Connecting more than one wire on or under a single lug or connection point is called “double lugging,” and it is ONLY allowed in line voltage wiring under one condition, according to NEC 110.14:

If the terminal, lug, or connector is specifically rated for more than one wire

In the case of a conductor splice, like a wire nut or a split bolt, they are only designed for two wires unless they specifically state otherwise on the box, on the connector itself, or in the instructions/product data.

That means that wiring in a surge protector under the same lugs as the main—or jamming as many wires as you can make fit under a split bolt or wire—may be common, but it is not allowable according to NEC 110.14.

HVAC techs and installers will often double-lug contactors when making a repair, or they will connect to the closest and easiest point when installing 120v or 240v accessories, such as a UV light, humidifier, or air purifier.

In all of these cases, it is best to take a few minutes and find an approved and permanent method of making the connection instead of taking the easy way out.

Why It Fails

A reader asked this in the original article, and it is a fair question.

A lug is built to clamp one conductor. The screw comes down on a single bundle of strands, the pressure spreads across it evenly, and the torque spec on the label assumes that is what is under the screw.

Put a second wire in, and the screw now bears on two bundles that are never quite the same height. One takes most of the clamping force. The other sits with less contact area than it needs, and the wire with less contact carries more resistance.

Resistance makes heat. Heat expands the metal, and when it cools, it does not return exactly to where it started, so the connection gets a little looser. Looser means more resistance, which means more heat. That loop does not correct itself.

The tell in the field is discoloration, insulation that has melted back off the copper, and strands that have gone black instead of bright. By then the damage is done, and the lug is scrap.

Nothing about a fresh double lug tells you which way it will go. That is the real reason to spend the extra few minutes.

The Exception Exists, But Not Where You Are Working

Terminals listed for more than one conductor do exist. They are on grounding and neutral bars, on some breakers, and on large gear. When a terminal is listed for two, the part says so. The screw terminals on a definite purpose contactor are single-conductor terminals.

If the part does not say otherwise, the answer is one wire.

It is also worth mentioning that some connections are rated for copper only and will be marked CU, while others designed for aluminum will be marked AL or ALR. Some will be marked as CU/AL, which means that either copper OR aluminum may be used but not necessarily that copper and aluminum may be MIXED.

There are very few connections that allow the mixing of copper and aluminum, and if they do, they must be specifically listed for that purpose.

—Bryan

Illustrations by JD Kelly