Transformer overcurrent protection is one of the more confusing areas of the NEC because the rules depend on multiple variables: transformer voltage (over or under 1000V), whether the location is supervised, whether there is primary-only or primary-plus-secondary protection, and the specific impedance of the transformer. Article 450.3 uses two tables, 450.3(A) for transformers over 1000V and 450.3(B) for transformers 1000V and below, with different columns for different protection configurations.
This guide focuses on Table 450.3(B), which covers the transformers most electricians and engineers encounter: dry-type and liquid-filled units at 600V and below. It walks through the primary-only and primary-plus-secondary protection options, explains the supervised vs unsupervised distinction, and provides practical sizing examples for common transformer configurations.
Primary-Only Protection: The Simple Case
When a transformer has overcurrent protection on the primary side only (no secondary OCPD), Table 450.3(B) limits the primary OCPD to 125% of the transformer's primary full-load current. If 125% does not correspond to a standard fuse or breaker size, you may round up to the next standard size per NEC 240.6.
Example: A 45 kVA, 480V primary, 3-phase transformer. Primary FLA = 45,000 / (480 × 1.732) = 54.1 amps. 125% of 54.1 = 67.7 amps. The next standard breaker size is 70 amps. A 70A breaker on the primary is code-compliant for primary-only protection.
There is an important exception: if 125% of the primary FLA does not correspond to a standard rating AND rounding up would exceed 125%, you can use the next lower standard size instead. In practice, this exception rarely applies because rounding up is permitted. However, if the calculated value is exactly a standard size (say exactly 100A), you cannot go to 110A. 100A is the maximum.
Limitation of primary-only protection: The primary OCPD protects the transformer from overcurrent but provides limited protection for the secondary conductors. A fault on the secondary side appears as a reduced current on the primary (reduced by the turns ratio), which means the primary OCPD may be slow to respond or may not trip at all for secondary faults, especially on large transformers with high secondary currents. For this reason, secondary protection is recommended even when primary-only is code-compliant.
Primary FLA = kVA × 1000 / (Vprimary × √3) [3-phase]
Primary FLA = kVA × 1000 / Vprimary [1-phase]
Max OCPD = 125% × Primary FLA
Round up to next standard size per NEC 240.6
Example: 75 kVA, 480V, 3-phase
FLA = 75,000 / (480 × 1.732) = 90.2A
125% = 112.8A → next standard = 125A primary breaker
Transformer Protection Calculator
Size transformer overcurrent protection per NEC 450.3. Primary and secondary fuse or breaker sizing for single-phase and three-phase transformers up to 1000V.
Primary-Plus-Secondary Protection: More Flexibility
When overcurrent protection is provided on both the primary and secondary sides of the transformer, Table 450.3(B) allows the primary OCPD to be sized up to 250% of the primary FLA (compared to 125% for primary-only). This is a significant relaxation. The secondary OCPD provides the fault protection that the oversized primary OCPD does not.
The secondary OCPD must not exceed 125% of the secondary FLA. If 125% does not correspond to a standard size, round up to the next standard size. This secondary OCPD directly protects the secondary conductors and the downstream distribution equipment.
Example: A 112.5 kVA, 480V to 208/120V, 3-phase transformer. Primary FLA = 112,500 / (480 × 1.732) = 135.3A. Secondary FLA = 112,500 / (208 × 1.732) = 312.2A. Primary OCPD: 250% × 135.3 = 338.3A → next standard = 350A. Secondary OCPD: 125% × 312.2 = 390.3A → next standard = 400A.
The advantage of this arrangement is that the primary OCPD does not nuisance-trip on transformer inrush current (the magnetizing current surge that occurs when the transformer is energized, typically 8 to 12 times FLA for the first few cycles). A 125% primary OCPD can trip on inrush; a 250% primary OCPD will not.
Primary OCPD: max 250% of primary FLA
Secondary OCPD: max 125% of secondary FLA
Both can round up to next standard size per NEC 240.6.
Standard sizes (NEC 240.6(A)):
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600...
Transformer Protection Calculator
Size transformer overcurrent protection per NEC 450.3. Primary and secondary fuse or breaker sizing for single-phase and three-phase transformers up to 1000V.
Supervised vs Unsupervised Locations
Table 450.3(B) has different columns for supervised and unsupervised locations. A supervised location is defined by the NEC as a location where maintenance and supervision ensure that only qualified persons monitor and service the installation. In practice, this means an electrical room in a commercial or industrial facility with restricted access, not a location accessible to unqualified persons.
In supervised locations, primary-only protection can use up to 250% of primary FLA (instead of 125%) when the transformer impedance is 6% or less. The logic is that qualified personnel will respond quickly to abnormal conditions, and the higher setting prevents nuisance trips. This option is useful for large transformers in industrial settings where inrush tripping is a problem.
For unsupervised locations (residential, most commercial, retail, and any location accessible to the general public), the rules are stricter. Primary-only protection is limited to 125%, and primary-plus-secondary protection allows 250% primary and 125% secondary. There is no additional relaxation for impedance or any other factor.
Most commercial and light industrial installations are treated as unsupervised unless the engineer or contractor can demonstrate that the electrical room meets the supervised criteria. When in doubt, design for unsupervised rules. They always satisfy the supervised requirements as well.
• Access restricted to qualified electrical personnel
• Conditions of maintenance and supervision ensure qualified monitoring
• Typically: dedicated electrical rooms with locked access
• NOT: areas accessible to general public, tenants, or unqualified staff
When in doubt, design as unsupervised. The stricter rules always satisfy both cases.
Common Transformer Protection Scenarios
Small dry-type (15-45 kVA, 480 to 208/120V): These are the most common transformers in commercial buildings, feeding panelboards for lighting and receptacles. Primary-only protection at 125% is the simplest approach. A 30 kVA transformer at 480V has a primary FLA of 36.1A; a 45A primary breaker is code-compliant. If inrush tripping is a problem, add a secondary breaker and size the primary up to 250%.
Medium dry-type (75-225 kVA, 480 to 208/120V): These transformers frequently require primary-plus-secondary protection because the 125% primary rule results in a breaker that trips on inrush. A 150 kVA, 480V transformer draws 180A FLA; a 225A primary breaker at 125% may still trip on 10x inrush (1,800A for several cycles). Using 250% primary (450A) with 125% secondary (519A → 600A) eliminates the inrush issue while providing proper protection on both sides.
Buck-boost transformers: Small autotransformers used for voltage adjustment (208V to 240V, 240V to 208V). Article 450.4 allows these to be protected by the primary OCPD at the load rating rather than the transformer kVA rating, because autotransformers share a common winding. This typically results in a smaller OCPD than the 125% rule would require for a standard transformer of the same kVA.
Panelboard-mounted main breakers: When the transformer feeds a panelboard with a main breaker, the main breaker serves as the secondary OCPD. Size the main breaker at 125% of secondary FLA or the panelboard bus rating, whichever is less. The primary OCPD can then be sized at 250% of primary FLA for inrush tolerance.
Transformer inrush is typically 8-12× FLA for the first 0.1 seconds. A primary OCPD at 125% of FLA can trip on this inrush if the breaker has a fast magnetic trip setting. Solutions:
1. Use a breaker with adjustable instantaneous trip (set above inrush)
2. Use primary + secondary protection (250% primary)
3. Use time-delay fuses (inherent inrush tolerance)
Transformer Protection Calculator
Size transformer overcurrent protection per NEC 450.3. Primary and secondary fuse or breaker sizing for single-phase and three-phase transformers up to 1000V.