Calculate maximum overcurrent protection device (OCPD) sizes for transformers per NEC 450.3 or CEC Rule 26-254. Supports single-phase and three-phase, supervised and unsupervised locations.
Based on NEC 450.3 / CEC Rule 26-254
When 125% does not correspond to a standard OCPD rating, the next higher standard size is permitted.
Primary FLA
90.21A
125% = 112.77A
Secondary FLA
208.19A
Calculation Breakdown
1.Phase: Three-phase
2.Primary FLA = 75 kVA x 1000 / (480V x 1.732) = 90.21A
3.Secondary FLA = 75 kVA x 1000 / (208V x 1.732) = 208.19A
4.Impedance: 5%
5.Location: Supervised
6.Protection: Primary Only
7.Applying NEC Table 450.3(B) (1000V and below transformers)
8.Impedance <= 6%: Primary max = 125%
9.Note: Next standard size up permitted if 125% does not match a standard OCPD rating
10.Primary max OCPD amps = 90.21A x 125% = 112.77A
11.Primary OCPD: next standard size up = 125A
Per NEC 450.3: Overcurrent protection for transformers. Table 450.3(A) applies to transformers over 1000V; Table 450.3(B) applies to 1000V and below.
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For Reference Only: This calculator is provided for educational and informational purposes only. It is not intended to be a substitute for professional engineering design, advice, or certification.
Code Compliance: Calculations are based on the 2023 National Electrical Code (NEC®). However, local codes, amendments, and Authority Having Jurisdiction (AHJ) requirements vary by location and may supersede standard NEC requirements.
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NEC Reference: NEC 450.3 / CEC Rule 26-254
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