3.5 Transformers and Separately Derived System Decisions

Transformer Decisions After the Service

Transformers often sit just downstream of service equipment, but they can change the entire grounding, bonding, and overcurrent analysis. A common arrangement is a 480Y/277 volt service feeding a dry-type transformer that supplies 208Y/120 volt panelboards. Another is a service feeding a separately derived 120/240 system for a specific load.

The master electrician must decide whether the transformer secondary is a separately derived system, where the system bonding jumper belongs, how the grounding electrode connection is made, how secondary conductors are protected, and what fault current is available downstream.

A separately derived system exists when derived conductors have no direct electrical connection to supply conductors except through grounding and bonding connections. A transformer with isolated primary and secondary windings usually qualifies. An autotransformer may not because it has an electrical connection between windings. A generator, UPS, or transfer equipment can also raise separately derived system questions depending on switching of the grounded conductor. Do not answer from equipment name alone; answer from the electrical connection shown.

Grounding and bonding are the core exam issue. At service equipment, the grounded service conductor is bonded to the service disconnect enclosure and grounding electrode system at the service bonding point. Downstream feeders normally separate grounded conductors from equipment grounding conductors. A separately derived system creates a new system bonding decision. The derived system grounded conductor is bonded to the equipment grounding conductors and grounding electrode connection at the permitted location, often at the transformer or first disconnecting means.

The exact location and conductor sizing follow the grounding and bonding article.

Use this comparison:

Transformer secondary conductor protection is another high-value topic. Primary overcurrent protection may not fully protect secondary conductors, especially when the transformer ratio changes voltage and current. Secondary conductors may need overcurrent protection at a specified location unless a tap rule or transformer rule permits the installation. Length, routing, ampacity, termination, and number of disconnects matter. The exam will often give a transformer rating, primary voltage, secondary voltage, conductor length, and overcurrent device location, then ask whether the secondary conductors are protected.

Calculate transformer current from kVA. For single-phase, amperes equal kVA times 1000 divided by voltage. For three-phase, amperes equal kVA times 1000 divided by voltage times 1.732. A 75 kVA, 480 to 208Y/120 three-phase transformer has secondary full-load current of about 208 amperes because 75,000 divided by 208 divided by 1.732 is about 208. This current is the starting point for secondary conductor and overcurrent decisions, not the end of the design.

Fault current on transformer secondaries can be very high close to the transformer. The approximate maximum secondary fault current can be estimated from full-load current divided by transformer impedance per unit, ignoring upstream impedance for a conservative initial screen. For example, a transformer with 208 amperes full-load current and 5 percent impedance has an idealized secondary fault current near 4,160 amperes. Utility contribution, primary system, conductor impedance, and motor contribution can affect the final result.

Main distribution equipment downstream must still be rated for the available fault current at its terminals.

Ventilation and installation environment also matter. Dry-type transformers generate heat and sound. They need working space where required, clearances per listing and instructions, suitable ambient conditions, and protection from physical damage. A transformer installed above a suspended ceiling, in a storage room, or near combustible material may raise accessibility, ventilation, and listing concerns. Master-level supervision includes verifying that the transformer can be maintained, that terminations can be torqued and inspected, and that the room can reject heat.

Inrush current affects overcurrent device selection. A transformer may draw high magnetizing inrush when energized, so primary protection must allow energization while still protecting conductors and equipment within code limits. Selecting too small a primary breaker can create nuisance tripping; selecting too large a device can leave conductors or transformer protection noncompliant. The answer is not guesswork. Use transformer overcurrent rules, manufacturer data, and the conductor protection path.

Exam questions often blur services and separately derived systems. If a transformer is downstream of service equipment, its secondary is not a new service. It may be a separately derived system, but it is not supplied directly by the serving utility as service conductors. That distinction affects disconnect terminology, grounded conductor bonding, feeder classification, and overcurrent protection. When in doubt, mark the service disconnect first, then mark the transformer, then decide whether the secondary is separately derived.

A field-ready transformer design note should state primary overcurrent device, primary conductor size, transformer kVA and impedance, secondary voltage, secondary full-load current, secondary conductor size and length, secondary overcurrent location, bonding point, grounding electrode connection, available secondary fault current, and equipment ratings. That level of detail is what separates a master-level design from a parts list.