Transformer Current and Overcurrent Protection Calculator
Calculate transformer primary and secondary rated current, review maximum overcurrent-device ratings, and compare manufacturer-specified RMS inrush data.
Nameplate details
Transformer protection method
Select only when the transformer manufacturer provides this coordinated protection arrangement.
Inrush current comparison
Use only an RMS inrush value for the selected transformer. Peak values and time-current coordination are not evaluated.
Transformer current and protection
Calculated at the entered winding voltages using the selected phase and kVA rating. These are rated-current estimates.
Maximum overcurrent-device ratings
Table 450.5(B), primary protection where separate secondary protection is provided.
Table 450.5(B), secondary protection at 9 A or more.
About this calculator
Enter a two-winding transformer's kVA rating, phase, and primary and secondary voltages to calculate rated line current. Select a primary-only or primary-and-secondary protection arrangement to see the corresponding maximum overcurrent-device ratings under Table 450.5(B). You can also compare a manufacturer-specified RMS inrush current with rated primary current.
This reference is limited to transformers covered by Table 450.5(B), with windings rated 1000 V or less. For motor-control-circuit transformers, verify the applicable Article 430.72 requirements before using these values. It does not select an installed device or determine conductor protection or protective-device coordination.
Method and assumptions
For single-phase transformers, rated line current = kVA × 1,000 ÷ winding voltage. For balanced three-phase transformers, rated line current = kVA × 1,000 ÷ (√3 × line-to-line winding voltage). The calculation uses the transformer's kVA rating on both sides and excludes losses and unbalanced loading. The optional inrush multiple = manufacturer-specified RMS inrush current ÷ rated primary current.
Table 450.5(B) primary-only protection uses 125% of rated primary current at 9 A or more, 167% below 9 A and at least 2 A, and 300% below 2 A. When primary and secondary devices are provided, the primary limit is generally 250%; the secondary limit is 125% at 9 A or more and 167% below 9 A.
For a 125% table calculation, the next higher standard rating is permitted only when the calculated rating is not standard and the device is a fuse or nonadjustable circuit breaker. This tool does not apply that allowance to adjustable breakers or to other table percentages.
A separate Table 450.5(B) provision can increase the primary limit when manufacturer-coordinated thermal overload protection interrupts primary current and secondary protection is also provided. For impedance of 6% or less, that limit is 600%; for impedance greater than 6% and not more than 10%, it is 400%. Enter impedance only when the equipment uses that exact manufacturer-provided arrangement. Above 10%, this exception does not apply.
The inrush ratio is a comparison only. The worksheet does not model current waveforms, inrush duration, protective-device response curves, or coordination.
Example
A 25 kVA single-phase transformer rated 480 V primary and 120 V secondary has rated currents of 52.08 A primary and 208.33 A secondary. With primary-only protection, the table maximum is 125% of primary current, or 65.10 A. Because that value is not a standard rating, the next higher standard rating permitted by the table note is 70 A for a fuse or nonadjustable breaker.
With separate primary and secondary protection, the primary table limit is 250%, or 130.21 A. The secondary limit is 125%, or 260.42 A; the next higher standard rating permitted by the note is 300 A. These are table limits, not device selections.
Interpret the results
Use the rated primary and secondary current for preliminary calculations. Compare each overcurrent-device rating with the maximum shown for the selected protection method. If the 125% note applies, the displayed next standard rating is a permitted upper bound, not an automatic recommendation.
Primary-only transformer protection does not by itself verify protection for secondary conductors. Check conductor rules, transformer type, grounding, available fault current, inrush, equipment instructions, and locally adopted requirements separately.
FAQs
Which voltage should I enter?
Enter the nameplate line-to-line voltage for each side. For a single-phase transformer, enter the voltage across the primary winding and the voltage across the secondary winding.
Does this calculate current for a three-phase transformer?
Yes. Select three-phase and enter the primary and secondary line-to-line voltages. The calculation assumes a balanced three-phase load.
Does Table 450.5(B) apply to motor-control-circuit transformers?
Do not assume it applies. Verify the applicable Article 430.72 requirements before using these Table 450.5(B) values for a motor-control-circuit transformer.
What is the difference between primary-only and primary-and-secondary protection?
Table 450.5(B) provides different maximum ratings depending on whether a separate secondary overcurrent device is provided. Primary-only protection does not establish protection for the secondary conductors; check their protection separately.
When can the next standard device rating be used?
The table note allows the next higher standard rating only for a 125% calculation that does not correspond to a standard fuse or nonadjustable circuit-breaker rating. The reference does not apply that allowance to adjustable circuit breakers or to the other table percentages.
When does transformer impedance affect the result?
Impedance affects the special primary-protection limit only when the manufacturer provides coordinated thermal overload protection arranged to interrupt primary current and separate secondary protection is selected. Enter the nameplate impedance and verify the complete arrangement with the manufacturer.
Does this select a conductor or device for installation?
No. It reports rated winding current and maximum table calculations. It does not determine secondary-conductor protection, select an installed device, or verify the transformer configuration, inrush, coordination, listing, and local requirements.
Why are primary and secondary currents different?
For the same kVA rating, current is inversely proportional to voltage. A lower-voltage secondary therefore carries more current than a higher-voltage primary.
Can I estimate transformer inrush from kVA or impedance?
No. Use an RMS inrush value supplied by the transformer manufacturer for the applicable test condition. This comparison reports a multiple of rated primary current but does not check protective-device coordination or transient performance.
Sources
- NFPA 70, National Electrical Code, 2026 edition — Section 430.72(C), Section 450.5(B), and Table 450.5(B). Verify the edition adopted by the authority having jurisdiction.
- NFPA Code-Making Panel 9 Second Draft Report, preliminary report — public code-development material documenting the 2026 Article 450 section and table numbering; it is not a substitute for the adopted final code.
- NFPA Code-Making Panel 11 Second Draft Report, preliminary report — public code-development material on motor-control-circuit transformer protection under 430.72(C); it is not a substitute for the adopted final code.
- Use the transformer nameplate and manufacturer instructions to confirm the winding ratings, impedance, and coordinated thermal-protection arrangement.