Splices and Terminations of Conductors
They may seem like just the finishing touches, but splices and terminations are critical components of any electrical system.
The Code requires you to follow manufacturer's instructions when installing electrical equipment (110.3), and this applies to terminal devices. For example, if the labeling on a terminal device states “Suitable for 18-2 AWG Stranded,” you can use only stranded conductors with that device. If it states “Suitable for 18-2 AWG Solid,” you can use only solid conductors with it. If it states “Suitable for 18-2 AWG,” you can use either solid or stranded conductors with the terminating device. Let's look at some other considerations.
In 404.14(C) and 406.2(C), the Code requires you to use terminals and splicing devices identified for use with the conductor material. Terminals and splicing devices suitable only for aluminum must be marked AL. Terminals listed for aluminum typically have an antioxidant to reduce aluminum oxide, which reduces the conductor/terminal contact resistance. Split-bolt lugs listed for aluminum-to-aluminum come with detailed instructions that vary among lug designs. All aluminum conductors require wire-brushing immediately prior to assembly.
Copper and aluminum conductors must not make contact with each other in a terminal or splicing device not specifically listed and identified for the purpose. Terminals and splicing devices suitable for both copper and aluminum must be marked as CU/AL or CO/ALR.
Terminals must ensure good connections without damaging the conductors. To get good connections, follow the manufacturer's torque specifications, which are often included with the equipment instructions. Generic torque tables do not account for all the variables that affect developing a torque value for a connection. Contrary to a popular myth, contact resistance rises dramatically with over-torque. If you have the device but not the torque value, contact the manufacturer.
There are several ways to damage a torque wrench, from storing it with the spring compressed to failing to exercise the spring, both of which can yield meaningless readings. Fortunately, these tools come with instructions that tell you how to get the right reading every time.
Follow the one-wire-per-terminal rule. Terminals listed for more than one wire must be identified for this purpose in the equipment instructions or on the terminal.
Be sure to match wire size and terminal ratings. What if your wire is too large for the terminal? The designer should order equipment with terminals that accommodate the larger wire. But if you are at the installation stage of the project, you have three choices for fixing the problem:
Contact the manufacturer or dealer and request the proper terminals, bolts, washers, and nuts.
Order a terminal device that crimps on the end of the conductor and terminates on the terminal.
Install a short lead wire on the terminal and splice it to the circuit conductors.
It's critical that you use the right splicing devices. If you don't use a listed splicing device, weld your connections exothermically. Special applications, such as underground burial, require splicing devices listed for those applications. But no matter what the application, you cannot use a u-bolt fitting designed to splice non-electrical cables.
Manufacturers of insulated twist-on wire connectors suggest you not hand-twist the wires together before applying the connector. The wires will bond properly if inserted straight, but you risk making a poor connection if you pre-twist. You must cover splices and the free ends of all conductors — used or not — with insulation at least equal to that of the conductor. Use twist-on wire connectors where necessary.
Placement of the splices is important, as well. Conductors must be continuous between all points of the system. You must install a box or conduit body at each conductor splice connection point, outlet, switch, junction, or pull point, with exceptions in these sections:
312.8 — Cabinet or Cutout Boxes
820.3 — CATV
725.3 — Class 2 and 3 Control and Signaling
314.16 — Conduit Bodies
770.3 — Fiber Optical Cable
410.31 — Luminaires
640.3 — Sound Systems
352.7 — Surface Raceways
800.52 — Telecommunications
110.14(B) and 300.5(E) — Underground
376.56 — Wireways
Sec. 314.5 doesn't allow splices in short radius conduit bodies. For other conduit bodies, observe the limitations on the number of splices in a given enclosure per 314.16(C).
Sec. 300.14 requires you to leave at least 6 in. of free conductor — measured from the point where the conductors enter the box — at each outlet, junction, and switch point for splices or terminations of luminaires or devices. Enclosures with openings less than 8 in. must have at least 3 in. of free conductor outside the opening, and no less than 6 in. of free conductor. Enclosures with an opening of 8 in. or more must have at least 6 in. of free conductor. However, conductors that pass through an enclosure and are not used for splices or termination do not require 6 in. of free conductor.
Incorrect pigtailing can cause problems. The removal of a wiring device, such as a receptacle, must not interrupt continuity for the grounded (neutral) conductor of a multiwire branch circuit. Thus at devices such as receptacles, you must splice the grounded (neutral) conductors together and provide a pigtail (separate lead wire) for device terminations. See 250.148(A) for the pigtailing requirements for equipment grounding conductors. On the other hand, opening the hot or grounded (neutral) conductor of a two-wire circuit during the replacement of a device doesn't cause a safety hazard, so you don't have to pigtail these conductors. Conductors need not be pigtailed to GFCI receptacles.
Proper terminations and splices are critical for system performance and safe operation. There are a lot of details to remember, but you'll be OK if you use the right equipment, follow the manufacturers' installation instructions, and observe the rules of splice locations. A large share of electrical failures arises from improper terminations and splices. Make sure your work isn't part of the problem.