How it works
FLA (3φ) = kVA × 1000 / (√3 × V). Max primary OCPD = 250% × FLA (primary+secondary) or 125% × FLA (primary-only). Max secondary OCPD = 125% × FLA.
Full-load amps come from the apparent power and the line-to-line voltage: for three-phase, FLA = kVA × 1000 / (√3 × V); for single-phase, drop the √3. The maximum overcurrent protection is a percentage of that FLA set by NEC Table 450.3(B): with primary + secondary protection you may run the primary device up to 250% of primary FLA and the secondary up to 125% of secondary FLA; with primary-only protection the single primary device is limited to 125%. Note 1 lets you round up to the next standard size in NEC 240.6(A) for the secondary device and for a primary-only device — but NOT for the 250% primary value, which must be rounded down to the next standard size at or below the limit. For transformers 1000 V and less a supervised location unlocks the higher-multiplier "Supervised locations only" columns of Table 450.3(B); those columns have no impedance condition (the 4%/6% impedance thresholds belong to Table 450.3(A), which governs transformers over 1000 V and is outside this tool's scope). The readout shows the standard (rounded) primary breaker as the headline number, with the secondary breaker, both FLAs, and the primary conductor ampacity as supporting metrics. The intermediate raw limits are the un-rounded products — for the boot 75 kVA, 480→208Y/120 V example the primary raw limit is 250% × 90.2 A = 225.5 A (rounds down to 225 A) and the secondary raw limit is 125% × 208.2 A = 260.2 A (rounds up to 300 A); the verdict names the 225 A and 260 A ceilings you must not exceed. The primary conductor is sized at 125% of primary FLA; the secondary conductor must be rated at least as high as the secondary OCPD per the transformer secondary tap rules of NEC 240.21(C) — the next-size-up allowance of 240.4(B) does not apply to it. Installation depends on type and size (NEC 450.21 / 450.23): a dry-type transformer ≤112.5 kVA may be installed indoors without a vault with the required clearances and combustible separation (450.21(A)); a dry-type >112.5 kVA needs a fire-resistant transformer room unless it is a listed fire-resistant or liquid-insulated type (450.21(B)); a liquid-filled transformer indoors generally requires a transformer vault (450.23).
Code references
- Transformers and transformer vaults — scope NEC 2023, Article 450
- Maximum OCPD, 1000 V and under (primary 125%/250%, secondary 125%) NEC 2023, Table 450.3(B)
- Rounding to the next standard size (round-up permission) NEC 2023, 450.3(B) Note 1
- Standard ampere ratings for fuses and breakers NEC 2023, 240.6(A)
- Transformer secondary conductor tap rules NEC 2023, 240.21(C)
- Next-size-up rule does not apply to secondary conductors NEC 2023, 240.4(B)
- Allowable conductor ampacities NEC 2023, Table 310.16
- Dry-type transformer installation (≤112.5 kVA and >112.5 kVA) NEC 2023, 450.21(A) and 450.21(B)
- Liquid-filled transformer installation NEC 2023, 450.23
FAQ
What is the maximum breaker size for a 75 kVA transformer?
For a 75 kVA, 480 V three-phase transformer the primary FLA is about 90.2 A. With primary + secondary protection, Table 450.3(B) allows the primary device up to 250% of FLA = 225.5 A, which rounds down to a 225 A standard breaker (Note 1 round-up is not permitted on the 250% primary value). On the 208Y/120 V secondary the FLA is about 208 A, and the secondary device is limited to 125% = 260 A. So: 225 A primary, 260 A secondary maximum.
Why can the primary be 250% but the secondary only 125%?
A transformer has a large magnetizing inrush when energized, so the primary device must tolerate that surge without nuisance tripping — Table 450.3(B) allows up to 250% of primary FLA when a separate secondary device handles the actual overload protection. The secondary device sits between the transformer and the load and is sized at 125% to actually protect the transformer winding and the secondary conductors against overload, so it is held to a tighter multiplier.
Why does the next-size-up rule (240.4(B)) not apply to transformer secondary conductors?
NEC 240.4(B) lets you protect a conductor with the next standard OCPD above its ampacity in many cases — but 240.4(B) explicitly excludes transformer secondary conductors. Those are governed by 240.21(C): the secondary conductor must be rated at least as high as the secondary overcurrent device (or fall under a specific 10-ft / 25-ft tap rule). Sizing a secondary conductor smaller than its OCPD and leaning on 240.4(B) is one of the most common field violations.
What does a "supervised location" mean in NEC 450.3?
A supervised location is one where the conditions of maintenance and supervision ensure that only qualified persons monitor and service the installation — typically an industrial facility with an on-staff electrical crew. For transformers 1000 V and less, Table 450.3(B) gives supervised locations a separate set of columns with a higher primary overcurrent multiplier (up to 250% for primary-only). Those supervised columns have no impedance condition — being a supervised location is the only gate. (The 4%/6% impedance thresholds you may have seen belong to Table 450.3(A), which applies to transformers over 1000 V and is outside this tool's scope.) This tool applies the supervised column whenever you set "Supervised location" to Yes.
How do I size a transformer from a panel schedule?
Switch the calculator to "Size from load," total the connected load in watts from the panel schedule, enter the power factor (0.85–0.9 for a typical mixed commercial load, 1.0 for purely resistive), and add a spare-capacity percentage for future growth. The tool computes the apparent power in kVA and snaps it up to the next standard dry-type size (15 / 30 / 45 / 75 / 112.5 / 150 / 225 kVA). Then switch to "Size the breaker" with that kVA to size the primary and secondary OCPD.
This calculator is provided for estimation purposes. Transformer overcurrent protection has additional conditions and exceptions in NEC 450.3 and its table notes. Always verify results against the current NEC edition, the transformer nameplate, and local amendments with a licensed electrician or electrical engineer before specifying equipment or overcurrent devices.