# Transformer overcurrent protection, 1000 V and less

> For a transformer rated 1000 V or less, NEC Table 450.5(B) (Table 450.3(B) in the 2023 NEC) sets the largest overcurrent device as a percentage of rated current. With primary protection only, the limit is 125% of a primary current of 9 A or more; with both primary and secondary protection, the primary may go to 250% and the secondary to 125% of a secondary current of 9 A or more.

Exams: Journeyman Electrician, Master Electrician. Transformers.

## Formula

    Primary only (Ip 9 A or more): Max primary OCPD (A) = Ip × 1.25, next standard size up allowed
    Primary and secondary: Max primary OCPD (A) = Ip × 2.50, no rounding up
    Primary and secondary (Is 9 A or more): Max secondary OCPD (A) = Is × 1.25, next standard size up allowed

## Variables

- **Ip**: Rated primary current, kVA × 1,000 ÷ Ep (÷ 1.732 for three-phase) (A)
- **Is**: Rated secondary current, kVA × 1,000 ÷ Es (÷ 1.732 for three-phase) (A)
- **OCPD**: Overcurrent protective device: fuse or circuit breaker (A)

## Open your 2026 NEC to

- Table 450.5(B) (2023: Table 450.3(B)): Maximum primary and secondary overcurrent protection, as a percent of rated current, for transformers 1000 V and less
- 240.6(A): Standard ampere ratings of fuses and inverse-time circuit breakers
- 240.21(C): Secondary conductors need their own protection; the transformer table does not size it

These rules protect the transformer itself. You can protect a transformer of 1000 V or less in one of two ways: with a device on the primary side only, or with devices on both the primary and the secondary. The table gives a maximum percentage of rated current for each case.

With primary protection only, the primary device may be up to 125% of the rated primary current when that current is 9 A or more. Smaller transformers with less than 9 A of primary current are allowed higher percentages; look those up in the table. When 125% does not land on a standard rating from 240.6(A), Note 1 of the table lets you go up to the next higher standard rating.

With both primary and secondary protection, the secondary device may be up to 125% of the rated secondary current (next size up allowed when that current is 9 A or more), and then the primary device may be as large as 250% of the rated primary current. The 250% value is a ceiling: Note 1 covers only the 125% values, so if 250% falls between ratings, use the standard rating below it. The secondary conductors still need protection sized for their ampacity under 240.4 and 240.21(C).

## Worked example

A 30 kVA three-phase transformer has a 480 V primary. It will have overcurrent protection on the primary side only. Using NEC Table 450.5(B), what is the largest standard rating allowed for the primary overcurrent device?

1. Rated primary current: 30,000 VA ÷ (480 × 1.732) = 36.09 A.
2. Only the primary is protected and its current is 9 A or more, so Table 450.5(B) limits the device to 125%.
3. Maximum: 36.09 × 1.25 = 45.11 A.
4. Note 1 of the table lets you go up to the next standard rating in 240.6(A): 50 A.

**Answer: 50 A**

## How to solve it on the exam

1. Find the rated current on each side you are protecting: kVA × 1,000 ÷ E, also ÷ 1.732 for three-phase.
2. Decide which method the question uses: primary protection only, or both primary and secondary protection.
3. Primary only: multiply the primary current by 1.25 (for a primary current of 9 A or more). If the result is not a standard 240.6(A) rating, go up to the next standard rating.
4. Primary and secondary: multiply the secondary current by 1.25 and go up to the next standard rating if needed. Multiply the primary current by 2.50 and, if it falls between ratings, go down to the standard rating below it.
5. Read the question carefully: "maximum" means the largest the table allows, not the size you might usually install.

## Common mistakes

- Rounding the 250% primary value up. The next-size-up allowance applies only to the 125% values.
- Using 250% on the primary when the secondary has no overcurrent device.
- Using the secondary current to size the primary device, or the other way around.
- Forgetting 1.732 when finding the rated current of a three-phase transformer.
- Using the transformer table to protect the secondary conductors. They need protection based on their own ampacity.

## Exam tips

- Most transformer protection questions take two lookups: Table 450.5(B) for the percentage and 240.6(A) for the standard rating. Find both in your code book before exam day.
- In the 2023 NEC this table was numbered 450.3(B). Many study guides and videos still use the old number.

## Questions people ask

### What size breaker or fuse goes on the primary of a transformer?

With primary protection only, the maximum is 125% of the rated primary current when that current is 9 A or more, and you may go up to the next standard size. A 45 kVA, 480 V three-phase transformer draws 54.1 A, and 54.1 × 1.25 = 67.7 A, so 70 A is the maximum standard rating.

### When can transformer primary protection be 250%?

When the secondary also has an overcurrent device rated no more than 125% of the rated secondary current. Then the primary device may be up to 250% of the rated primary current, and that value is a ceiling with no rounding up.

### Where is transformer overcurrent protection in the 2026 NEC?

It is in 450.5, with Table 450.5(B) for transformers 1000 V and less. In the 2023 NEC the same rules were in 450.3 and Table 450.3(B).

### Can you round transformer overcurrent protection up to the next size?

Yes, for the 125% values when the current is 9 A or more: if 125% is not a standard rating, the next higher standard rating is allowed. The 250% primary limit has no such allowance.

## Sources

- [Schneider Electric FAQ: transformer primary and secondary overcurrent protection](https://www.se.com/nz/en/faqs/FA97084/)
- [AutomationDirect: Recommendations for overcurrent protection (UL and CSA)](https://cdn.automationdirect.com/static/specs/overcurrentstandards.pdf)
- [EC&M: Article 450, transformers and transformer vaults](https://www.ecmweb.com/code-basics/article-450-transformers-and-transformer-vaults)
- [EC&M: Applying NEC Art. 450 requirements for distribution transformers](https://www.ecmweb.com/design/article/55352096/applying-nec-art-450-requirements-for-distribution-transformers)

Interactive practice: https://texaselectricianexam.com/calculations/transformer-protection/