Transformer Sizing Calculator

About this calculator

How to size the primary fuse or breaker

Find the transformer rated current, then apply NEC Table 450.3(B) for transformers 1000 volts and less. Primary protection only is 125 percent of primary current when that current is 9 amperes or more. Note 1 lets you take the next standard rating from 240.6 when 125 percent does not land on a standard fuse or breaker.

• Primary current, three-phase: kVA × 1000 ÷ (volts × √3).
• Primary current, single-phase: kVA × 1000 ÷ volts.
• If the secondary is also protected, Table 450.3(B) allows a higher primary percentage. Read the row that matches your scheme. Do not use the 125 percent primary-only number when you are actually using primary and secondary protection.

Worked example

A 75 kVA, 480 volt, three-phase transformer, primary protection only:

I = 75,000 ÷ (480 × 1.732) = 90.2 A
125% × 90.2 A = 113 A
Next standard device at or above 113 A is 125 A

A 125 ampere primary fuse or breaker is the Note 1 result for this primary-only case. Secondary protection, continuous load, and the conductor ampacity are separate checks.

Common questions

How do I size a transformer primary breaker?

Divide the transformer kVA by the primary voltage (and by √3 for three-phase) to get rated current, multiply by the Table 450.3(B) percentage for your protection scheme, then pick a standard fuse or breaker. For primary-only protection at 9 amperes or more, that percentage is 125, and Note 1 allows the next size up.

Does the same percentage size the secondary breaker?

No. When both sides are protected, the secondary device is its own row in Table 450.3(B), commonly 125 percent of secondary current when that current is 9 amperes or more. Conductors are sized from the load and 310.16, not by copying the transformer percentage.

For design, installation, or safety decisions, verify assumptions against the applicable code edition, equipment documentation, and site requirements.