If you’ve ever worked with high-frequency electronics, power conversion systems, or compact motor windings, chances are you’ve crossed paths with Flat Litz Wire. As a supplier that’s specialized in this product line for over a decade, I’ve had more conversations with engineers, design leads, and production managers about wire performance than I can count. Most of those conversations circle back to one question that never goes out of style: how does insulation thickness on Flat Litz Wire actually impact how it performs? It’s not the sexiest part of the design, I’ll admit—when everyone’s obsessing over core material, winding geometry, or switching frequency, insulation thickness gets written off as a trivial detail. But over years of troubleshooting field failures, optimizing production runs, and testing prototype designs, I’ve learned that this single variable can make or break a part’s efficiency, reliability, and overall cost. Let’s break this down like we would in our warehouse on a busy Monday morning, no overly academic jargon, just the real-world stuff that matters. Flat Litz Wire
First, let’s ground ourselves in what Flat Litz Wire actually is, for anyone who might be newer to this. Unlike round Litz wire, which is made of twisted strands of insulated round conductors, Flat Litz is composed of multiple flat, rectangular copper or aluminum strands, each wrapped in its own thin insulation, arranged side-by-side into a flat, ribbon-like shape. That flat geometry is why it’s so popular for high-frequency applications—round Litz can leave dead space in tightly packed windings, but flat Litz conforms much better to the tight slots of high-power transformers, EV motor stators, and RF inductors. The individual strand insulation prevents the skin effect and proximity effect, the two big culprits of high-frequency AC loss that make regular solid wire heat up like a pan on a stove when pushed past certain frequencies. But the outer insulation layer—wait, no, let’s clarify: every single strand in Flat Litz has its own insulation, and then there’s usually a top and bottom outer insulation layer that binds the whole flat ribbon together, right? That outer layer is what most engineers refer to when they talk about “insulation thickness” for Flat Litz, though we also have to consider the individual strand insulation, because that’s part of the same equation.
Let’s start with the most obvious performance metric: AC loss. This is where insulation thickness hits hardest, especially at frequencies above 10 kHz, which is where most modern power electronics operate. When you have thinner insulation on the outer layers of the Flat Litz ribbon, the distance between the top/bottom strands’ insulation and the winding slot’s core steel or adjacent conductors is smaller. That proximity effect gets stronger the closer conductors are to each other, right? The magnetic fields from adjacent current-carrying strands distort the current flow, pushing more of it to the outer edges of each conductor, increasing effective resistance, and turning more electrical energy into unwanted heat. I remember a project we had a few years back with a team designing a 50 kW fast charger transformer. They’d originally sourced a competitor’s Flat Litz with 0.02 mm outer insulation, and when they tested the prototype at 50 kHz, the winding was running 12°C hotter than their target. They came to us frustrated, thinking they needed to switch to a higher-grade copper, but we suggested testing our standard product with 0.05 mm outer insulation. The heat drop was immediate—down to 7°C over the target. The thicker insulation increased the distance between adjacent strands and the slot walls, weakening the proximity effect enough to cut AC loss by almost 18%. It’s not magic, it’s basic electromagnetism, but that’s the kind of real-world impact that makes insulation thickness matter.
But wait—thicker insulation isn’t always a win, and that’s where a lot of engineers get tripped up. Because Flat Litz is designed to fit into tight, standardized winding slots, the outer dimensions are non-negotiable almost every time. If you increase insulation thickness, even by a tiny amount, you reduce the total cross-sectional area of copper in the same ribbon width. Let’s do a quick math example to make this concrete: a standard 10 mm wide Flat Litz ribbon, with eight 0.5 mm thick strands each with 0.02 mm strand insulation, and original outer insulation of 0.02 mm. Total copper cross-section here is roughly 10 mm wide * (8 strands * 0.5 mm strand copper thickness) = 40 mm². If we bump outer insulation to 0.08 mm to cut proximity effect, the ribbon’s total thickness goes up by 0.12 mm (0.06 mm on top, 0.06 mm on bottom) so we have to compensate by either making the strands slightly thinner or the whole ribbon narrower. If we narrow the ribbon to 9.8 mm to keep the total height the same, copper cross-section drops to ~39.2 mm², a 2% loss. If strands get thinner, say 0.49 mm each, copper drops to ~39.6 mm². That 2% might sound trivial, but when you’re building 100,000 chargers a year, that’s thousands of dollars in extra copper cost, and it can also push current density up just enough to negate some of the heat gain from thicker insulation. It’s a balancing act, not a one-size-fits-all rule.
Next up, reliability and dielectric performance—this is where insulation thickness gets way more critical, especially for high-voltage applications. I’ve seen too many field failures where a transformer arced because the insulation was too thin. Let’s take EV traction inverters, for example, which run at 400V or 800V these days, and have winding slots that are tight enough that any minor variation can cause a voltage breakdown. The outer insulation on Flat Litz has to not only bind the ribbon together during winding but also act as an electrical barrier between adjacent layers of windings, between the Flat Litz ribbon and the core steel, and between the ribbon and any adjacent conductors. If insulation is too thin, even a small scratch from winding, a burr on a slot edge, or thermal expansion of the wire can punch through the insulation and cause a short circuit. Last year, we got a call from an industrial motor manufacturer who had 200 motors fail in the field within three months of launch. They’d used a very thin, low-cost insulation to save a few cents per meter on Flat Litz, and when we tore down a failed unit, we found insulation punctures along the top edges of the Flat Litz ribbons from the sharp slot edges. They switched to our standard medium-voltage Flat Litz with 0.07 mm outer insulation, and the failure rate dropped to zero in their next production run. That’s not about performance efficiency—that’s about warranty costs, brand reputation, and getting a product to market on time.
On the flip side, too-thick outer insulation can also hurt reliability, in a less obvious way. Flat Litz wire is flexible, which is why it’s so easy to wind into tight slots. Thicker insulation adds stiffness to the ribbon, making it harder to bend without cracking or fraying at the edges. Cracks in insulation are just as bad as too-thin insulation—they create weak points where voltage can break down over time, especially as the wire cycles through heat and cold during operation. I remember a medical device client that was winding Flat Litz into a compact RF coil for an MRI machine. They’d tested a thick 0.12 mm insulation to get maximum dielectric strength, but when they tried to bend it into a tight 2 cm radius for the coil, the outer insulation cracked along the entire length of 30% of the ribbons. We suggested a thinner 0.05 mm insulation with a slightly higher dielectric rating (we use a modified polyimide that’s more flexible than standard polyester) and they got the same dielectric performance without the cracking, cutting their failure rate from 15% to less than 1% during winding. That’s another key point: insulation thickness isn’t the only variable when it comes to dielectric strength or flexibility. The material of the insulation plays a huge role too, but thickness is the primary lever most designers can adjust.
Then there’s thermal management, which ties back to both loss and reliability. The thicker the insulation, the lower the thermal conductivity between the copper strands and the surrounding environment (slot walls, cooling fluid, etc.). Copper is an amazing heat conductor, but insulation is a heat barrier. So even if thicker insulation cuts AC loss, if it traps more heat in the winding, you might end up with higher overall operating temperatures. That’s why we always work with clients to test thermal impedance alongside electrical loss. For example, in a liquid-cooled transformer, the slot is filled with coolant, so heat transfers more easily, so a slightly thicker insulation can work well because the heat doesn’t get trapped. In an air-cooled motor, though, where heat has to dissipate through the slot walls, a thicker insulation might create a thermal bottleneck, leading to higher overall temperatures despite lower AC loss. We had a client a few years back who tested both insulation thicknesses on an air-cooled HVAC motor. The 0.05 mm outer insulation ran at 62°C, while the 0.08 mm insulation ran at 65°C, even though the AC loss was 10% lower for the thicker insulation. The extra heat from the insulation barrier outweighed the loss gain, so they went with the thinner insulation, and ended up with a more reliable, cooler-running motor. That’s the kind of tradeoff that only comes from real testing, not just textbook equations.
Now, let’s talk about a lesser-known area: processability and cost, which impact your end product’s profitability. Thinner insulation is cheaper, right? Less material per meter, easier to process, less chance of cracking during winding. But as we saw with the charger example, thinner insulation can lead to higher AC loss, which means bigger heat sinks, more cooling components, or a larger transformer to handle the extra heat. Those extra components add way more cost than the 2-cent per meter saved on cheaper wire. On the other end, thicker insulation is more expensive per meter, and can lead to higher copper cross-section loss, as we calculated earlier. So the sweet spot is almost never the thinnest or thickest insulation—it’s the one that balances electrical performance, thermal performance, reliability, and cost for your specific application. That’s why we don’t just sell Flat Litz wire off the shelf as “thick” or “thin”; we work with each client to match insulation thickness to their operating frequency, voltage level, slot geometry, cooling method, and production process.
I want to be clear here: this isn’t a rule that applies across all Flat Litz wire. For low-frequency applications, like power distribution transformers running at 50 Hz, insulation thickness barely matters at all, because AC loss from proximity effect is negligible. The main concern there is dielectric strength, so you’d go for the thickest insulation possible to avoid breakdown. For high-frequency applications like RF coils, where frequency is in the MHz range, strand insulation thickness (not just outer) becomes way more critical than outer insulation, because skin effect is the main loss driver, not proximity. But for most of the work we do at our company—high-frequency power electronics, EV components, industrial motors, medical devices—insulation thickness is one of the first variables we tweak when optimizing a design.
Over the years, I’ve seen too many engineers skip this step, defaulting to standard insulation thickness because it’s what they’ve always used. I get it—time is tight, design cycles are short, and there’s always a dozen other problems to solve. But the 1-2% gain in efficiency, the reduction in field failures, the lower production costs that come from matching insulation thickness to your application—those add up fast. I’ll give you a recent example: a renewable energy client designing a 100 kW grid-tied inverter. They’d been using a competitor’s Flat Litz with 0.03 mm outer insulation, and their inverter had an efficiency of 97.2%, which was just barely under the efficiency threshold they needed for government incentives. We suggested testing our product with 0.06 mm outer insulation, optimized for their 20 kHz switching frequency. Not only did the efficiency jump to 97.8%, which got them the full incentive, but their winding temperature dropped by 8°C, which extended the lifespan of the inverter’s internal components. All that from changing a 0.03 mm layer of insulation.
Let’s wrap this up with what I wish every engineer knew about insulation thickness on Flat Litz wire: it’s not a trivial detail, it’s a core design variable that connects electrical performance, thermal reliability, production cost, and end-product lifespan. There’s no universal “best” thickness—what works for a 50 Hz transformer won’t work for an 800 V EV motor, and what works for an air-cooled motor won’t work for a liquid-cooled charger. The key is to test, iterate, and partner with a supplier that’s not just selling you wire, but helping you navigate those tradeoffs.
If you’re currently designing a new component, troubleshooting a field failure, or just want to optimize your existing Flat Litz wire setup, I’d be happy to walk through your specific requirements, run small prototype tests, or answer any questions you have about how insulation thickness will impact your performance. We’ve worked with everything from small startup teams to large multinational OEMs, and we tailor every Flat Litz order to the unique needs of your application. Don’t leave insulation thickness as an afterthought—let’s work together to get it right for your project.
Enameled Flat Wire References
- McCluskey, F., & Wang, H. (2019). High-Frequency AC Losses in Litz and Flat Litz Conductors for Power Electronics. IEEE Transactions on Power Electronics, 34(10), 9872-9885.
- Skutt, G., & Sullivan, C. R. (2007). Effect of Insulation Thickness on Proximity Loss in Litz Wire Windings. Proceedings of the 2007 IEEE Power Electronics Specialists Conference, 2345-2351.
- Underwriters Laboratories. (2021). Standard for Insulated Wire and Cable for Use in Electrical Equipment, UL 1001.
- Bosch, J. (2022). Thermal Management of High-Density Power Electronics: The Role of Conductor Insulation. Journal of Electronic Packaging, 144(3), 030904.
- Electric Power Research Institute. (2020). Insulation System Design for Medium-Voltage Motor Windings. EPRI Technical Report 3002001247.
Tianjin Jingwei Power Technology Co., Ltd.
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