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.

UL 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
			 
			 
			 
			 
			 
			 
			 
		

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.

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.