When I first started training my team to test transformers back in 2012, one of the most common questions new techs asked was, “If we’re transformer testers, can we actually check the load loss?” At the time, I had to pull from years of hands-on experience and industry manuals to explain it—it wasn’t a straightforward yes or no. Now, as a supplier of transformer testing equipment, I get this question at least a dozen times a week, from small utility companies to large industrial manufacturing plants. Let me break this down the way I do with every customer who reaches out: grounded in real testing processes, not just textbook theory. Transformer Tester
First, let’s define what load loss is, because it’s the foundation of the answer. Load loss (also called copper loss) is the power lost in a transformer’s windings when current flows through them. It’s generated by the electrical resistance of the copper or aluminum wire used in the windings, plus stray losses from eddy currents and stray flux around the core. Unlike no-load loss (which happens when the transformer is energized but not carrying a load), load loss is directly tied to the amount of current passing through the transformer—so it’s a critical metric for efficiency, especially for units operating at high load for extended periods.
A lot of people assume load loss testing is reserved for specialized electrical labs, only accessible with bulky, custom equipment. That’s where the confusion comes in. In reality, transformer testers—the portable, field-ready units my company designs and builds—are not only capable of measuring load loss, they’re often more practical for routine testing than lab equipment. Here’s how it works: when you perform a short-circuit test (the standard method for measuring load loss), you apply a reduced voltage to the primary winding of the transformer and short the secondary winding. At this point, the only current flowing is the load current, and the power consumed is almost entirely load loss (stray losses add a tiny fraction, which modern testers can account for).
Now, I’m not saying every off-the-shelf transformer tester will do this. Early models we sold in the mid-2010s, for example, had basic power measurement capabilities but lacked the precision to separate load loss from background electrical noise, especially in older transformers installed in industrial plants with heavy machinery. Back in 2016, a customer in Detroit told us their old tester would give wildly inconsistent load loss readings on their 15MVA distribution transformers, because it couldn’t filter out the harmonics from their welding equipment. That feedback pushed our team to redesign our core tester models to include advanced harmonic analysis, a feature that’s now standard in our current line.
Here’s where being a transformer tester supplier gives me insight into what works for real customers, not just lab scenarios. For a utility company performing routine maintenance on 50+ transformers a year, a field-ready tester cuts down on downtime drastically. Instead of disconnecting each transformer, hauling it to a lab, and waiting weeks for results, a technician can set up our tester on-site in 15 minutes, run the short-circuit test, and get a load loss reading in less than 10 minutes. That’s not just convenient—it saves thousands of dollars in labor and transportation costs per transformer.
But let’s address the caveats, because no testing is one-size-fits-all. A transformer tester can measure load loss accurately, but it depends on two key factors: the tester’s power resolution, and the test setup. First, power resolution: load loss is usually a small percentage of the transformer’s rated capacity. For a small distribution transformer (500kVA), load loss might be as low as 2kW, while for a large power transformer (100MVA), it could be 100kW or more. A low-end tester might not have the sensitivity to pick up that 2kW loss accurately, especially when testing small transformers. Our entry-level transformer testers, for example, have a power measurement resolution of 0.1W, which is more than enough for even the smallest distribution units, while our high-end models can handle up to 1000V of test voltage, making them suitable for large power transformers used in grid systems.
Second, the test setup has to be correct. A common mistake I see new technicians make is not accounting for the winding resistance before running the load loss test. Load loss is calculated as I²R (current squared times resistance), so if you don’t measure the winding resistance first, your load loss reading will be off. Modern transformer testers automate this step now—when you run a short-circuit test, the tester will first measure the winding resistance, apply the test voltage, and calculate load loss automatically, eliminating human error from manual calculations. That’s a feature we added after a customer in Texas told us their techs were missing a 15% load loss discrepancy because they forgot to adjust for resistance in a manual test.
Another point: load loss testing with a transformer tester isn’t just for commissioning new units. It’s also critical for fault detection. A sudden spike in load loss can indicate problems like loose connections in the windings, damaged insulation, or even shorted turns in the winding. Last year, a customer in Canada used our tester to detect a faulty winding in a 20MVA transformer that was causing 30% higher load loss than rated. If they hadn’t run load loss tests, they would have had the transformer fail during peak winter demand, which would have cost them an estimated $200,000 in lost production at their pulp and paper mill. That’s the real value of load loss testing with a transformer tester—it’s not just a efficiency metric, it’s a predictive maintenance tool.
I’ve had customers argue with me that they should just use a power analyzer instead of a transformer tester for load loss testing. And while power analyzers can measure power, they’re not designed for transformer-specific tests. A transformer tester is calibrated to handle the specific conditions of transformer testing—like short-circuit test configurations, winding resistance measurements, and temperature compensation (which is vital, because load loss changes with winding temperature). Our testers automatically adjust load loss readings to 75°C, the standard reference temperature for transformers, so customers don’t have to do the math themselves. A power analyzer might give a raw power reading, but it won’t translate that into a standardized load loss value that aligns with IEEE or IEC standards.
Speaking of standards, it’s important to note that modern transformer testers are aligned with the major industry standards for load loss testing. IEEE C57.12.90, the standard for transformer test code, outlines the procedures for short-circuit tests to measure load loss, and our tester models follow those procedures exactly. IEC 60076-1, another key standard, also specifies the requirements for load loss measurement, and our testers are certified to meet those requirements. That’s why our customers trust our testers for both in-house testing and compliance reports they need for regulatory audits.
Now, let’s talk about the evolution of transformer testing equipment, because as a supplier, I’ve seen how far we’ve come. Ten years ago, the only way to run a load loss test was with a large, heavy unit that required two people to carry, and it would take hours to set up. Today, our testers are portable, battery-powered, and weigh less than 20 pounds. We’ve also added features like Bluetooth connectivity, so technicians can view load loss readings on their phones while the test runs, and data logging, which stores test results for long-term tracking. That’s a big deal for customers who monitor load loss over time—they can track how a transformer’s efficiency changes with age, and plan maintenance before a fault occurs.
I want to be clear, though: if someone is looking to test load loss on a transformer, a transformer tester is a better choice than general electrical testing equipment. The specialized design means fewer errors, faster testing, and compliance with industry standards. That said, there are still cases where a lab test is needed—for example, when testing load loss on a custom-built power transformer with non-standard winding configurations. But for 95% of routine load loss testing, whether it’s for distribution transformers, industrial transformers, or grid power transformers, a transformer tester is more than capable.
I can’t end this blog without talking about what this means for you, the person who’s responsible for transformer maintenance or procurement. If you’re wondering whether your current tester can measure load loss, check these three things: does it have a short-circuit test function, does it measure winding resistance accurately, and does it calculate load loss at a standardized temperature (like 75°C)? If not, it might be time to consider an upgrade. A bad load loss test isn’t just a missed metric—it can lead to unexpected downtime, wasted energy costs, and even safety hazards.
As a transformer tester supplier, my goal isn’t just to sell equipment—it’s to make sure you have the right tools to keep your transformers running efficiently and safely. Load loss testing is one of the most important tests you can run, and transformer testers are built to make that test easy, accurate, and fast. If you’re not sure whether your current equipment is up to the task, or if you’re looking to upgrade to a more reliable tester, reach out to our team to discuss your specific needs. We’ve worked with utilities, industrial plants, and electrical contractors for years, and we can help you find the right tester for your transformers.
Secondary Current Injection Testing Equipment References
IEEE Std C57.12.90, Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers
IEC 60076-1, Power Transformers – Part 1: General
Transformers and Inductors for Power Systems: Design, Applications and Testing, by K. Chandrasekhar
Wuhan Jiuhua Jingce Power Equipment Co., Ltd.
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