The "1.5mm²" Power Strip Cords Got Too Hot to Touch
Power Cord Wire Gauge & Copper Verification Guide - Why Power Cords and Power Strips That Passed Inspection Overheat and Soften in Use, Marked Cross-Section vs Actual Size, Pure Copper vs CCA (Copper-Clad Aluminum) and Recycled Copper, Checks Such as Conductor Resistance, Strand Count and Diameter, Insulation Thickness, Scraping and Weight Compared, Approximate Criteria, Gaps in Factory Reports and Jacket Printing, Supplier Substitution Such as Copper Samples Only, Undersized Conductors, Fewer Strands and Thinner Insulation, Spec Sheet Items, Stage-by-Stage Checks and a Checklist

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A household goods seller imported 5,000 power strips with individual switches from a factory in Cixi (慈溪), Zhejiang. They were rated 16A, the cord jacket was clearly printed "3×1.5mm²," and the pre-shipment inspection passed them on appearance, continuity, switch function and packaging. The factory said the cords were "national standard (国标) pure copper" and even attached a material test report.

The trouble started when winter arrived. Customers who plugged in electric heaters and fan heaters began reporting that "the cord is too hot to touch" and "the jacket near the plug has gone soft." Close to twenty complaints came in within two weeks. When the seller stripped the cords from remaining stock, the copper-colored strands were silver inside. It was CCA wire: aluminum with a thin copper coating. The actual cross-section, calculated from the strand diameter, was about 1.0mm², and the conductor resistance per meter was more than double the limit for 1.5mm² pure copper. Higher resistance means much more heat at the same current. A few hours with a 2kW heater was enough for the cords to overheat.

What makes wire problems tricky is that nothing looks different from the outside. CCA wire and undersized wire look exactly like genuine pure copper once they are jacketed, and they pass continuity checks without trouble. The difference shows up as heat only after high current has flowed for hours. This article covers how to check wire size and material, approximate criteria, the substitutions factories commonly use, and how to add wire checks to your spec sheet and inspection checklist, all from a practical point of view.

The "1.5mm²" printed on the jacket and a material test report are not enough. Set conductor material, cross-section, strand construction and a maximum conductor resistance in numbers in your spec sheet, and cut and measure the cords of products you pick yourself from the production lot. That is how you prevent overheating claims and safety incidents.

1. Why Cords Overheat and Insulation Melts

When current flows through a wire, conductor resistance turns some of it into heat. Heat equals current squared times resistance, so at the same 16A a wire with double the resistance produces double the heat. Resistance is set by the conductor's material and cross-section. Use a material that conducts worse than copper, or shrink the cross-section, and resistance goes up.

Aluminum has about 1.6 times the resistance of copper. CCA wire is an aluminum core with a thin copper coating, so it looks like copper but behaves electrically much like aluminum. Use a thinner wire than marked on top of that and resistance climbs further. If a wire marked 1.5mm² is really 1.0mm², that alone raises resistance by 1.5 times. Add CCA and it ends up well over double that of the genuine product.

A high-resistance cord gives no sign of trouble when it powers a phone charger or a desk lamp. Plug in something that draws high current for a long time, such as an electric heater, air fryer or electric cooktop, and the cord heats up. The insulation softens and then melts, which can lead to short circuits or fire.

Wire size and material problems usually come from five causes.

• Material substitution: CCA, copper-clad steel (CCS) or impure recycled copper (杂铜) instead of pure copper.
• Undersized conductors: marked 1.5mm² but actually equivalent to 1.0mm² or 1.25mm². The trade calls this "non-standard" (非标) wire.
• Fewer strands: cutting the strand count and thickening the insulation so the cord looks the same size, or shaving a little off each strand's diameter.
• Thinner insulation: reducing insulation and sheath thickness to cut cost, then adding filler to keep the outer diameter.
• Unauthorized change of wire supplier: mass producing with cheaper wire from a different supplier than the one used for the approval sample.

We covered importing home appliances in general in Episode 75 and dielectric strength testing in Episode 138. This article focuses on checking that the conductor inside the cord is what the marking says.

2. Checking Methods — From Jacket Printing to Conductor Resistance

If you only say "please check the wire spec," the factory will often send a photo of the jacket printing or the wire supplier's report and leave it there. Each method tells you something different, so decide first how you will check.

Jacket printing is not evidence

The most common mistake is to see "3×1.5mm²" and a certification mark on the sheath and stop there. Jacket printing is just ink-jetted text added during extrusion and has nothing to do with what is inside. Plenty of non-standard wire carries national standard sizes and well-known wire makers' names. Treat the printing as a starting point only, and always cut and measure.

Conductor resistance is the final test

Measuring conductor resistance is more reliable than checking material and size one by one. CCA, thin wire and recycled copper all end up showing as higher resistance. Cut at least 1m of conductor, take a four-wire measurement with a low-resistance meter, record the temperature and convert to 20°C. If the resistance is within the standard limit, the wire passes electrically, whatever was done to its material or size.

3. What Criteria to Set

Maximum conductor resistance by cross-section is set in IEC 60228 and in GB/T 3956, which carries the same content. Approximate values at 20°C for flexible stranded conductors (Class 5) used in power cords are below. Set actual criteria with input from a test lab based on cable type, rating and certification requirements.

Read the table backwards and it works as a screening tool too. If a wire marked 1.5mm² measures around 19 Ω/km, it is really 1.0mm² class; above 20 Ω/km it is almost always undersized or CCA. For insulation, IEC 60227-type PVC cords of 0.75-1.5mm² specify an average insulation thickness of about 0.6-0.7mm, so cut a cross-section and look for visibly thin spots as well.

4. Why You Should Not Take Factory Reports and Certificates at Face Value

When you ask for evidence on the wire, the factory usually sends the wire supplier's outgoing test report or a copy of a KC or CCC certificate. These documents have gaps.

5. Common Supplier Substitutions

Copper makes up a large share of a power cord's cost, so when copper prices rise or price pressure is heavy, the wire is the first place factories cut. Here are the patterns we have actually run into in consultations.

The most common pattern is genuine copper in the samples and different wire in production. The product looks fine and passes continuity and dielectric tests, so unless the inspector cuts the cord, there is no way to notice. Simply cutting 1m from the cord of a randomly picked product and measuring its resistance catches a large share of cases. We explained the same principle of covert material substitution for plastics in Episode 125 and stainless steel in Episode 150.

6. The Damage Wire Defects Leave Behind

Wire defects do not show at shipment and only appear when customers draw high current, but the damage spreads far beyond the product's price.

• Fire and burn risk: overheated cords can melt their insulation and cause short circuits or fire, and injuries turn into product liability issues.
• Certification withdrawal and sales stop: if a KC-certified product turns out to use different wire than at certification, you can face certificate cancellation, a sales ban and even a recall order.
• Mass recall: the whole lot was made with the same wire, so if one unit is bad you often have to recall everything.
• Platform penalties: as safety reviews and reports pile up, marketplaces delist the product and lower your seller rating.

See Episode 53 for product liability and recalls and Episode 10 for product certification.

7. What to Put in the Spec Sheet

A single line like "3×1.5mm² pure copper wire" gets read differently by every factory. Put the items below in a table and attach it to your spec sheet. See Episode 59 for how to write a spec sheet.

Writing the maximum conductor resistance as a number may seem fussy, but it does the most work. Words like "pure copper" and "1.5mm²" leave room for interpretation, while a resistance value gives a clear answer with one meter. See Episode 119 for how to handle changes in wire supplier or spec.

8. How to Add Wire Checks to Your QC Checklist

Sample stage

When the approval samples arrive, sacrifice one cord: cut 1m and record conductor resistance, strand count, strand diameter and insulation thickness with cross-section photos. Run the remaining samples near rated current for two to three hours and measure cord and plug temperature. These records become the baseline for comparing production units.

Production stage

During production, get the lot number, the wire maker's report and the factory's own conductor resistance measurement when wire arrives at the factory. If the results show the same number every time with no measurement temperature, start by asking whether anyone actually measured. If possible, also get photos of the labels on the incoming wire reels. See Episode 102 for production tracking.

Pre-shipment inspection stage

The inspector picks packed products personally, cuts the cord, scrapes the strands to check the color inside, tries a magnet and measures 1m conductor resistance. If bringing a low-resistance meter is impractical, seal randomly picked samples and send them to a third-party lab. Also check that the jacket printing and plug markings match the certificate. See Episode 100 for pre-shipment inspection and Episode 47 for using third-party inspection.

9. Sample Wording to Send the Factory

Putting the wording below in your purchase order or spec sheet takes the choice of wire out of the factory's hands and makes it easier to assign responsibility if overheating claims arise.

"本产品电源线须使用纯铜多股导体H05VV-F 3G1.5mm²（与认证报告所列规格及线材供应商一致），严禁使用铜包铝（CCA）、铜包钢（CCS）、再生铜或非标线。导体电阻须符合GB/T 3956或IEC 60228第5类导体要求，20℃换算值不得超过13.3Ω/km；绝缘及护套厚度须符合IEC 60227要求。未经我方书面同意不得更换线材供应商、线材规格或生产工厂。每批须提交线材入库批号、线材供应商出厂报告，以及注明测量长度、温度和设备的导体电阻实测记录和截面照片。检测样品由我方验货员从成品中随机抽取并剪线测量。如有争议，以第三方实验室按GB/T 3048.4或IEC 60228测得的导体电阻为准。如有不合格，须加抽复测；复测仍不合格的批次暂停出货，并更换合格线材返工。"
(The power cord of this product shall use pure copper stranded conductor H05VV-F 3G1.5mm² (identical in spec and wire supplier to the certification report); copper-clad aluminum (CCA), copper-clad steel (CCS), recycled copper and non-standard wire are prohibited. Conductor resistance shall meet GB/T 3956 or IEC 60228 Class 5 requirements and shall not exceed 13.3 Ω/km at 20°C; insulation and sheath thickness shall meet IEC 60227. The wire supplier, wire spec and production factory may not be changed without our written consent. Each lot shall be accompanied by the incoming wire lot number, the wire supplier's outgoing report, and measured conductor resistance records stating length, temperature and equipment, with cross-section photos. Test samples shall be picked at random from finished products by our inspector and the cords cut for measurement. In case of dispute, conductor resistance measured by a third-party lab under GB/T 3048.4 or IEC 60228 shall prevail. If any sample fails, additional samples shall be drawn and retested; lots that still fail shall be held from shipment and reworked with conforming wire.)

10. Three Real Cases

These are anonymized versions of patterns we see often in consultations. Amounts and figures are examples.

Case 1 — 16A power strips with individual switches

This is the case from the start of the article. When the seller confronted the factory with the measurements, the factory blamed its wire supplier for sending something different. It turned out the wire supplier used for the certification sample differed from the one used in production; the factory had switched to a supplier 30% cheaper without saying so. Sales suspension, voluntary recall, replacements and certification-related work added up to a loss of about 48 million won, and the factory covered only part of it. From the next order, the wire maker and maximum conductor resistance are in the spec sheet, and cords are cut and measured at pre-shipment inspection.

Case 2 — Electric heating mat power cords

A winter goods brand imported 2,000 electric heating mats whose cords were printed "2×0.75mm²" and looked copper-colored all the way through when scraped. Yet complaints that "the cord connection gets warm" kept coming, and a lab measured conductor resistance at 39 Ω/km, 1.5 times the limit. The color was copper, but the wire was impure recycled copper with strands thinner than standard. Scraping and a magnet could not have caught it, so the brand now requires measured conductor resistance for every lot.

Case 3 — USB fast-charging cables

A phone accessories company sold 10,000 USB-C cables as "100W fast charging," and reviews saying "charging is slow and the cable gets hot" kept piling up. Cutting the cables open showed CCA power wires thinner than the marked spec. Under high current the voltage drop was large enough that chargers cut their output. Data transfer and short charging tests worked fine, which is how the cables passed inspection. The company has since added power wire spec and resistance to the spec sheet and a check that charges at rated current for a set time while measuring cable temperature.

11. Common Mistakes

• Passing products based only on the size and certification marks printed on the jacket
• Writing only "pure copper" or "national standard wire" without a maximum conductor resistance in numbers
• Assuming copper color after scraping means genuine, and missing recycled copper or undersizing
• Not checking that the certification sample and production use the same wire supplier
• Running only continuity and dielectric tests without checking heating at rated current
• Checking only the wire and never opening up the plug and socket connections

12. Wire Gauge and Material Control Checklist

Specification stage

• Set cable type, size and conductor material (pure copper)
• Wrote the maximum conductor resistance as a value at 20°C
• Wrote strand construction and insulation thickness criteria
• Fixed the wire supplier and set a change approval procedure
• Confirmed the spec sheet matches the wire spec listed in the certificate

Sample and production stage

• Recorded the approval sample's conductor resistance, strands and insulation thickness with cross-section photos
• Checked cord and plug temperature on approval samples under load
• Received incoming wire lot numbers and the wire maker's reports
• Received the factory's own conductor resistance measurements with temperature

Pre-shipment inspection stage

• Cut cords from random samples and checked core color and magnet response
• Measured 1m conductor resistance or sent sealed samples to a lab
• Checked that jacket printing and plug markings match the certificate
• Opened plug and socket connections to check crimping and soldering

Final Thoughts - Check Resistance, Not Printing

When power cords leave the factory, genuine copper and CCA look equally neat and smooth. The difference only shows up as heat after a customer has run a heater for a few hours. Writing a maximum conductor resistance in the spec sheet and cutting 1m of cord per lot to measure it does not cost much. Put the price of one low-resistance meter next to tens of millions of won in recall and certification costs, and it is clear which is cheaper.

There is something you can do right now. Ask the factory currently in production to send the name of the wire maker it is using, the incoming lot number, measured conductor resistance with temperature, and photos of the wire cross-section. If the reply is a single line saying "it's national standard pure copper," make a wire control conditions sheet and send it before your next order.

StartMade supports you on the ground with spec sheets that set wire spec and conductor resistance criteria, wire measurement records for approval samples, checks on incoming wire during production, and pre-shipment inspections where cords are cut and measured. If you have already received overheating claims, we start by measuring the wire in your remaining stock; if you are preparing a new product, we start with wire spec and supplier selection.