Three-Phase Transformer Wiring Trainer
Follow a dry-type step-down transformer from the 480V panel to the energized 208Y/120V panelboard, one conductor at a time, in the order the crew lands them.
This animation follows the install in our three-phase transformer guide from the 480V panel to the energized 208Y/120V panelboard: the primary overcurrent device, conductors landing on H1, H2 and H3, the delta primary and wye secondary windings, X1, X2 and X3 out to the secondary main, and the three connections that land on X0 — the neutral to the panel, the system bonding jumper to the enclosure ground bar, and the grounding electrode conductor down to the electrode. A legend gives the conductor colors, and each connection is drawn in the order the crew makes it.
The code behind it is Article 450 for the transformer and 250.30 for the separately derived system it creates. Table 450.3(B) sets the primary and secondary overcurrent protection; 240.21(C) governs the secondary conductors between the transformer and the first device; 450.9 and 450.13 cover ventilation clearances and accessibility. On the grounding side, 250.30(A)(1) requires the system bonding jumper — one, sized from Table 250.102(C)(1) — 250.30(A)(4) names the electrode, and 250.30(A)(5) sizes the grounding electrode conductor from Table 250.66. The neutral downstream of this transformer is created inside it; that is why inspectors read this section closely.
Common mistakes the animation makes visible: bonding the secondary neutral to ground a second time in the downstream panel; leaving the equipment grounding conductor off the primary feeder because “the secondary is grounded”; and sizing the secondary conductors from the primary breaker instead of 240.21(C).
Simplified concept tool for training and illustration — not a construction document. Equipment layouts and bender take-ups vary; manufacturer instructions and the locally adopted code govern.
This page frames an interactive trainer that does not print. Open https://www.aztucsonelectricalsolutions.com/trainer-three-phase-transformer in a browser to use it.
Text description of this trainer
Simplified connection diagram. A 480V three-phase panel feeds a three-pole primary overcurrent protective device, then conductors land on transformer terminals H1, H2 and H3. The delta primary and wye secondary windings are shown symbolically. Secondary terminals X1, X2 and X3 feed a secondary main breaker and a 208Y/120V panel. Terminal X0 has three connections: the neutral continuing to the panel neutral bar, the system bonding jumper to the transformer enclosure ground bar per NEC 250.30(A)(1), and via that bar the grounding electrode conductor per 250.30(A)(5) down to a grounding electrode per 250.30(A)(4). A green equipment grounding conductor runs with the primary feeder to the transformer enclosure, and another runs with the secondary conductors to the panel ground bar. A legend gives conductor colors.
How to use it in a class
Play it through once, then replay it with the room talking: have students name each conductor before it is drawn and say which table sizes it. The X0 terminal is the teaching moment — three conductors land there for three different reasons, and a student who can explain all three understands separately derived systems. Follow with the Transformer Sizing Calculator: give the class a load in kVA or amps and have them size the unit, the primary device and the secondary device per 450.3(B), then come back to the animation and check where each device sits.
Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction and its amendments govern, and the AHJ has the final say. The trainer is a teaching simplification of work for qualified, licensed electricians — on a real installation, de-energize, apply lockout/tagout, verify absence of voltage with a tested meter, and wear PPE selected per NFPA 70E before touching anything.
Learn it here, do it with us
Arizona Electrical Solutions trains apprentices on exactly this work — services, grounding and bonding, transformers, conduit — on commercial projects across Arizona. If you are planning the work itself rather than learning it, send us the job.
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The guide this trainer came from, the calculator that pairs with it, and the rest of the set.