10 13 18 26 39 52 78 104          
12 7 10 10 16 26 36 52 78 104              

 

Conclusion

While you may be tempted to leave off the secondary protection to save money, including it allows you to use a much larger primary protection which will eliminate nuisance trips.  In the electrical portion of our articles section you will find many other articles on electrical sizing such as:

Next Steps

Go to the Control and Power Wiring Lesson series to select your next lesson.  There are also many other Lesson Series on PLC Programming and Industrial Automation.

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