Hey there, it’s Jake from your go-to Flat Litz Wire supplier, and I get it—when you’re spec’ing wire for your latest project, you’re not just picking parts off a shelf. You’re chasing less heat, better efficiency, and that edge that keeps your design ahead of the pack. I swear, over the 10 years I’ve been in this game, the question I hear most after “does flat litz even beat regular wire?” is this: “How does the strand diameter actually mess with resistance?” Flat Litz Wire
Let’s cut the jargon first—no stuffy engineering textbooks here, okay? Flat Litz Wire isn’t a single solid chunk of metal; it’s a bunch of tiny, insulated copper strands pressed flat into a ribbon shape. The strand diameter is how thick each of those tiny individual wires is. Think of it like a bag of M&Ms: if you use mini M&Ms vs. the regular ones, they fit together differently, right? Well, same vibe here, and that difference directly hits resistance.
First, let’s recap the basics most folks forget about resistance. Resistance is just how much a wire fights the flow of electricity. The formula for that is R = ρL/A—ρ is the resistivity of copper (that’s a constant, like how dense lead is), L is the length of the wire, and A is the cross-sectional area. But here’s the big twist with litz wire: regular solid wire uses the full A, but litz wire has to deal with something called the skin effect. Ever turn on a big power transformer and feel the unit get warm? That’s because at high frequencies (which are super common now in EV chargers, induction heaters, or even small consumer electronics), electricity doesn’t spread evenly across a solid wire. It clings to the outer “skin” of the wire, so the actual area carrying current is way smaller than the total cross-section. That’s the #1 culprit for extra resistance and wasted heat.
Now, where does strand diameter come in? If you use bigger strands, guess what? The skin effect hits them harder. Let’s say you have a strand that’s 0.5mm thick. At, say, 100kHz (that’s a common frequency for compact power supplies), the skin depth in copper is only about 0.2mm. That means the inner 0.1mm of that 0.5mm strand is totally unused—no current flows there. So that strand’s effective area isn’t 0.5mm, it’s a thin shell around it, so the resistance jumps way more than you’d expect from just the total copper volume.
But if you go smaller—way smaller strands, like 0.05mm or even 0.02mm thick—now the strand’s diameter is smaller than that skin depth. The entire cross-section of the tiny strand is carrying current! No unused copper, no wasted space. That’s the magic of small strands in flat litz: they beat the skin effect flat. I’ve seen this play out firsthand with a client who was making a 50kW EV on-board charger. They originally used 0.3mm strands and were seeing 12% higher resistance than their simulation at 80kHz—all because the inner 60% of each strand was dead weight. Switched to 0.07mm strands, and that resistance dropped to within 2% of their number, no extra cooling needed. Total game-changer for their design, and they’ve been our repeat client ever since.
Wait, hold on—this isn’t a “smaller = always better” thing, though. I see a lot of new designers get this wrong. Super tiny strands have a catch: proximity effect. That’s like the skin effect’s annoying cousin. When strands are packed close together (which flat litz does, since they’re pressed flat), the current in adjacent strands pushes against each other. If strands are too small, or too many, the magnetic fields between them build up and restrict current even more. Plus, smaller strands mean way more insulation material. Every tiny strand has its own thin coat of enamel, right? So if you have a litz wire that needs a total copper cross-section of 1mm², using 0.5mm strands would take ~5 strands, using 0.05mm strands would take ~500 strands. All that extra insulation takes up space, so the overall copper fill factor (the actual % of the flat ribbon that’s copper, not insulation) drops. Lower fill factor means lower total copper area, which means higher resistance. Duh. So that’s the tradeoff.
I’ve had a customer recently come to me panicking because they thought “smaller strands = zero resistance” and tried 0.02mm for a medical induction coil. Their fill factor was only 72%, because the tiny insulation layers couldn’t be compressed when pressing flat, and their total resistance ended up 8% higher than when we sized them for 0.08mm strands. We adjusted the strand count to hit the right fill factor for their coil’s width, and they got exactly the resistance they needed without overcomplicating things.
Another thing to consider: frequency. That’s the big driver here, and it’s tied straight to strand diameter. If your project is running at low frequencies, like 50/60Hz (the grid frequency), the skin depth is almost 1mm. So a 0.5mm strand is totally fine—no skin effect, no extra resistance. You don’t need tiny strands here, and you’d just be wasting money on super fine wire that’s harder to source and more expensive. But crank that frequency up to 200kHz for a fast charger, and skin depth drops to ~0.05mm. Now 0.05mm strands are perfect, and 0.1mm strands would have 50% unused copper, leading to way higher resistance. It all lines up.
Let’s also talk about practicality beyond the numbers, because that’s what you care about, right? Super small strands (below 0.05mm) are way more fragile. If you’re winding them tight into a transformer or coil, they can break during manufacturing, adding defects and downtime. I’ve had a client that switched to 0.03mm strands to squeeze out every last drop of resistance, only to have 15% of their coils fail due to broken strands. Going back to 0.06mm fixed that, and their resistance was only 3% higher, which was totally acceptable for their cost target.
What about if you have a custom flat litz wire you’re working on? Let’s say you need a 10mm wide, 1mm thick flat ribbon for a 150kHz inverter. Let’s do the quick math: skin depth at 150kHz is ~0.065mm. So you want strand diameter a little smaller than that, like 0.06mm, so the whole strand is used. If you go too big, say 0.1mm, each strand has a core that’s 0.02mm unused, so total resistance jumps by around 20%—that’s massive for a power inverter where efficiency is everything.
Another common mistake I see: people mixing strand diameters. Like, using a mix of big and small strands to “save money.” Don’t do that. The bigger strands will get hit way harder by skin/proximity effect, creating hot spots and uneven resistance. We had a guy try that for a motor winding once, and after 200 hours of testing, his litz wire had a dead spot that burned out the whole motor. We straightened him out, and since then he’s only spec’d uniform strand diameter, and his failure rate dropped to almost zero.
At the end of the day, the sweet spot is matching strand diameter to your operating frequency, balancing skin/proximity effect, fill factor, mechanical durability, and cost. For low frequencies (under 1kHz), stick to strands 0.2mm to 0.5mm. For mid-range (1kHz to 100kHz), 0.08mm to 0.2mm works great. For high frequencies (over 100kHz, like fast chargers or RF coils), you’ll want strands 0.02mm to 0.08mm. That’s the range we see 90% of our customers land in, and it’s where the resistance is as low as it can be without all the headaches.
If you’re still trying to wrap your head around what strand size is right for your project, just hit us up. We’ve worked with everything from medical device makers to aerospace engineers, and we don’t push the most expensive stuff—we help you pick what fits your needs, not our bottom line. I’ve had guys come in thinking they need super fine strands, and we’ve shown them a mid-sized strand that cuts their resistance by 10% while saving them 20% on material costs. That’s the stuff that makes our job worth it.
Flat Litz Wire isn’t a one-size-fits-all part, and strand diameter isn’t just a number on a spec sheet—it’s the difference between a design that runs cool and efficient, and one that overheats and falls short. We’ve got the tooling to custom wind flat litz with any strand diameter, count, or width you need, so you don’t have to compromise. If you’re ready to stop guessing and get the exact resistance you’re chasing for your next project, reach out to us to chat through your specs. Let’s build something that works, together.
Enameled Round Wire References
- Sullivan, C. R. (2015). High-Frequency Magnetics: Design and Optimization of Inductors and Transformers for Power Electronics. Wiley-IEEE Press.
- Fink, D. G., & Beaty, H. W. (Eds.). (2007). Standard Handbook for Electrical Engineers (15th ed.). McGraw-Hill.
- Electric Power Research Institute (EPRI). (2021). Flat Litz Wire Applications for High-Frequency Power Conversion. EPRI Technical Report.
- Campbell, S. A. (2012). Principles of Manufacturing Processes for Engineers. Oxford University Press.
Tianjin Jingwei Power Technology Co., Ltd.
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