450.3
Overcurrent Protection for Transformers
Transformers need overcurrent protection to prevent damage from sustained overloads and to clear faults. But unlike a simple branch circuit, a transformer has two sides with different voltages and currents, so the protection scheme is more complex. Table 450.3(B) — the table you will use most often in everyday commercial and industrial work — covers transformers rated 1000 volts and below. It has two rows. The top row is for primary-only protection: if you are only protecting the transformer on the primary side (no secondary OCPD), the primary device can be no larger than 125 percent of the transformer's rated primary current when that current is 9 amps or more. If the primary current is between 2 and 9 amps, you can go up to 167 percent. If it is less than 2 amps, you can go up to 300 percent. The bottom row is for primary-and-secondary protection: when you install protection on both sides, the primary device can be up to 250 percent and the secondary device up to 125 percent of the respective rated currents. A critical detail is Note 1, which says that when 125 percent of the rated current does not correspond to a standard OCPD size listed in 240.6(A), you are permitted to round up to the next higher standard size. However, when using the 250 percent multiplier for primary-and-secondary protection, Note 1 does NOT apply — you cannot round up. Table 450.3(A) covers transformers over 1000 volts and provides higher percentages (up to 300 percent for circuit breakers, 250 percent for fuses) because high-voltage transformer inrush currents are more severe and the protection philosophy relies more heavily on relay coordination.
When You Need This
- Sizing the primary fuse or breaker for a control transformer in a motor control center or industrial panel
- Installing a step-down transformer for a lighting panel and selecting both primary and secondary overcurrent protection
- Determining whether you can protect a small transformer (under 9 amps primary) with primary-only protection and a higher percentage device
- Designing protection for a medium-voltage transformer (over 1000V) using Table 450.3(A)
- Preparing for a licensing exam — transformer protection sizing using Table 450.3(B) is one of the most common calculation topics
- Troubleshooting a transformer primary fuse that keeps blowing — the fuse may be undersized relative to the allowable percentage
Key Points
Common Mistakes
Applying Note 1 rounding to the 250% primary value in the primary-and-secondary row — Note 1 only applies to the 125% values, not to 250%
Forgetting the special rules for small transformers — if the primary current is less than 9 amps or less than 2 amps, higher OCPD percentages are permitted
Calculating the transformer rated current incorrectly — always divide the VA rating by the voltage to get the rated current for each side (primary current = VA / primary voltage)
Using the primary-only protection percentages when secondary protection is also installed — the two rows in Table 450.3(B) are mutually exclusive; pick one scheme
Confusing transformer overcurrent protection (450.3) with conductor overcurrent protection (240.4) — the transformer OCPD protects the transformer, and separate rules may apply to the conductors
Not verifying that the secondary conductors are properly protected — if you use primary-only protection, the primary OCPD must be sized to adequately protect the secondary conductors as well
Exam Tip
The exam will give you a transformer kVA rating and voltages, then ask you to size the primary and/or secondary OCPD. The steps are: (1) calculate primary and secondary rated currents (I = VA / V), (2) determine if the question uses primary-only or primary-and-secondary protection, (3) apply the correct percentage from Table 450.3(B), (4) check if Note 1 allows rounding up to the next standard size. Remember: 125% gets Note 1 rounding, 250% does not.
Frequently Asked Questions
Small transformers have a very high ratio of magnetizing inrush current to normal running current. A 125% device would trip on every energization. The 300% allowance prevents nuisance tripping while still providing fault protection. This is similar in principle to why motor branch-circuit OCPDs are sized well above the running current.
Primary-and-secondary protection is preferred when the secondary conductors are long or when you want tighter protection on the secondary side. It also allows you to size secondary conductors based on the secondary OCPD rating rather than the transformer capacity, which can save on conductor cost for large transformers with lightly loaded secondaries.
No. Note 1 specifically applies where 125 percent of the rated current does not correspond to a standard OCPD size. It does not apply to the 167%, 250%, or 300% values. For the 250% primary protection in the primary-and-secondary row, you must round down if the calculated value is not a standard size.
CEC Rule 26-256 covers overcurrent protection for transformers. The CEC approach is similar in structure, with different protection percentages for primary-only versus primary-and-secondary protection and special rules for small transformers, though the specific percentage values may differ from the NEC tables.
Related Code Sections
This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.