480v To 120v Transformer Wiring Diagram

Wiring a 480V to 120V Step-Down Transformer

The basic job is taking a three-phase 480-volt service and dropping it to 120 volts for control circuits, lighting, or receptacles in an industrial setting. It sounds simple enough on paper, but the actual wiring varies depending on the transformer type you are working with, and getting it wrong means either a dead load or a blown fuse you did not deserve. I spent the better part of a Tuesday morning on a job where someone had crossed two primary leads on a delta-connected transformer and we ended up with 840 volts trying to push into a 480-volt primary winding. The unit smoked but did not fail catastrophically, mostly because the overcurrent protection was sized correctly. That is the kind of thing that makes you more careful than you probably need to be. A 480V to 120V transformer typically handles either single-phase or three-phase conversion depending on your source and your load. For most small to medium commercial applications, a single-phase transformer is the standard choice. The primary side connects to two legs of 480V phase-to-phase. The secondary provides 120 volts between the output terminals, and many units include a center-tapped secondary that gives you 120/240 volts if you ever need both. When you are dealing with three-phase, you usually either use three single-phase transformers tied into an open delta or Wye configuration, or you buy a single three-phase unit out of the box. The wiring diagram on the transformer nameplate covers the connection details, but it is often so cramped and printed in tiny type that reading it requires a flashlight and decent eyesight. I always photocopy the nameplate diagram before I start removing any wires, because once you have everything disconnected, getting it back exactly right becomes a guessing game if you are relying on memory alone. Let me explain the connection process first since that is what actually matters on the job. You start by verifying that the incoming voltage is truly 480V and not something else. I have seen contractors measure 415V on a supposedly 480V system and just proceed anyway because they assumed the label on the panel said 480 and moved on. That assumption got them a transformer that ran warm and a motor that ran slow. Use a true RMS multimeter. Check phase-to-phase on all three legs if it is three-phase. Then check phase-to-neutral. Make sure your source voltage is within 10 percent of the transformer's rated primary before you close any disconnects.

Here is how I wire a standard single-phase 480V to 120V transformer that shows up on most job sites. The primary terminals are labeled H1 and H2. Connect your 480V hot leads to those terminals. If you have a three-phase delta system and need single-phase 120V, you take two phases off the 480V delta. Some people mistakenly take one phase and neutral, but that only gives you 277 volts, not 480. The transformer will see a voltage mismatch and behave unpredictably. I learned that the hard way on a project where the schematic showed a 480V delta and the field was actually a 480V Wye with a neutral. Someone just grabbed a phase and the neutral without checking the system configuration first. On the secondary side, the terminals are labeled X1 and X2 for a simple 120V output. If your transformer has a center tap, it will be labeled X0 or sometimes just marked with a white stripe on the winding. Connect your 120V load between X1 and X2, or between X1 and X0 if you only need 120 volts and want to leave the X2 terminal unused. When using the center tap for a 120/240V split-phase output, remember that the current rating on each half is the same as the full winding rating, not double. A 30 amp transformer does not give you 60 amps from the center tap setup. It gives you 30 amps maximum per leg to neutral, with the total capacity still being 30 amps. That is a mistake I see make-or-break more often than you would think.

The 480v To 120v Transformer Wiring Diagram for Three-Phase

Three-phase transformers are wired in either Wye-Wye, Delta-Delta, or Wye-Delta configurations. The most common for stepping 480V down to 120V in industrial work is a Delta primary to Wye secondary arrangement. The primary side connects line-to-line across the three phases with no neutral reference needed. The secondary produces 120 volts phase-to-neutral and 208 volts line-to-line because a Wye-connected 120V secondary inherently creates 208V between any two secondary lines. Wait, that might be the point of confusion for some people. If you specifically need 120 volts line-to-line on the secondary, a Delta-Wye transformer will not give you that. You would need a Delta-Delta transformer with a high-leg or a different winding arrangement entirely. Most of the time though, people who think they need 120V line-to-line actually just need 120V to neutral and are fine with the Wye secondary output. Clarify that early and save yourself a return trip to the supplier. I ran into a situation last year where a contractor ordered a 480V delta to 120V Wye transformer for a machine tool that specifically required 120V line-to-line power. The machine would not operate correctly on 120V phase-to-neutral because the internal control circuit expected a true two-wire 120V source without a neutral reference. We ended up wiring an autotransformer arrangement to create the proper voltage relationship, which added cost and complexity that the original transformer selection should have avoided. Reading the equipment nameplate and understanding the actual electrical requirements before purchasing the transformer would have prevented that entire headache. When wiring the three-phase unit, connect your three primary lines L1, L2, and L3 to the H1, H2, and H3 terminals respectively. If the transformer has a grounded neutral on the primary side, connect it to the system neutral. Most 480V delta systems do not have a neutral, so that terminal may be left unconnected or capped. On the secondary Wye side, connect your loads between any secondary line (X1, X2, X3) and the neutral (X0). The neutral should be grounded at the transformer if the system design requires it, but do not bond the neutral to ground at the transformer if your facility uses a separately derived system with a different grounding scheme. The grounding decisions here depend entirely on your local code and the overall system design, not just the transformer itself.

Overcurrent Protection and Conductor Sizing

You need overcurrent protection on both the primary and secondary sides. NEC Article 450 covers this and the general rule is that the primary OCPD should be sized at no more than 125 percent of the transformer's rated primary current, unless the transformer has built-in thermal protection or the next standard size up is required by code allowances. For a 10 kVA single-phase transformer on a 480V primary, the rated primary current is about 20.8 amps. Multiplying by 125 percent gives you approximately 26 amps, so a 25-amp or 30-amp breaker would be appropriate depending on what standard sizes your panel offers and whether the transformer is protected thermally. The secondary OCPD should be sized at no more than 125 percent of the rated secondary current as well. A 10 kVA transformer at 120V secondary draws about 83.3 amps, so your secondary protection would be in the 100-amp range. Conductor sizing follows from the ampacity requirements. Primary conductors should be sized for at least 125 percent of the rated primary current. Secondary conductors follow the same rule based on secondary current. Use THHN or THWN-2 in conduit for most indoor industrial installations. Make sure your disconnecting means is within sight of the transformer and rated for the circuit voltage and current. I once worked on a facility where the transformer was fed from a distant panel with no local disconnect, which violated code and created a maintenance hazard. Someone had to climb a ladder to pull a plug-style disconnect that was mounted six feet above the floor. That is not the kind of thing you want to be dealing with during an emergency.

Testing After Installation

Once everything is wired, verify your connections before energizing the transformer. Double-check that H1 and H2 are connected to the correct phase or phases, and that X1 and X2 have your load connected properly. Use a multimeter to confirm there are no shorts between primary and secondary windings, and no shorts to ground. Then energize the primary and measure the secondary voltage with no load connected. It should read very close to 120 volts, usually within a couple of percent. If it reads significantly higher or lower, check your primary voltage and your tap settings. Many transformers have adjustable taps that allow you to compensate for incoming voltage variations. A 480V transformer might have taps at 440V, 460V, 480V, 500V, and 520V. Make sure the tap selector matches your actual incoming voltage. Running a 480V transformer on a 440V tap when your supply is 480V will produce a secondary voltage that is roughly 9 percent too high. That might not seem like much, but it adds up over time on sensitive equipment. After verifying the no-load voltage, connect your load and monitor the transformer for a few minutes. It should run cool to the touch, not hot. A slight warmth is normal, but if the casing gets uncomfortably hot within five minutes, you likely have an overload or a wiring error. Check your load current against the transformer's kVA rating. A 10 kVA transformer at 120V can handle about 83 amps continuously. If your connected load exceeds that, you need a larger transformer or you need to redistribute some of the load elsewhere. Transformers can handle short-term overloads, but continuous operation above nameplate rating will degrade the insulation and shorten the transformer's life significantly. I have pulled transformers that were installed at 150 percent of their rated capacity and the internal winding insulation was brittle and cracked. That transformer had been running like that for about three years before anyone noticed. The whole process from start to finish on a straightforward single-phase installation like this usually takes about 45 minutes to an hour if you know what you are doing and have the right parts on hand. Adding in the testing and documentation time brings it closer to two hours. That is including the time I spend rereading the nameplate diagram three times to make sure I am not missing a detail that some manufacturer included in a font size smaller than typical.