4.6 Transformers and Separately Derived Systems
Key Takeaways
- A separately derived system has no direct electrical connection, including a grounded circuit conductor, to another supply system except through grounding and bonding connections.
- Transformer secondary grounding and bonding depend on whether the secondary is separately derived and whether it is required or permitted to be grounded.
- The system bonding jumper, grounding electrode conductor, and equipment grounding conductor have separate roles at transformer installations.
- Transformer overcurrent protection, conductor sizing, ventilation, and working space often appear with grounding and bonding facts in the same question.
Why transformers change the grounding question
A transformer can create a new electrical source for the secondary system. When the secondary has no direct electrical connection to the primary system conductors, other than grounding and bonding connections, it is usually a separately derived system. Common examples include many dry-type transformers that step 480 volts down to 208Y/120 volts for panels, receptacles, and lighting. Autotransformers are different because they have a direct electrical connection between primary and secondary windings.
Once a system is separately derived, the exam question shifts. You must decide whether the secondary must be grounded, where the grounded conductor is bonded, how the grounding electrode conductor is connected, and how the secondary conductors are protected. Do not answer a transformer question as if it were only a feeder.
Code-navigation table
| Question clue | Code path | Key decision |
|---|---|---|
| Dry-type transformer, primary 480 V, secondary 208Y/120 V | Article 450 and Article 250 SDS rules | Is the secondary separately derived and grounded? |
| Autotransformer | Transformer definitions and Article 450 | It may not be a separately derived system. |
| Secondary neutral bonded in transformer or first disconnect | Article 250 SDS bonding | Is the bonding location permitted and singular? |
| Secondary conductor protection | Article 240 and Article 450 | Are secondary conductors protected by primary protection or secondary OCPD rule? |
| Transformer room clearances | Article 110 and Article 450 | Working space, ventilation, access, and guarding. |
| Grounding electrode connection | Article 250 SDS GEC rules | Which electrode is used and where is the connection made? |
Separately derived system checklist
Use this short checklist before sizing anything:
- Does the secondary have a direct electrical connection to the primary circuit conductors?
- Is it an autotransformer or another arrangement that is not separately derived?
- Is the secondary system required or permitted to be grounded?
- Where is the first disconnecting means for the secondary?
- Where is the system bonding jumper installed?
- Where does the grounding electrode conductor connect?
- How are secondary conductors protected from overcurrent?
This checklist matters because a transformer installation can have correct ampacity but wrong bonding. It can also have correct bonding but inadequate secondary conductor protection.
System bonding jumper
For a grounded separately derived system, the system bonding jumper connects the grounded conductor of the derived system to the equipment grounding conductors and metal enclosures. It performs a role similar to the main bonding jumper at service equipment, but it is not the main bonding jumper. It belongs to the derived system. It may be installed at the transformer, at the first system disconnecting means, or at another permitted point depending on the NEC rule and the installation.
Only one bonding point is normally established for the derived system grounded conductor. If the neutral is bonded in the transformer and again in the secondary panel, normal neutral current can flow on raceways and equipment grounding conductors. That is the same objectionable-current problem seen in feeder panels, but now the source is a transformer secondary rather than the utility service.
Grounding electrode conductor for SDS
A separately derived system that is grounded must connect to a grounding electrode system according to Article 250. The GEC for the derived system is not the same as the primary equipment grounding conductor. The primary EGC bonds the transformer enclosure back to the supply source. The SDS GEC connects the derived system grounded conductor or bonding point to the grounding electrode system. The system bonding jumper ties the derived grounded conductor to the equipment grounding paths.
A common field arrangement is a 480 volt feeder to a dry-type transformer, a 208Y/120 volt secondary to a panel, and a grounding electrode conductor from the derived system bonding point to building steel or another permitted electrode. The neutral is bonded at the selected SDS bonding point and isolated downstream. The transformer case is bonded by the primary equipment grounding conductor and by secondary bonding as applicable.
Transformer overcurrent and conductors
Article 450 covers transformer protection, but conductor protection may send you to Article 240 and the tap rules. The exam may give a transformer kVA rating and ask for primary current, secondary current, overcurrent protection, or conductor ampacity. Start with the formula:
Single-phase: I = VA / V
Three-phase: I = VA / (1.732 x V)
For a 75 kVA, 480 volt to 208Y/120 volt three-phase transformer, primary current is 75,000 / (1.732 x 480), about 90 amps. Secondary current is 75,000 / (1.732 x 208), about 208 amps. That arithmetic does not by itself select the breaker. You still apply transformer and conductor protection rules, standard sizes, terminal ratings, and any permitted primary-only protection logic.
Field case
A shop installs a 45 kVA dry-type transformer fed from a 480 volt panel. The transformer supplies a 208Y/120 volt panel nearby. The installer runs phase conductors and a neutral to the secondary panel, bonds the neutral in the panel, and also installs a bonding strap in the transformer. During testing, current is measured on the secondary raceway under normal load. The duplicate neutral bond created a parallel path. The correction is to establish the system bonding jumper at one permitted point and isolate the neutral elsewhere.
Another case involves a transformer secondary with no grounding electrode conductor. The primary feeder includes an equipment grounding conductor, and the transformer case is bonded, but the derived system grounded conductor is not connected to the grounding electrode system as required. The installation may seem complete because every enclosure is bonded, yet the SDS grounding electrode connection is missing.
Exam traps
Do not call every transformer a separately derived system. Check for direct electrical connection. Do not call the system bonding jumper a main bonding jumper; the main bonding jumper is a service term. Do not use the primary breaker alone to answer every secondary conductor protection question. Do not bond the secondary neutral in both the transformer and the first panel unless a specific permitted arrangement and bonding method supports it.
When a question mixes transformer kVA with grounding facts, solve identity first, calculation second, protection third. That order keeps you from doing accurate math on the wrong system.
Which transformer arrangement is commonly a separately derived system?
For a grounded separately derived system, what does the system bonding jumper do?
What is the secondary current of a 75 kVA, 208 volt, three-phase transformer before applying overcurrent rules?