How to Select a DT Copper Lug Properly

The DT copper lug is the purple-copper connector terminal you see every day at the end of a cable inside a switchgear cabinet. The “DT” in the name is simply short for a copper connection terminal — D stands for crimping, T stands for copper. The full name is the DT series copper connection terminal, and it is one of the most fundamental — and least forgiving — parts of any electrical connection.

It is normally extruded in one piece from T2 copper bar, with copper content above 99.9%. Solid and seamless, it conducts well, resists tensile stress, and does not deform easily after crimping. It looks insignificant, but in a high-current application it carries the connection reliability of the entire circuit.

The defining feature of the DT is that the tail is closed — the well-known “oil-block” design. When a cable termination is made, this closed tail prevents insulating oil from seeping out behind the terminal. There is a similar-looking sibling, the DTG, which is stamped from copper tube with an open tail and no oil blocking, and costs considerably less. So if you are building high-voltage cable terminations or working in an oil-immersed environment, never substitute a DTG for a DT — they are not interchangeable.

The model numbers are easy to read: the number is the cable cross-section. DT-70, for example, is for a 70 mm² copper cable. From DT-10 through DT-630 and DT-800 the range is essentially complete. As a rule, DT is used for copper cables of 10 mm² and above; below that a crimp ferrule is used. Genuine DT terminals must conform to GB/T 14315-2008, so it is worth checking for that when you buy.

Crimping is a craft and cannot be done with a single casual squeeze. The strip length has to match the barrel length so that the insulation butts right up against the terminal. The oxide layer on the copper strands is best sanded off, otherwise the contact resistance rises. In practice you crimp once near the tail of the terminal and then again near the root where it meets the busbar, using a matching hydraulic crimper and die so the conductor is fully enclosed. After crimping, fit a heat-shrink tube or wrap the insulation back with tape.

Two surface finishes are available. Plain copper is cheaper and suits dry indoor environments, but it will oxidise and darken over time. Tinned copper costs more but is used far more in damp, outdoor or photovoltaic applications, because the tin layer prevents copper oxidation and keeps the contact resistance stable.

Here are a few real-world situations that show you exactly where this part lives.

Switchgear incoming and outgoing feeders. This is DT’s main battlefield. In a low-voltage switchgear cabinet the incoming cable comes from the transformer at 240 mm², 300 mm² or more. Once the cable is stripped, the copper core is pushed straight into the DT barrel and crimped solid with a hydraulic tool, and the round hole at the other end is bolted onto the breaker or busbar. Practically every main-circuit connection inside the cabinet relies on this part.

Box substations and transformers. Space inside a box substation is tight, and the cable termination between the transformer outgoing side and the low-voltage cabinet is almost always made on site. This is where the oil-block structure of the DT earns its place — especially with oil-immersed transformers, where the cable termination contains insulating oil. If the tail of the terminal is not sealed, the oil slowly seeps out and the insulation properties degrade over time. For transformer outgoing terminations, DT is the standard and DTG must not be used.

Photovoltaic DC side. With more and more photovoltaic plants being built, there are large volumes of PV-specific cable running from module strings to inverters and from inverters to combiner boxes. Most of these cables are copper or tinned copper, and the joints use tinned DT terminals. Outdoors, exposed to sun and rain, plain copper oxidises and turns black within months; the tin layer holds the process back and keeps the contact resistance stable. On photovoltaic projects, therefore, tinned DT is the default choice even though it costs more.

Industrial power cables. For factory motors, pumps and fans, the power cable runs from the switch room to the machine and is crimped with a DT at both ends. The currents are large and the vibration is heavy, so poor crimping quality leads to heating after a period of operation and in the worst case burns the terminal red. Industrial projects therefore impose stricter requirements on crimping: the hydraulic die must match the terminal size, and after crimping the joint has to be checked for off-centre or cracked deformation.

Cable termination construction. When making 10 kV or 35 kV cable terminations, the DT is the critical connection element. From stripping the outer sheath and armour through to crimping the DT terminal and restoring the insulation, every step has its own requirements. Here the DT does not merely conduct — it also carries the mechanical pull of the cable itself, so its tensile strength directly determines whether the termination keeps working reliably over the long term.

EV charging and energy storage. EV chargers and battery storage cabinets also contain a large number of busbar and cable connections. The gun leads of charging piles and the battery-module connections inside storage cabinets frequently use DT terminals or similar crimped terminals. These applications combine high current with frequent mating and unmating, so they demand very high standards for conductive stability and vibration resistance.

Ultimately, the DT copper lug is the “screw” of electrical connection — unremarkable in appearance, but indispensable in switchgear, transformers, photovoltaics, industry and charging infrastructure. Choose it well and crimp it properly, and it will work safely for decades; choose wrong or crimp it carelessly, and overheating, burning and outages are all its doing.