Terminations

Most burned-up connections come down to a loose screw, the wrong terminal or a nicked conductor. Here's how to get them right.

Terminal temperature ratings

A conductor's ampacity is limited by the coolest-rated part of the connection, usually the terminal, not the wire's insulation.

100 A and less, or 14 through 1 AWG: use the 60 °C ampacity, unless the equipment is listed and marked for 75 °C.
Over 100 A, or larger than 1 AWG: use the 75 °C ampacity, unless marked for more.
90 °C wire (THHN, XHHW-2) still helps: you can start derating from its 90 °C ampacity, as long as the final answer doesn't exceed the terminal's column.
Most breakers, panels and devices today are marked 60/75 °C. Read the marking.

The wire size calculator applies these limits for you.

NEC: 110.14(C), 310.15

When the equipment gives a torque value (on the label, the lug or in the instructions), tighten to it with a calibrated torque screwdriver or wrench. It's a code requirement, not a suggestion.
"Tight enough" by feel is usually wrong in both directions: too loose heats up; too tight cracks lugs and cuts strands.
Retightening good connections isn't routine maintenance. It can over-stress the conductor. Use infrared scans to find hot connections, then fix those.
Mark the screw with a paint pen after torquing so anyone can see it was done.

NEC: 110.14(D)

Strip to the strip gauge on the device. No copper showing past the terminal, and none of the insulation under the clamp.
No nicks. A nicked conductor is a hot spot and a break waiting to happen. Use the right gauge hole on the strippers.
Under a screw: loop the wire clockwise, three-quarters of the way around, so tightening pulls it in.
One wire per terminal unless the terminal is marked for two. Pigtail instead.
Neutrals: one per terminal on a panel's neutral bar. Grounds can sometimes share, if the panel is marked for it.
Back-stab (push-in) holes often take only 14 AWG solid. Back-wire clamps (screw-tightened) are a better choice.
Wire nuts: strip to the length on the package, push the wires in even, twist until the insulation starts to twist, then tug each wire.
Fine-stranded wire (welding cable, flexible cords, some building wire) needs terminals or ferrules listed for it. A standard set-screw lug can cut and loosen it.
Compression lugs: use the tool and die the lug maker lists, the right number of crimps, and check the die marks.

NEC: 110.14(A), 110.14(B), 408.41

Land aluminum only on terminals marked AL/CU (or AL7CU, AL9CU on large lugs).
15 and 20 A receptacles and switches on aluminum branch wiring need devices marked CO/ALR, or pigtail to copper with connectors listed for aluminum to copper.
Wire-brush the strands and apply the antioxidant compound if the lug maker calls for it. Many compression lugs come pre-filled.
Torque matters even more with aluminum. Follow the value on the lug.
Aluminum is larger for the same ampacity: check that the lug range and conduit fill work for the size you need.
Don't mix copper and aluminum in one terminal or splice unless it's marked for that.

NEC: 110.14, 406.3(C), 404.14(C)