Transformer Sizing and kVA Calculations

Transformer questions come in three types: find the current from the kVA rating, find the kVA needed for a load, and size the overcurrent protection. All three use the same short formulas. This guide covers them with single-phase and three-phase examples, plus the protection rules in 450.3(B).

The formulas

E is the line-to-line voltage on the side you are calculating. A transformer has a different current on each side, so always say which side you are working on before you start.

Example 1: single-phase currents

A 25 kVA single-phase transformer has a 480 V primary and a 240 V secondary. Find the full-load current on each side.

The voltage ratio is 480 / 240 = 2, and the current ratio is the inverse: the low-voltage side carries twice the current. Power in equals power out (ignoring losses), so when voltage goes down, current goes up by the same ratio.

Example 2: three-phase currents

A 75 kVA three-phase transformer has a 480 V delta primary and a 208Y/120 V secondary.

Use 208 V (line to line) for the secondary, not 120 V. The 120 V is line to neutral and is only used for single-phase loads connected phase to neutral.

Example 3: sizing a transformer for a load

A panel will serve a calculated three-phase load of 180 A at 208 V. What size transformer is needed?

Transformers are built in standard ratings. Three-phase dry-type units are commonly made in sizes such as 45, 75 and 112.5 kVA, so the next common size above 64.8 kVA is 75 kVA. The NEC does not list transformer sizes. On the exam, the answer choices will give you the options, and you pick the smallest one that is not below your calculated value.

Single-phase version: a 240 V single-phase load of 160 A needs 240 × 160 / 1,000 = 38.4 kVA, so a 50 kVA unit (a common single-phase size above 37.5 kVA) would be chosen.

Example 4: unbalanced single-phase loads

A three-phase 208Y/120 V transformer often feeds 120 V loads connected phase to neutral. If those loads are not balanced, the most heavily loaded phase decides the size, because each of the three windings can only carry one third of the transformer's rating.

If you had sized from the 19 kVA total, phase A would be overloaded. When a question gives loads per phase, check the largest one before you add them up. The other fix is to balance the panel by moving circuits from phase A to phase C.

Overcurrent protection: 450.3(B)

Table 450.3(B) covers transformers rated 1,000 V and less. It gives two options.

When 125% does not land on a standard rating, a note to the table allows the next higher standard size. Standard ratings are listed in 240.6(A).

Applying it to the 75 kVA transformer

Notice the 250% primary value is a ceiling, not a 125% value, so the next-size-up note does not apply to it. Round that one down to the standard size that does not exceed it.

Protecting the transformer is a separate question from protecting the conductors. The primary and secondary conductors still need ampacity and protection that meet their own rules.

Secondary conductors

Conductors leaving the secondary are often not protected at their source, because there is no device there. 240.21(C) has the rules for where secondary conductors may run before they reach an overcurrent device, including length limits of 10 ft and 25 ft under specific conditions. If a question gives a secondary conductor length, that is the section to open.

Turns ratio and voltage

On a delta-wye transformer, the line-to-line voltage ratio is not the same as the turns ratio of each winding. Each delta primary winding sees 480 V, while each wye secondary winding sees 120 V (phase to neutral), so the winding turns ratio is 480 / 120 = 4 : 1. Read which ratio the question is asking for.

Checklist for transformer questions

1. Single-phase or three-phase? That decides whether 1.732 is in the formula.
2. Which side? Use that side's line-to-line voltage.
3. For sizing, pick the smallest rating that is not below the calculated kVA.
4. For protection, find the primary current, pick the method, and apply the right percentage.
5. Round up only where the 125% note allows it. Round down for ceilings like 250%.

Transformer secondaries often feed long runs where voltage drop matters. Check one with the voltage drop calculator, and see Ohm's law and power formulas for the basics behind these formulas.