Transformer size calculator

How the transformer size and protection are calculated

This page picks a transformer size from a load and then reads Table 450.3(B) of the NEC for the largest breaker or fuse on each winding. The size is the smallest kVA at or above the load in the lists that Hammond Power Solutions prints on its full-load-current cards, and the rated current of each winding follows from the kVA and the voltage. Table 450.3(B) gives each limit as a percentage of that rated current. Standard device ratings come from Table 240.6(A). The page covers transformers of 1,000 V and less, and it does not size the conductors, allow for inrush or apply the secondary conductor rule of 240.21(C).

Load in kVA

kVA = kW / power factorA transformer is rated in kVA, so a load given in kW is divided by its power factor. 40 kW at 0.9 is 44.44 kVA. Extra capacity is an allowance of your own, and the size is picked for the load times one plus that percentage.

Transformer size

size = smallest listed kVA at or above the loadHammond Power Solutions prints three-phase sizes of 3 to 3,000 kVA and single-phase sizes of 0.5 to 333 kVA, and the page uses those lists. A 44.44 kVA load takes the 45 kVA size, and 60 kVA takes 75 kVA.

Rated current

single phase I = kVA x 1,000 / V; three phase I = kVA x 1,000 / (V x sqrt(3))The voltage is line to line for three phase. 45 kVA at 480 V is 54.13 A, and at 208 V it is 124.91 A. Mike Holt prints 54 A for the primary using 1.732 for the square root of 3, and Hammond's card prints 125 A at 208 V.

Primary protection only

maximum device = 125% of primary current, 167% from 2 to 9 A, 300% under 2 AWith protection on the primary alone, Table 450.3(B) allows 125 percent of the rated primary current when it is 9 A or more, 167 percent when it is under 9 A and 300 percent when it is under 2 A. Where 125 percent is not a standard rating, Note 1 permits the next standard rating up. 125% of 54.13 A is 67.66 A, so Mike Holt's example takes 70 A. The table asks for no device on the secondary in this case.

Primary and secondary protection

primary maximum = 250% of primary current; secondary maximum = 125% of secondary current, 167% under 9 AWith devices on both sides, the primary may be 250 percent of its rated current at any current, and the secondary 125 percent when its current is 9 A or more, or 167 percent when it is under 9 A. The page rounds the 250 percent figure down to a standard rating and takes the next size up on the 125 percent figure. For 45 kVA the primary limit of 135.32 A is a 125 A device, and the secondary limit of 156.13 A allows up to 175 A.

Standard ratings

device = standard rating of Table 240.6(A), with 1, 3, 6 and 601 A also for fusesThe device ratings come from Table 240.6(A), the same list as the standard breaker and fuse size calculator. A breaker has no standard rating under 10 A, so a small primary limit gives a fuse of 1, 3 or 6 A and no breaker rating. In editions before 2023 the smallest standard breaker was 15 A.

Worked examples from published transformer data

The default case is the 45 kVA, 480 V transformer of Mike Holt's EC&M article, with a 208 V secondary that the article does not give. The primary current is 54.13 A, 125 percent of it is 67.66 A, and the next standard rating up is 70 A. With devices on both sides, 250 percent of the primary current is 135.32 A, which allows 125 A, and 125 percent of the 124.91 A secondary current is 156.13 A, which allows 175 A.

The table below sets each figure that the sources print beside the value this page computes. Sources print their currents rounded to the nearest amp or tenth of an amp, and Mike Holt and ExpertCE use 1.732 for the square root of 3, so the computed values differ in the last digit. Device ratings agree exactly. EntoGo takes 250 percent of 180.42 A, 451.05 A, and uses 450 A.

Eight cases below run through the same functions the form uses. A 44.44 kVA load from 40 kW at 0.9 takes the 45 kVA size, and 60 kVA steps up to 75 kVA. The 150 kVA case is the EntoGo example, where the primary limit of 451.05 A allows 450 A and the secondary limit of 520.45 A allows 600 A. In the single-phase cases the voltage is the full winding voltage.

Standards and data sources

• Mike Holt, NEC Requirements for Transformers, EC&M, October 13, 2022 (2020 NEC) - The worked example of a 45 kVA, three-phase, 480 V transformer on primary protection only, with a primary current of 54 A, 125 percent of it at 68 A, and a 70 A device as the next size up. It states that Note 1 of Table 450.3(B) permits the next size up and does not require it. The article gives no secondary voltage, so the 208 V secondary of the default is this page's own choice.
• Schneider Electric, FAQ FA97084, transformer overcurrent protection under Table 450.3(B) - Primary protection at 125 percent of the rated current when the primary current is 9 A or more, with no secondary protection required by the table. A primary device up to 250 percent of the rated current when the secondary current is 9 A or more and secondary protection limited to 125 percent is provided.
• EntoGo, Sizing transformer overcurrent protection (NEC & CEC) - Table 450.3(B) for transformers of 1,000 V and less, with the 125, 167 and 300 percent entries for primary-only protection and the 250 percent primary and 125 and 167 percent secondary entries for protection on both sides. Its worked example of a 150 kVA, 480 V to 208 V, three-phase transformer gives 180 A and 416 A, a 450 A primary device at 250 percent, and a 600 A secondary device at 125 percent. It uses 451 A as 450 A, which is why this page rounds the 167, 250 and 300 percent entries down.
• ExpertCE, article on sizing primary and secondary transformer fuses - A 75 kVA, 480 V, three-phase example with a primary current of 90.2 A, 112.75 A at 125 percent, and a 125 A device as the next standard size up.
• LornMac, NEC Table 450.3(B) - The grid of Table 450.3(B) in rows and columns. Primary-only protection reads 125, 167 and 300 percent by current band, and protection on both sides reads 250 percent on the primary with 125 and 167 percent on the secondary.
• Hammond Power Solutions, Full Load Currents cards for single-phase and three-phase dry-type transformers, September 2023 - The kVA sizes this page lists, 0.5 to 333 kVA single phase and 3 to 3,000 kVA three phase, and the relations amps = kVA x 1,000 / volts for single phase and kVA x 1,000 / (volts x 1.732) for three phase. Every printed rated-current cell at 208 and 480 V three phase and at 240 and 480 V single phase is reproduced in the tests. The cards are one maker's list and not a standard of their own.

Limitations of this calculator

• Transformers of 1,000 V and less only. Table 450.3(B) covers them, and a transformer over 1,000 V on either side falls under Table 450.3(A), which this page does not apply. Autotransformers are not covered.
• The ratings are ceilings from the table and not a design. A smaller standard device is permitted, and the load, the inrush of the transformer and coordination with the devices around it decide the device chosen. No source read here gives a number for inrush, so none is applied.
• Note 1 is read as applying to the 125 percent entries only. No source read here prints the table's notes in full, so the rounding down of the 167, 250 and 300 percent entries rests on the EntoGo worked example, which uses 451 A as 450 A. Check the notes in the edition you work under.
• With primary protection only, the table puts no device on the secondary, but the secondary conductors still need protection under 240.21(C). This page does not apply that section. Conductor sizes are on the wire size calculator.
• The kVA sizes are the ones Hammond Power Solutions prints for dry-type transformers, 3 to 3,000 kVA three phase and 0.5 to 333 kVA single phase. Other makers, and oil-filled and pad-mounted units, list other sizes. Take the size and its impedance from the manufacturer's data.
• No continuous-load factor, demand factor, harmonic loading, K-factor, temperature rise, altitude or ambient derate is applied, and the extra capacity field is only your own percentage. A load with a high starting current, such as a large motor, needs a check against the transformer's data. Any current above 4,800 A on either winding is refused.
• The sources read are written against the 2014 and 2020 editions of the NEC. NFPA committee material for the 2026 cycle shows Article 450 text with a Table 450.5(A) for transformers over 1000 V, so the numbering may differ in the 2026 edition. Confirm the table and its notes in the edition your jurisdiction has adopted.
• Device ratings are the standard sizes of Table 240.6(A), with 15 A as the smallest breaker before the 2023 edition and 10 A in 2023. The page uses the 2023 list. Non-standard and adjustable ratings are not covered.
• This is a planning estimate, not a design drawing. A licensed electrician or engineer, the manufacturer's data and the authority having jurisdiction decide the transformer, the protection and the installation.

Common transformer sizing mistakes

1. Sizing the transformer on kW. Transformers are rated in kVA. 40 kW at 0.9 power factor is 44.44 kVA, and the 45 kVA size carries it at 98.8 percent. At 0.7 the same 40 kW is 57.14 kVA and takes the 75 kVA size.
2. Using one voltage for both windings. Each winding's current uses its own voltage. 45 kVA is 54.13 A on a 480 V primary and 124.91 A on a 208 V secondary. Taking the secondary current for the primary device gives a rating about 2.3 times too large.
3. Taking 250 percent without a secondary device. The 250 percent primary limit applies only when the secondary has its own protection at 125 percent or less of its rated current, or at the table's higher percentages for small currents. With 45 kVA, 250 percent allows 125 A on the primary, and without a secondary device the limit falls to 125 percent, which is 70 A.
4. Rounding the 250 percent figure up. Note 1 permits the next size up on the 125 percent entries. EntoGo's 150 kVA example takes 250 percent of 180.42 A, which is 451.05 A, and uses 450 A, not 500 A. This page rounds the 167, 250 and 300 percent entries down.
5. Reading the next size up as a requirement. Note 1 permits the next size up and does not require it. 125% of 54.13 A is 67.66 A, so 70 A is the largest permitted, and 60 A is also allowed when the load and the inrush leave room for it. Mike Holt's article makes the same point.
6. Leaving out the secondary conductors. With primary protection only, the table puts no device on the secondary, so the secondary conductors need their own protection under 240.21(C). The 45 kVA secondary carries 124.91 A, and the conductors are sized and protected as a separate step.

Common questions about transformer size

Related calculators

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Used on the same job

• Wire size calculator The conductors on each side, from the device rating.
• Voltage drop calculator Voltage drop on the feeder from the secondary.
• Point-to-point short circuit calculator The fault current at the secondary, which the protection must be rated to interrupt.