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Transformer breaker and fuse size calculator

Enter the transformer's rated currents and how it's protected. You get the maximum primary and secondary breaker or fuse rating from Table 450.5(A) or 450.5(B).

Built on the 2026 NEC · Free, no sign-up · Updated October 2026

Rated primary current.

Required when protection is primary and secondary.

Required for over 1000V. Table 450.5(A) covers up to 10%.

Required for over 1000V. Table 450.5(A) has separate breaker and fuse columns.

Required for over 1000V with secondary protection.

Applies only to over 1000V.

The result updates as you type.

Result

Primary ≤ 31 A (450.5(B))

How it was calculated

  1. Primary max OCPD = 25 × 125% = 31.25 A.
  2. Secondary OCPD: not required.
  3. Select a standard OCPD rating (240.6) at or below the maximum; Note 1 allows the next higher standard rating for the 125% values.

Assumptions

  • low-voltage transformer → Table 450.5(B).
  • Primary current 25 A → 125% (scheme: primary-only).
  • No secondary OCPD required by the chosen scheme.
  • The calculated value is the maximum OCPD rating. Where a 125% value does not match a standard rating, Note 1 permits the next higher standard rating (240.6); for the other percentages, select a standard rating at or below the value.

Sections used

450.5Table 450.5(B)

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

Transformer protection is 450.5 in the 2026 code

In earlier editions, transformer overcurrent protection was 450.3. In the 2026 NEC it is 450.5, with Table 450.5(A) for transformers over 1000 V and Table 450.5(B) for 1000 V and less. Old notes and exam books that say 450.3 point to the same rule.

Transformers 1000 V and less

  • Primary-only protection: 125% of rated primary current when it is 9 A or more, 167% from 2 A to under 9 A, and 300% under 2 A.
  • Primary and secondary protection: the primary can go up to 250%, and the secondary is 125% when the secondary current is 9 A or more, 167% when it is less.

Table 450.5(B) notes let you round up to the next standard size in some cases. Read them before you finalize.

Protecting the transformer isn't the whole job

Table 450.5 protects the transformer. The primary and secondary conductors still need their own protection under 240.4 and 240.21(C). Check the secondary conductors with the tap rule calculator.

2023 to 2026: our list has 1 change in Article 450. See the changes

Worked example

A transformer under 1000 V with a 25 A primary, primary-only protection:

  • Voltage class: Low-voltage (≤1000V)
  • Primary FLA (amps): 25
  • Protection scheme: Primary only
  • Secondary FLA (amps): 100
  • OCPD type: Fuse
  • Secondary voltage: Secondary ≤1000V
  • Supervised location (450.5(A) supervised rows): no

Result

Primary ≤ 31 A (450.5(B))

How it was calculated

  1. Primary max OCPD = 25 × 125% = 31.25 A.
  2. Secondary OCPD: not required.
  3. Select a standard OCPD rating (240.6) at or below the maximum; Note 1 allows the next higher standard rating for the 125% values.

Assumptions

  • low-voltage transformer → Table 450.5(B).
  • Primary current 25 A → 125% (scheme: primary-only).
  • No secondary OCPD required by the chosen scheme.
  • The calculated value is the maximum OCPD rating. Where a 125% value does not match a standard rating, Note 1 permits the next higher standard rating (240.6); for the other percentages, select a standard rating at or below the value.

Sections used

450.5Table 450.5(B)

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

Questions electricians ask

What does NEC 450.5 cover?

NEC 450.5 covers overcurrent protection for transformers in the 2026 code. Table 450.5(A) is for transformers over 1000 V and Table 450.5(B) is for 1000 V and less. The calculator on this page finds the maximum primary and secondary rating from the rated currents you enter.

What size breaker do I need for a 45 kVA transformer?

Work out the rated primary current first: kVA × 1000 ÷ (voltage × 1.732) for three-phase. A 45 kVA at 480 V three-phase is about 54 A. Enter that in the calculator. With primary-only protection, the maximum is 125%, or about 68 A. The table's notes let you go to the next standard size in some cases.

Where did 450.3 go?

In the 2026 NEC, transformer overcurrent protection is in 450.5. The tables are now 450.5(A) and 450.5(B).

Does the secondary always need its own breaker?

Not always for the transformer, but the secondary conductors need protection under 240.21(C). Some of those rules require them to terminate in a single overcurrent device.

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How this calculator was built

The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.

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