DLO vs THHN vs Bare Copper Jumpers: Which Conductor and Lug to Use

Three conductor types cover most custom jumper work: Type DLO flexible cable, THHN/THWN-2 building wire, and bare copper. They can share an AWG size and a lug pattern and still behave very differently once installed. The right choice depends on how much the jumper has to bend, what environment it sits in, what voltage it sees, and which termination hardware is approved for its stranding. This article compares the three and explains why the lug choice follows the conductor choice.

Stranding classes: the detail that drives everything

Copper conductors are made in standardized stranding classes. NEC Chapter 9, Table 10 lists Class B and Class C as the reference stranding for building wire. Finer classes are used where flexibility matters:

• Class B and Class C: concentric-lay stranding with relatively few, larger strands. This is standard for building wire such as THHN.
• Class I: rope-lay stranding with bunched members. Southwire and other manufacturers specify Class I tinned copper per ASTM B172 for Type DLO.
• Class K and Class M: finer still. Industry references describe Class K as built from 30 AWG wires and Class M from 34 AWG wires, used for welding and other high-flex cable.

More strands of smaller diameter make a cable more flexible. That same construction changes how the conductor behaves inside a compression barrel, which is why NEC 110.14 requires connectors and terminals for conductors finer than Class B and Class C to be identified for the specific class.

Type DLO: flexible power cable

DLO (diesel locomotive cable) is a single-conductor flexible power cable. A typical 2 kV DLO construction, as described on manufacturer data sheets, includes:

• Class I rope-lay tinned copper conductor
• EPDM insulation with a CPE jacket
• 2,000 V rating
• 90 °C continuous rating in wet and dry locations, with higher emergency overload and short-circuit ratings
• Dual marking as Type RHH/RHW-2 on cables listed that way, which lets the cable be used where those NEC wire types are permitted

Where it is used: battery energy storage cabinets, UPS battery strings, generator and load bank connections, rectifiers, and any location where a jumper has to make a tight bend between fixed terminals or take vibration. Its flexibility reduces the bending stress transferred to battery posts and bus connections. The tinned conductor resists corrosion in battery rooms and outdoor enclosures.

What it requires: lugs identified for fine-strand conductors. More on that below.

THHN/THWN-2: building wire

THHN/THWN-2 is single-conductor building wire with PVC insulation and a nylon jacket. It is rated 600 V and 90 °C in dry locations as THHN and in wet locations as THWN-2. Larger sizes use Class B stranding.

Where it is used: jumpers inside switchgear, panelboards, and control enclosures where the routing is fixed and the bend is gentle. It is also used when a project spec calls for building wire type insulation throughout, or when the jumper must match the conductors it connects to. THHN has a smaller outside diameter than DLO of the same size, which helps in crowded gutters.

What it requires: standard compression lugs rated for Class B and Class C copper. No special lug class is needed.

Limitations: it is stiffer than DLO, so tight bends put more stress on terminals. The 600 V rating rules it out of higher-voltage DC circuits where DLO's 2 kV rating applies.

Bare copper: grounding and bonding

Bare copper jumpers have no insulation. They are used for equipment grounding and bonding: rack and cabinet bonds, ground bar connections, and telecom grounding where a two-hole lug on 5/8 in centers is common.

What it requires: standard compression lugs for the conductor's stranding. Two-hole lugs are preferred where the connection must not rotate.

Limitations: it is not used where an insulated conductor is required. Identification of grounding conductors follows NEC 250.119, so check the project spec for any marking requirements.

Comparison table

Why DLO needs flex-rated lugs

A compression lug is a system made of a barrel, a die, and a conductor. Standard lugs are designed and tested around Class B and Class C stranding. A fine-strand conductor of the same AWG has a different strand count and packing, so the standard die and barrel can produce a termination that is under-compressed or that lets strands migrate under thermal cycling. The result is higher contact resistance, heat, and eventually a loose or failed connection. Terminations on fine-strand cable are a known failure point in the field.

The code and the product standard address this directly:

• NEC 110.14: connectors and terminals for conductors more finely stranded than Class B and Class C must be identified for the specific conductor class or classes.
• UL 486A-486B: connectors tested with conductor classes other than Class B, Class C, or single input wire must be marked with those classes on the connector, its unit container, or an information sheet packed with it. Unmarked connectors are limited to the standard classes.

The practical rule: if the cable is DLO, welding cable, or any other fine-strand construction, the lug must be identified for that class. In the Conversions Tech Custom Cable Builder this is not optional. Choosing DLO assigns flex-rated tinned copper long-barrel lugs, crimped twice, and the engineering drawing lists the lug's conductor class. Flex lugs are priced 10% above standard lugs. That cost is small compared with a failed termination in a battery cabinet.

THHN and bare copper builds use standard tinned copper long-barrel compression lugs, also crimped twice. You can browse both lug families in the copper battery cable lug collection, and the lug applications guide covers hole patterns and stud sizes.

Choosing by application

• BESS and UPS battery strings: DLO with flex lugs. The voltage rating, flexibility, and tinned conductor suit tight cabinet routing and repeated thermal cycling.
• Generator, load bank, and rectifier connections: DLO, especially where vibration is present.
• Switchgear and panel jumpers with fixed routing: THHN/THWN-2 when the bend radius is manageable and the project calls for building wire.
• Grounding and bonding: bare copper with two-hole lugs where anti-rotation matters.
• Mixed projects: use the same builder for all three. Each build gets its own part number and drawing, so a submittal can show every jumper type in one consistent format.

Configure and order

The Custom Cable Builder prices DLO and bare copper jumpers from 8 AWG to 4/0, and THHN to 500 kcmil, instantly and shows NEC 310.16 ampacity at 75 °C and 90 °C for the size selected. Every build generates a permanent reorderable part number and an engineering drawing. Volume breaks apply at 10, 25, and 100 pieces. Approved trade accounts can order on PO and net terms. Assemblies are Assembled in USA from UL listed components.

FAQ

Is DLO allowed as building wire?

DLO that carries the RHH/RHW-2 marking is recognized as those NEC wire types and can be used where they are permitted. Check the cable marking and the project spec.

Can I put a standard lug on welding cable or DLO if it fits?

A lug that physically fits is not necessarily identified for the conductor class. NEC 110.14 requires the termination to be identified for fine-strand classes. Use lugs marked for the class.

Why choose THHN over DLO for a switchgear jumper?

THHN has a smaller outside diameter and uses standard lugs. It fits fixed routing in tight gutters when the project calls for building wire. DLO is better where flexibility, vibration resistance, or a higher voltage rating is needed.

Do bare copper jumpers need heat shrink?

Not for insulation. Some sites use colored shrink or labels for identification. You can add printed labels in the builder.

Which sizes are priced instantly?

DLO and bare copper from 8 AWG through 4/0, and THHN from 8 AWG through 500 kcmil. DLO from 262 to 777 kcmil and THHN at 750 kcmil are quoted, with a budget price shown during configuration.