You're right to be concerned about running THHN wire directly through a mechanical lug. Here's a breakdown of why, and what you should be doing instead

The Problem: Mechanical Lugs and Wire Types
Mechanical lugs are generally designed for specific conductor types and sizes. While they create a tight connection through clamping force, the contact surface isn't necessarily optimized for all types of wire. THHN (and similar insulation types like THWN, THWN-2) are designed for specific operating temperatures and environments. They have a specific insulation thickness. Direct contact between the wire and lug could lead to



Damage to the insulation

The sharp edges of the lug, or the pressure exerted by the screw, could nick, cut, or compress the insulation. This compromises the wire's protection against short circuits and ground faults.

Overheating

An improper connection (even if it seems tight) can create resistance, leading to heat buildup. This can further damage the insulation and potentially cause a fire.

Corrosion

Dissimilar metals (the wire and lug) in contact with moisture can create corrosion, weakening the connection over time.

Code Violations

Most electrical codes (like the NEC in the US) have specific requirements for terminations, and simply shoving a wire through a lug that's not designed for that purpose is almost certainly a violation.

Why "Open-Type" Lug is Generally NOT OK

An "open-type" lug (if you mean a lug that's simply a metal block with a hole through it) is even more problematic. There's no clamping force to ensure a secure, low-resistance connection.

What IS Legal and Safe


Use a Listed Connector



The best and safest practice is to use a connector that's specifically listed and labeled (UL, ETL, CSA, etc.) for the type and size of wire you're using, and for the specific application. This means the connector has been tested and certified to provide a safe and reliable connection when used according to the manufacturer's instructions. The connector should be rated for the wire type (e.g., THHN) and the wire gauge (e.g., 10 AWG).2. Use Properly Sized Lugs with Ferrules (Often Recommended for Stranded Wire)

If you're using stranded wire (common for larger gauges), using a

ferrule is often the best practice. Ferrules are metal sleeves that are crimped onto the end of the stranded wire. They create a solid, gas-tight connection, making it easier and safer to insert the wire into a lug. The lug should still be listed for use with ferrules.3. Use Compression Lugs



Compression lugs are designed for larger wire sizes and provide a very secure connection. They require a special crimping tool to compress the lug onto the wire. Again, make sure the compression lug is rated for the wire type and size.

Key Considerations



Manufacturer's Instructions

Always, always, always follow the manufacturer's instructions for the lug or connector you're using. They will specify the wire types and sizes the connector is designed for.

Electrical Codes

Adhere to all applicable electrical codes (NEC in the US, CEC in Canada, etc.). These codes are designed to ensure safety. Your local electrical inspector is the final authority.

Torque Specifications

Many lugs require a specific torque when tightening the screw. Using a torque wrench ensures the connection is tight enough but not so tight that it damages the wire or lug.

Stranded vs. Solid

The type of wire (stranded or solid) can affect the suitability of a lug.

Aluminum vs. Copper

When connecting aluminum and copper wires, you must use connectors that are specifically listed and labeled for that purpose (AL/CU rated). Using an oxide-inhibiting compound is also crucial. Dissimilar metals can corrode and cause connection failures.

In short: Don't run THHN wire (or similar) directly through a mechanical lug unless the lug is explicitly* listed and labeled for that use, AND you're following the manufacturer's instructions. When in doubt, use a listed connector or a ferrule/lug combination that's designed for your wire type and size. When working with electricity always consult a qualified electrician. Flag for review