8.6 Transformer Primary, Secondary, and Grounding Decisions

Transformers Create A New System Boundary

A transformer changes voltage, current, and often grounding relationships. It may also create a separately derived system because the secondary conductors are not solidly connected to the primary circuit conductors. That means a transformer question is not just an ampacity question. It can involve primary overcurrent protection, secondary overcurrent protection, secondary conductor protection, grounding electrode connection, system bonding jumper, equipment grounding conductors, ventilation, working space, fault current, and identification of grounded conductors.

Start with the nameplate and system diagram. Record kVA, primary voltage, secondary voltage, phase, frequency, impedance, connection type, temperature rise, enclosure type, and location. Then write whether the question is asking about primary conductors, primary overcurrent protection, secondary conductors, secondary overcurrent protection, grounding and bonding, or available fault current. The biggest transformer exam errors come from applying a primary rule to secondary conductors or assuming the primary breaker protects everything downstream.

Basic Current Formulas

Use kVA formulas before code tables. For a single-phase transformer, current equals kVA x 1000 divided by volts. For a three-phase transformer, current equals kVA x 1000 divided by volts x 1.732. Use primary voltage for primary current and secondary voltage for secondary current.

Example: a 75 kVA, 480 V to 208Y/120 V, three-phase transformer has primary current of 75,000 / (480 x 1.732), about 90 A. The secondary current is 75,000 / (208 x 1.732), about 208 A. Those are different currents because power is transferred at different voltages. Do not size the secondary conductors from the primary current.

Primary And Secondary OCP

Transformer overcurrent protection rules depend on voltage, impedance, whether primary-only protection is allowed, whether secondary protection is provided, transformer type, and conductor protection. A small transformer may have different permitted percentages than a large transformer. The exam often gives answer choices that are close standard sizes. Keep the calculated current and code-permitted percentage separate, then apply standard-size rules only where allowed.

The primary overcurrent device protects the primary conductors and transformer under its rule. It does not automatically protect secondary conductors, especially where the secondary current is much higher. Secondary conductors may be protected by a secondary overcurrent device or may qualify under specific secondary conductor rules with length, routing, ampacity, termination, and location conditions. These are sometimes called transformer secondary conductor rules, but the important point is that they are conditional.

If the secondary conductors are too long, routed improperly, or terminate incorrectly, the exception may not apply.

Secondary Conductor Trap

A 75 kVA transformer with 208 A secondary current feeding a panel 40 ft away cannot be approved by saying the 480 V primary breaker is already installed. The secondary conductors need protection according to the transformer and conductor rules. If a secondary breaker is mounted at the transformer or the conductors meet a permitted secondary conductor rule, the installation may be acceptable. If not, those conductors may be underprotected.

On exam scratch paper, write: primary OCP, transformer, secondary conductors, secondary OCP. Then place each device and conductor physically on the one-line. This prevents a common false assumption that current magically stays the same across the transformer.

Separately Derived System Grounding

Most common dry-type transformers used to create a 208Y/120 V system from a 480 V primary are separately derived systems. The secondary neutral must be bonded to the equipment grounding system at one permitted point by a system bonding jumper. A grounding electrode conductor must connect the derived system to the grounding electrode system as required. Equipment grounding conductors must provide the effective fault-current path back to the derived source.

Bonding at the wrong point can create objectionable current on metal raceways and equipment grounding conductors. Failing to bond the derived neutral can prevent a secondary ground fault from clearing promptly. The main bonding jumper at the service does not bond the separately derived secondary unless the required connection is made for that derived system.

Where the transformer secondary is delta, corner-grounded delta, ungrounded delta, or high-leg delta, identification and grounding rules change. A high-leg conductor in a 4-wire delta system has special identification requirements and cannot be casually used for 120 V loads. Corner-grounded delta systems have one phase intentionally grounded and require careful marking and overcurrent device selection. Ungrounded systems may require ground detectors and maintenance procedures.

Location, Ventilation, And Working Space

Transformers produce heat and noise. Dry-type transformers need ventilation clearance and must be installed in locations suitable for their enclosure and temperature. Do not block ventilation openings or bury a transformer above an inaccessible ceiling. Working space rules apply to equipment likely to require examination, adjustment, servicing, or maintenance while energized. Seismic support, vibration isolation, fire-rated rooms, and environmental exposure may matter under the adopted codes and specifications.

Transformer rooms also raise conductor and raceway issues. Large secondary conductors may have significant bending radius needs. Short secondary conductor rules may require conductors to be protected from physical damage and terminate in a single overcurrent device or grouped devices. A field reroute around an obstruction can turn a compliant secondary conductor length into a violation.

Fault Current And Equipment Ratings

Transformer impedance determines available fault current on the secondary. A low-impedance transformer can deliver high fault current that exceeds downstream equipment ratings if not checked. Available fault current is roughly related to full-load current divided by per-unit impedance, before utility contribution and conductor impedance refinements. For exam purposes, recognize that transformer replacement with a larger or lower-impedance unit can increase fault current and require panel SCCR or AIC review.

Exam Method

Use the transformer one-line as your map. Label primary voltage and current, primary OCP, transformer kVA, secondary voltage and current, secondary conductors, secondary OCP, neutral bonding point, grounding electrode conductor, and equipment grounding path. Then answer the specific question. If it asks for primary conductor ampacity, do not solve the grounding problem. If it asks whether the secondary neutral is bonded, do not size the primary fuse. Master-level speed comes from knowing which side of the transformer you are on.