8.5 Transformer Sizing, Protection, and Grounding

Transformer question map

A transformer changes voltage, but an exam question about a transformer usually asks one of four things: current, overcurrent protection, conductor sizing, or grounding. Treat those as separate paths. Article 450 covers transformer protection and installation rules. Article 240 covers conductor overcurrent protection, including transformer secondary conductor rules. Article 250 covers grounding and bonding for separately derived systems. Article 310 supplies conductor ampacity.

Use this map before doing math:

Current formulas

For single-phase transformers:

I = VA / V

For three-phase transformers:

I = VA / (1.732 x V)

Remember that kVA must be converted to VA:

45 kVA = 45,000 VA

Example 1: A 25 kVA single-phase transformer has a 240 V secondary.

I = 25,000 VA / 240 V = 104.2 A

Example 2: A 75 kVA three-phase transformer has a 480 V primary.

I = 75,000 VA / (1.732 x 480 V) = 90.2 A

Those current values are not automatically breaker sizes. They are starting points. The question must say whether it wants current, conductor ampacity, overcurrent device rating, or equipment grounding conductor size.

Transformer overcurrent protection

Transformer overcurrent protection is addressed in Article 450. The allowed primary and secondary protection depends on transformer type, voltage, current, and whether protection is primary-only or primary-and-secondary. The table navigation is more important than memorizing every percentage. Read the row and notes carefully.

A common error is applying ordinary conductor protection rules directly to the transformer without checking Article 450. Another error is stopping at Article 450 and forgetting that the conductors connected to the transformer have their own protection requirements. Transformer protection and conductor protection overlap, but they are not identical.

When the calculated overcurrent size does not match a standard rating, Article 450 and Article 240 rules may allow a next standard size in specific circumstances. Do not automatically round up every transformer number. The permissive rule must exist for the condition in the stem.

Secondary conductors

Transformer secondary conductors are a major trap. The primary overcurrent device protects the transformer primary circuit and may protect the transformer in some arrangements, but it does not automatically protect every secondary conductor from overload or short circuit in the way a branch-circuit breaker protects downstream conductors. Article 240 has specific secondary conductor rules with length limits, installation conditions, termination requirements, and overcurrent protection placement.

Case: A 480 V to 208Y/120 V transformer feeds a panelboard 8 feet away. The secondary conductors may qualify under a transformer secondary conductor rule if all conditions are met. Now move the panelboard 40 feet away through a warehouse. The answer may change because length and protection conditions change. On the exam, pay attention to the exact conductor length.

Separately derived system grounding

A transformer secondary that has no direct electrical connection to the primary conductors is commonly a separately derived system, except for equipment grounding and bonding connections. That means the secondary system needs a grounding and bonding arrangement under Article 250. The system bonding jumper connects the grounded conductor to the equipment grounding conductors and transformer or first disconnect enclosure at the permitted location. A grounding electrode conductor connects the derived system to the grounding electrode system as required.

The usual grounding question is: where is the neutral bonded? It should be bonded at the separately derived system location permitted by Article 250, not repeatedly bonded at every panel downstream. Multiple neutral-to-ground bonds on the load side can place normal neutral current on metal raceways, equipment grounding conductors, and building steel.

Transformer cases

Case 1: A 45 kVA three-phase transformer supplies a 208Y/120 V panel. Secondary current is:

45,000 / (1.732 x 208) = 124.9 A

The candidate then checks conductor ampacity, secondary conductor protection, panel rating, and grounding. The answer is not complete at 125 A unless the question asks only for secondary current.

Case 2: A dry-type transformer is installed in a storage room. Besides electrical sizing, check ventilation, working space, physical protection, sound/vibration concerns, and suitability for the environment. NEC installation rules and manufacturer instructions matter.

Case 3: A separately derived 120/208 V system has the neutral bonded in the transformer and again in the downstream panel. That is a classic objectionable-current trap unless the specific system arrangement allows it. Usually one bonding point is intended for the derived system.

Study workflow

Build a transformer worksheet with four lines: primary current, secondary current, transformer OCP, secondary conductor rule. Add a fifth line for SDS grounding: bonding jumper location, grounding electrode conductor, equipment grounding conductor, and panel neutral isolation. Use the worksheet every time, even for short questions. Consistency prevents mixing Article 450, 240, 250, and 310 in the wrong order.