Can I connect two different sizes of copper core cables together?
When a customer messages me out of the blue asking this question, I know exactly what they’re dealing with. As a copper core cable supplier who’s been in this game for over 18 years—first working with installation teams across residential build sites, then moving into commercial and industrial projects—this is one of the most common, and most misunderstood, questions we get. Most folks reach for whatever tools are handy or go with the first connector they find in their van, but that shortcut often leads to bigger problems down the line. Today, let’s break this down properly. Copper Core Cable
First, let’s get clear on what “different sizes of copper core cables” actually means here. We’re talking about two cables with different cross-sectional areas—like a 14 AWG (American Wire Gauge) and a 12 AWG, or a 6 mm² and a 10 mm². The smaller number means a thicker copper core, right? So a 12 AWG cable has more copper conducting material than 14 AWG. That difference is the root of everything you need to think through.
Let’s start with the science. Copper’s job is to carry electrical current, and every material has a limit to how much current it can handle safely. That limit is called ampacity, and it directly ties to the size of the copper core. A thicker core can carry more current without overheating because it has more space for electrons to flow, and the heat generated is dispersed over a larger surface area. A thinner core can’t do that—run too much current through it, and the wire’s insulation melts, the copper oxidizes, or even worse, you get a fire.
If you connect two different sizes, the smaller one will act as a bottleneck for the entire circuit. Let’s say you have a 10 mm² cable running from your main breaker, and you splice on a 16 mm² cable for a branch circuit. Wait, no—actually, it’s the other way around. If your main run is the thicker 16 mm² (ampacity ~100 amps) and you connect a 10 mm² branch (ampacity ~55 amps), that branch wire will only be able to carry 55 amps, no matter how much bigger the main wire is. If someone tries to pull 70 amps through that, the 10 mm² wire will overheat. I’ve seen this happen on job sites too many times—installers thought a thicker wire meant they could just splice any size to it, not realizing they were basically turning their branch circuit into a tiny, fire-prone wire.
But before you think “just never do this,” let’s get to the part where it is possible. It’s not a hard no—it’s a “do it right, or don’t do it at all.” The key rule here is: the conductor that will carry the lower current (the smaller size) has to be protected by an overcurrent device, like a circuit breaker or fuse, rated for its ampacity. That’s your non-negotiable. If you have a 12 AWG cable (ampacity ~20 amps) and you want to connect it to a 10 AWG cable (ampacity ~30 amps), you can do that, but you have to put a 20-amp breaker on the 12 AWG circuit. The 10 AWG can handle up to 30 amps, but since your smaller wire can only take 20, the breaker will trip before the 12 AWG overheats. That’s the safety check that makes it work.
Now, how do you actually make the connection between two different copper core cables? That’s another area where mistakes happen. The old way of twisting two bare wires together and taping them with electrical tape? That’s a recipe for corrosion and resistance, especially with different sizes. A loose connection creates “hot spots”—places where electrical resistance is higher, which generates heat, and if the connection isn’t tight enough, oxygen gets in and the copper oxidizes over time, making the resistance even worse. Eventually, that hot spot can melt the insulation, or even arc, which is how fires start.
The right methods fall into two categories, depending on the project. First, for low-voltage applications (like residential branch circuits, 120/240V), use a UL-listed wire connector specifically designed for multiple copper conductor sizes. These are not the cheap twist-on caps you see at hardware stores for single-size wires. Look for ones labeled for “range of conductor sizes”—so a cap that says it works for 14 to 10 AWG, for example. These connectors have internal metal springs that grip each wire evenly, even if they’re different thicknesses, and they’re sealed to keep moisture out. I always tell our customers that spending an extra 50 cents on a proper connector is way cheaper than redoing a splice in a year because it failed.
For higher-voltage or industrial applications, like splicing a 6 mm² control cable to a 16 mm² power cable, compression lugs are the way to go. Compression lugs create a permanent, low-resistance connection that’s way more reliable than twist-on caps. The process is: strip the insulation from both ends of the cables, slide the lug onto the thicker conductor, crimp it with a hydraulic tool (not a hand crimp—hand tools don’t create enough pressure for a solid connection), then you can either extend the splice with a pigtail of the smaller conductor that matches the circuit’s ampacity, or use a two-hole lug that accepts both wires. Wait, hold on—two-hole lugs for different sizes? Yes, but only if you follow the same ampacity rule: the smaller wire has to be sized for the breaker. And make sure the lug is rated for both conductor sizes, not just one. I’ve seen installers grab a 16 mm² lug and try to cram a 10 mm² wire into it, which leaves too much space, so the connection is loose. That’s a waste of time and money, and dangerous.
Now, what about the common myths I hear all the time? Let’s bust them one by one. First: “The bigger cable will carry the current, so the smaller one doesn’t matter.” No, that’s not how current works. Current follows the path of least resistance, but if one wire is smaller, its resistance is higher, so more voltage drops across that wire—meaning less voltage at the end of the circuit, and more heat in the splice. I tested this once in our warehouse: we spliced a 12 AWG and 10 AWG wire, ran 25 amps through them, and the splice on the 12 AWG got 10 degrees hotter than the 10 AWG. Multiply that by 8 hours a day, and that insulation will degrade 10 times faster.
Second myth: “If I use a bigger connector, it will work for any size.” Connectors are designed for specific ranges. A connector for 14-12 AWG won’t grip a 16 AWG wire tight enough, and it will crush a 10 AWG wire, damaging the copper core inside. I’ve had a customer bring back a connector he used for a 10 and 14 AWG splice—he bought a connector rated for 12-10 AWG, and it flattened the 14 AWG wire so bad it split when he pulled it. That’s not a “one-size-fits-all” part.
Third myth: “It only matters for residential work, not commercial.” Wrong. We supply cables to a lot of industrial facilities, and I’ve seen a manufacturing plant lose power for 3 hours because a spliced connection between a 10 mm² and 16 mm² cable overheated and melted. That shutdown cost them over $50,000. The initial splice cost him $2 in connectors—he cut corners, and it bit him hard. Size matters everywhere, not just at home.
Another thing to consider: different insulation types. If one cable is THHN (thermoplastic high heat-resistant nylon) and the other is NM (non-metallic sheathed), even if their copper sizes are the same, you have to make sure the connector is compatible. But when sizes are different, insulation is less of a factor than the conductor’s ampacity rating. Still, never splice a cable that’s rated for a lower temperature with one rated for higher, unless you’re planning to upgrade the insulation later. That’s a mistake I see new installers make all the time.
Wait, let’s talk about exceptions. Is there any time connecting different size copper core cables is not recommended? Yes—if you’re connecting a wire with a lower temperature rating to one with a higher one, and the smaller wire is near its limit. For example, a 14 AWG NM cable (rated for 60°C) and a 12 AWG THHN (rated for 90°C). If you run 15 amps through them, the 14 AWG will be at its limit, and even though the splice is inside a wall where temperature is low, the insulation on the NM wire will degrade faster. In that case, you should either use a pigtail that matches the lower temperature rating, or run a new wire of the same size for that circuit. It’s not that you can’t do it, but you have to account for all variables, not just the copper size.
Now, as someone who’s on the ground with installers every week, let’s give you a practical checklist if you decide to go through with this. First, check the ampacity of both wires. The smaller one has to have an ampacity that’s equal to or lower than the circuit breaker or fuse it’s connected to. If your breaker is 20 amps, your smaller wire can’t be larger than 12 AWG (which is rated for 20 amps at 60°C, per NEC standards). Second, choose a connector rated for both wire sizes. Don’t guess—check the label on the connector or the product sheet. Third, strip the insulation the right length. Too much insulation left, and you’ll have bare copper exposed; too little, and the connector won’t grip the wire. For twist-on caps, strip about ½ inch of insulation; for compression lugs, strip about ¼ inch more than the lug’s entry point to make sure the whole conductor is inside. Fourth, use the correct tool. For twist-on caps, twist them until they’re snug, then give the wire a gentle tug to make sure it’s not loose. For compression lugs, use a tool with the right die for the conductor size—using the wrong die will create a cold crimp, which is just as bad as a loose connection.
I should also mention: never splice wires inside a wall, ceiling, or floor unless you’re using a listed junction box. A lot of DIYers think “a good splice is enough,” but code requires all splices to be accessible in a junction box. That way, if something goes wrong (which it will, eventually), you can get to it without tearing out drywall. I see this on residential jobs all the time—someone splices two different wires in the attic, tucks it away, and 5 years later, there’s a short circuit and they have to cut out half the ceiling to find it. It’s not worth the hassle.
Now, let’s talk about when you should just walk away from the idea. If you’re connecting a wire that’s too small for the circuit—say, a 16 AWG (15 amp) wire to a 30-amp breaker—don’t do it. Even if you use a proper connector, the 16 AWG wire will overheat. There’s no way around that. The ampacity is too low, the circuit is pulling more current than the wire can handle, and that’s a fire hazard. We get a lot of inquiries from DIY homeowners who want to extend an outdoor light circuit and think “thicker is better,” but they grab a too-small wire because it was cheap at the store. That’s when we tell them to go for the same size, or upgrade the breaker if they need a bigger circuit.
Another scenario: connecting a solid copper cable to a stranded copper cable. Wait, that’s another “different size” situation people ask about. Solid and stranded copper are both copper, but their construction is different. Solid is a single piece, stranded is multiple thin wires twisted together. Can you connect them? Yes, but you have to be careful. Stranded wires can fray if you don’t grip them properly, and solid wires can split if you over-crimp. The solution is to use a connector that’s rated for both solid and stranded conductors, or tin the stranded wire before crimping it to make it sturdier. Tinning means coating the stranded wire with solder so the individual strands stay together—this prevents fraying and creates a more solid connection. I always recommend tinning for stranded wires when splicing to solid, especially in outdoor or high-vibration applications.
Let me share a recent story that illustrates why this matters. A contractor came to us last month, panicking because he had a job where he needed to connect 100 feet of 6 mm² cable to a 10 mm² cable for a pool pump. He had already used a cheap twist-on cap, and when we tested the connection, the resistance was 5 times higher than a proper compression lug. We showed him the right way: he cut the splice, used a compression lug rated for both 6 and 10 mm², crimped it correctly, and tested it again. The resistance dropped to normal levels. He later told us that his client had noticed the lights dimming when the pool pump turned on— that was the high resistance from the bad splice causing a voltage drop. If he had left it, the pump would have burned out in a few months, and he would have had to cover the cost of a new pump. That’s the kind of mistake that a little bit of knowledge could have prevented.
So, to circle back to the original question: can you connect two different sizes of copper core cables? The short answer is: yes, but only if you follow the rules. It’s not a free pass to mix and match any size, and it’s not a “do it yourself and forget it” job. It requires checking ampacity, using the right connectors or lugs, making a proper low-resistance connection, and ensuring everything complies with electrical codes (like the National Electrical Code in the US, or IEC standards internationally).
As a copper core cable supplier, I’ve seen firsthand what happens when you cut corners on these splices. Fires, burned equipment, upset customers, and contractors dealing with costly reworks. That’s why we never sell a cable without reminding our customers to plan their splices properly, and we always have a team on hand to answer questions before someone makes a dangerous mistake.
If you’re working on a project and you’re not sure if you can connect two different copper core cables, or you need the right connectors or lugs to make the connection safely, reach out to our team to talk through your project details. We don’t just sell cables—we help you make sure your connections are safe, compliant, and built to last. Don’t risk a shortcut turning into a big problem. Let’s work together to get your splice right the first time.
References
Aluminum Alloy Cable National Electrical Code (NEC) Article 110-14: Ampacity of Conductors
Underwriters Laboratories (UL) Standard 486A: Wire Connectors and Soldering Lugs for Use with Copper Conductors
International Electrotechnical Commission (IEC) 60227: Polyvinyl Chloride Insulated Cables of Rated Voltage Up to and Including 450/750 V
Hebei Fengbo Electrical Equipment Co., Ltd.
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