What a Transformer Nameplate Is Telling You

Why this matters

Techs read a transformer nameplate for one number, the kVA, and treat it as a capacity ceiling: under it fine, over it not fine. That reading throws away the field that actually decides whether the supply behaves, which is the percent impedance. Two transformers with identical kVA and identical voltages can behave completely differently under the same load, and the nameplate says so plainly if you know which line to read. Stiffness and capacity are two different questions, and the shop that can answer the first one stops arguing about equipment that only misbehaves when something else in the building starts.

The nameplate answers a stiffness question, not just a size question

The load-bearing idea is this: kVA tells you how much continuous heat the transformer can shed, and percent impedance tells you how hard the secondary voltage falls when current is drawn and how much current is available if the secondary is shorted. Those are the same physical property viewed from opposite ends. A low-impedance transformer holds voltage well and lets a great deal of fault current through. A high-impedance transformer sags more under motor starting and limits fault current. Neither is better in the abstract; the equipment and the protective devices decide which one you want.

The fields, and what each one is actually promising

• kVA. A continuous thermal rating, not a trip point. It is stated at a specific ambient and a specific winding temperature rise, and both of those appear elsewhere on the plate. Nothing on the transformer enforces it.
• Primary and secondary voltage, with the configuration. Written as something like 480 delta primary and 208Y/120 secondary. The Y or the delta is not decoration; it determines which line-to-ground readings mean anything, and a sibling article covers that.
• Percent impedance (%Z). The percentage of rated primary voltage required to circulate rated current with the secondary shorted. This is the stiffness number.
• Temperature rise class and maximum ambient. For dry types the rise classes commonly seen are 80, 115 and 150 degrees C, over a stated maximum ambient, most often 40 degrees C with a lower daily average. A 150 C rise unit is not hotter-running by accident; it is built with an insulation system rated for it.
• Frequency and phase. Worth reading before you assume, particularly on equipment that arrived with imported machinery.
• Taps. A list of fixed ratio positions, usually a small number of steps above and below nominal. Taps shift the whole voltage curve, they do not improve regulation, and a sibling article covers why that distinction matters.
• Insulation class or system, and the enclosure rating. These decide where the unit is allowed to live and how much of it you are allowed to change.

Two of those fields carry arithmetic you can do on the spot, and they are the two the rest of this article uses: kVA with the voltages gives you rated current, and %Z with rated current gives you both the fault-current ceiling and the bound on voltage sag.

The two calculations the plate supports

Rated current. On a three-phase transformer, rated current on either side is the kVA divided by the product of 1.732 and that side's rated voltage, with kVA converted to VA. The same formula runs on the primary and the secondary; you get two different currents because the voltages differ, and the volt-amperes match.

Fault-current ceiling. Rated secondary current divided by the percent impedance expressed as a decimal gives the symmetrical fault current available at the secondary terminals assuming an infinite primary source. That assumption is why this is a ceiling rather than an answer: the real primary has impedance of its own, so the true figure is lower, and the number that goes on an equipment label or into an interrupting-rating decision comes from a short-circuit study by an engineer with the utility's available fault data, not from a nameplate on a wall. What the ceiling is good for is a sanity check, and it is a good one: it tells you within a minute whether a panel's interrupting rating is in the right order of magnitude for what feeds it.

Sag. The percent impedance bounds the full-load voltage drop rather than equalling it. At a unity power factor load the drop is dominated by the resistive part of the impedance and lands well under %Z; at a lagging power factor near 0.8, which is where a building full of motors sits, the drop approaches %Z. So a 5 percent impedance transformer feeding motor load can be expected to give up something close to 5 percent of secondary voltage between no load and full load, and that expectation is what you compare an actual measurement against.

Worked example: reading a plate you have never seen before

A dry-type transformer in a mechanical room, feeding a subpanel that serves rooftop equipment. Take the plate data as read, with these values illustrative of a common configuration rather than a specification for any product:

Rated secondary current. 45,000 VA divided by (1.732 x 208 V) = 45,000 / 360.3 = 124.9 A. That is the current at which the transformer is at its rated continuous heat load, not the current at which anything trips.

Rated primary current. 45,000 / (1.732 x 480) = 45,000 / 831.4 = 54.1 A. Check the volt-amperes both ways and they agree, which is the fastest way to catch a misread voltage on a dirty plate.

Fault-current ceiling. 124.9 A divided by 0.05 = 2,498 A, call it 2,500 A symmetrical at the secondary terminals with an infinite primary assumed. The subpanel that transformer feeds needs an interrupting rating comfortably above that figure, and if the panel's label reads well under it you have found something worth escalating rather than something to solve on site.

Expected sag. At 5.0 percent impedance and a motor-heavy load near 0.8 lagging, expect the secondary to give up something approaching 5 percent between no load and full load, so roughly 10 V on a 208 V nominal secondary. Now that number is useful in the field: if you measure 211 V at that panel with the building idle and 201 V with the rooftop units running, the transformer is behaving exactly as its plate says it should and nothing upstream is faulty. The complaint, if there is one, is that a 5 percent unit was specified for a load that needed a stiffer one.

Where the load actually sits. If the rooftop equipment's combined nameplate current is, say, 96 A on the 208 V side, that is 96 / 124.9 = 77 percent of the transformer's rated current. Under its thermal rating, and the sag figure above should be scaled accordingly rather than taken at full value: roughly three-quarters of the full-load drop, so nearer 7 to 8 V than 10 V. Measuring 10 V of sag at 77 percent load means the drop is larger than the plate predicts, and the extra is in the conductors and connections between the transformer and your probes, not in the transformer.

What the plate does not tell you

It does not tell you what else is connected. A transformer at 77 percent on the equipment you came for can be at 110 percent once the loads you did not inventory are running, and only a clamp on the secondary conductors during a busy hour settles that.

It does not tell you the ambient it is actually living in. The rise class is stated over a maximum ambient, and a unit in a closed mechanical room in August is not in that ambient. A sibling article covers what happens when the sum of ambient and rise exceeds what the insulation system was built for.

It does not tell you whether a modification is permitted. Re-tapping, re-enclosing, adding forced ventilation or relocating a unit can affect its listing, and whether a given change voids that listing is a question for the manufacturer and the authority having jurisdiction, not a call a technician makes on site.

Verifying you read it right

Reading the plate is not live work, but every check below that involves a probe or a clamp is. 29 CFR 1910.333(a)(1) permits energized work only where the employer can demonstrate that de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations, and measuring a transformer's loaded secondary is that case, because de-energizing removes the very condition you are measuring. Work it under your employer's electrical safety program, with boundaries and PPE established on the basis in NFPA 70E-2021, 130.5 and 130.7, which is a consensus standard binding through that program or a contract rather than on its own, and with a meter and leads rated CAT III or better at or above the voltage present. If the check requires opening an enclosure that is not designed for live inspection, that is a change of task, not a change of care. When you move from measuring to working on the transformer or its conductors, open and lock the primary disconnect under 29 CFR 1910.333(b)(2), prove dead using the live-dead-live sequence at NFPA 70E-2021, 120.5, and remember that 29 CFR 1910.147 excludes this electrical exposure at (a)(1)(ii)(C), with 29 CFR 1926.417 as the construction counterpart for lockout and tagging of circuits.

Three confirmations, in the order that costs least:

1. Cross-check the two rated currents. If primary VA and secondary VA do not agree, you misread a voltage or a kVA digit on a plate that has been painted over. This costs nothing and catches the most common error.
2. Clamp the secondary conductors during a loaded hour and compare against the rated current you calculated. This is the only way to know the loading rather than to assume it.
3. Take the no-load and loaded secondary voltage at the transformer terminals themselves rather than at the panel, so the reading contains the transformer's regulation and nothing else. Compare the measured sag against the %Z-derived expectation scaled to the actual loading. A measured sag materially larger than the scaled prediction means either more load than you counted or a connection problem between the windings and your probes.

References

• 29 CFR 1910.333(a)(1) and (b)(2) for energized work and de-energizing; 29 CFR 1926.417 for lockout and tagging of circuits in construction
• NFPA 70E-2021, 120.5, 130.5 and 130.7, applied through an employer electrical safety program or contract
• NEC Article 450 as adopted in the edition in force in your jurisdiction, for transformer installation and protection requirements
• Manufacturer loading and application data for the specific unit, which owns ambient, rise and any question about modifying a listed assembly
• See related: What a Secondary Voltage Depends On; What Happens When a Transformer Is Loaded Past Its Rating