Electrical connectors use colour coding for practical reasons. Color tells the installer at a glance what wire gauge the connector is built for, which speeds selection and reduces the risk of a mismatched splice. Three connector families use standardized or semi-standardized color systems: twist-on wire connectors (wire nuts), insulated crimp terminals, and wire ferrules. Each system works differently, and the colors do not translate across families. 

This guide covers all three. Wire insulation color coding, which governs which conductor is hot, neutral, or ground under the National Electrical Code, is a separate topic and is not covered here.

Wire nuts (twist-on wire connectors)

Wire nuts are the most common splice connector in North American residential and commercial wiring. The color indicates the size of the internal spring and the range of wire combinations the connector is rated to hold. Manufacturers follow a strong industry convention, but the system is not formally standardized across brands. Final selection should be based on the wire range printed on the package, not on color alone.

Wire nuts are built for building wire in the 22–6 AWG range, both solid and lightly stranded copper. The most common cables landed with wire nuts are NM-B (Romex) for residential branch circuits, THHN and THWN-2 pulled through conduit, UF-B for direct burial applications inside weatherproof boxes, and the internal THHN conductors of MC and AC cable. Light-gauge fixture wire, such as TFFN, TFN, and SF-2, is typically joined with gray, blue, or orange wire nuts inside fixtures and appliances. Wire nuts are not appropriate for USE-2, service entrance cable, welding cable, fine-stranded flexible cordage, or MTW inside industrial control panels.

Green wire nuts are reserved for grounding splices and often have a hole at the top to allow a single conductor to pass through as a pigtail. Purple wire nuts are filled with anti-oxidant compounds and are the only wire nuts rated to splice aluminium to copper. Standard colour-coded wire nuts are not rated for aluminium conductors.

Wire nuts sold in North America are typically listed under UL 486C. Installations must comply with NEC Article 110.14, which requires that connectors be identified by the conductor material.

Insulated crimp terminals (NEMA standard)

The color code for insulated crimp terminals is the closest thing to a universal standard in the connector world. Red, blue, and yellow map to fixed AWG ranges, and the same colors appear on the corresponding cavities of the crimping tool. This scheme applies across ring terminals, spade terminals, fork terminals, butt splices, quick-disconnects, and heat-shrink versions of all of the above.

Crimp terminals are built for stranded conductors, which is why they dominate automotive, marine, and equipment wiring rather than residential building wiring. Common cables landed with crimp terminals include automotive primary wire (SGT, GPT, TXL, GXL, SXL) in the 22–10 AWG range, marine-grade tinned battery cable and primary wire under UL 1426 and ABYC guidelines, MTW machine tool wire on industrial terminal strips, stranded THHN landing on screw terminals or bus bars, and portable cord such as SOOW and SJOOW at strain-relief points. Heat-shrink butt splices with adhesive-lined insulation are the standard termination in marine applications where moisture and salt exposure are constant. Larger battery cable, welding cable, and other conductors above 8 AWG terminate with uninsulated lugs identified by stamped markings rather than color.

For conductors 8 AWG and larger, terminals are typically uninsulated and identified by stamped markings rather than color. Some manufacturers repeat the red-blue-yellow sequence for larger conductors. The practice is not consistent across brands, so verify the size from the datasheet rather than the color. Crimp terminals should not be used on solid conductors. The barrel is designed to deform around stranded wire; solid wire either does not compress properly or fatigues at the crimp exit.

The three-color system is published by the National Electrical Manufacturers Association (NEMA) and referenced in installation guides from every major terminal manufacturer.

Wire ferrules (DIN 46228-4)

Wire ferrules, also called bootlace ferrules or cord end terminals, are the standard termination for stranded conductors entering screw or spring terminal blocks. They are the dominant termination method in European control panels and are increasingly common in North American industrial work.

Color coding for insulated ferrules is governed by DIN 46228-4. Because ferrules cover a much wider range of cross-sections than crimp terminals, the same color repeats at multiple sizes. Cross-section is the primary means of selecting a wire ferrule, and the color is present only for confirmation.

Ferrules are built for fine-stranded flexible conductors and are almost never used with building wire. The most common cables landed with ferrules are European fine-stranded building wire (H05V-K, H07V-K), fine-stranded MTW inside UL 508A control panels, instrumentation and control tray cable (PLTC and ITC in class 5 and 6 constructions), flexible control cables such as ÖLFLEX, LiYY, and LiYCY, individual conductors inside VFD cable landing at the drive, and signal conductors from servo and encoder cable landing at controller terminal blocks. Ferrules are also used on fine-stranded pigtails at the load side of DIN-rail-mounted breakers, contactors, and terminal blocks. The threshold for when a stranded conductor requires a ferrule varies by terminal block manufacturer, but any class K or finer stranding typically calls for one.

The application follows the size band. Cross-sections of 0.25 to 0.5 mm² handle signal wiring. Cross-sections from 0.75 to 2.5 mm² handle control wiring. Cross-sections of 4.0-6.0 mm² for handle panel wiring. Cross-sections of 10.0 mm² and above handle power.

Two other color systems for ferrules exist. The French standard NF C 63-023 uses a different sequence, and Phoenix Contact, Weidmüller, and several other manufacturers offer UL, AWG, JIS, and multi-standard variants that deviate from DIN. The relevant compliance standard for bare and covered ferrules in North America is UL 486F. When sourcing across markets, verify the color-to-size mapping on the datasheet before ordering in bulk.

Governing standards

• UL 486A-486B covers wire connectors for copper and aluminum conductors.

• UL 486C covers splicing wire connectors, including wire nuts.

• UL 486F covers bare and covered ferrules.

• NEC Article 110.14 covers electrical connections generally and requires connectors to be identified for the conductor material.

• DIN 46228-1 covers uninsulated ferrules. DIN 46228-4 covers insulated ferrules.

• NEMA publishes the red-blue-yellow color convention for insulated crimp terminals.

Color coding accelerates selection, but it does not replace the manufacturer's wire range specification. For any critical, inspected, or high-current application, verify the wire size and rating from the printed package data or the product datasheet before installing.

Wires and cables for these connectors at Nassau National Cable

For wire nut splices in residential and light commercial work, Nassau National Cable stocks NM-B Romex in solid copper for indoor branch circuits, UF-B underground feeder cable for direct burial runs, THHN/THWN-2 building wire in solid and stranded from 14 AWG through 4/0, and Type MC metal-clad cable for commercial and light industrial branch circuits.

For crimp terminal applications, NNC carries battery cable for automotive and equipment power, welding cable in Class K fine-stranded construction, SOOW portable cord for tool and equipment leads, and marine and shipboard cable for corrosive salt-air environments where tinned heat-shrink butt splices and ring terminals are standard.

For ferrule terminations inside control panels and industrial equipment, NNC stocks VFD cable for variable frequency drive installations, tray cable in TC-ER and shielded constructions for control, power, and signal circuits, and MTW-rated conductors within its industrial cable inventory. Matching cable connectors and terminations are also available for direct pairing with the cables above.

Nassau National Cable serves as both a primary and backup supplier for utilities, EPC contractors, industrial buyers, and electrical distributors across all fifty states, with cut-to-length availability from 25 feet up through full master reels.