Red, blue and yellow crimp terminals are colour coded by the wire size they fit. Choosing the right colour, the right terminal type and the right tool gets you most of the way to a joint that will not pull out or work loose. This guide covers the colour code, how to match it to your wire, and how to make and check a good crimp.
In this guide
The red, blue and yellow colour code
The coloured sleeve on an insulated terminal tells you which wire sizes its metal barrel is made to grip. The same three colours are used across butt connectors, spade terminals, ring terminals and bullet connectors.
| Colour | Wire size | AWG | Typical uses |
|---|---|---|---|
| Red | 0.5 to 1.5mm² | 22 to 16 | Signal wires, lighting, small accessories, switches and relays |
| Blue | 1.5 to 2.5mm² | 16 to 14 | General 12V vehicle and boat circuits, pumps, fans, solar controllers |
| Yellow | 4 to 6mm² | 12 to 10 | Heavier power feeds, inverters, winches and battery leads within the terminal's range |
Yellow ranges vary by brand. Some manufacturers rate yellow terminals from 2.5mm² and some to 6.5mm². The red and blue ranges are very consistent. Always check the range printed on the packet or product page, not just the colour.
The ranges overlap at 1.5mm². Wire right on a boundary usually fits either colour. Choose the one where the stripped conductor fills the barrel snugly without forcing strands.
Ferrules use a different colour system. Bootlace ferrule colours follow more than one standard, so a red ferrule is not necessarily the same size as a red crimp terminal. Go by the size printed on the ferrule packet, such as E1508 for 1.5mm².
Work out your wire size
The colour code only works if you know the conductor size. Look for printing on the cable jacket first. Automotive and hookup wire is often marked in AWG, for example 16 AWG or AWM 16-14, while New Zealand building and appliance cable is usually marked in mm².
| AWG | Approximate mm² | Terminal colour |
|---|---|---|
| 22 | 0.33 | Red (at the small end) |
| 20 | 0.52 | Red |
| 18 | 0.82 | Red |
| 16 | 1.31 | Red or blue |
| 14 | 2.08 | Blue |
| 12 | 3.31 | Yellow, if the terminal's range starts low enough |
| 10 | 5.26 | Yellow |
If there is no marking, compare the stripped conductor with wire of a known size, or measure the bare conductor diameter and look it up. Do not judge by the outside of the insulation. Thick insulation on thin wire is common on cheap leads, and it can fool you into choosing a terminal that is too big.
Choose the right terminal type
| Terminal | Use it to | Choose by |
|---|---|---|
| Butt connector | Join two wires end to end permanently | Wire size |
| Female spade | Push onto a flat tab on a switch, fuse holder, relay or appliance | Wire size and tab width, most often 6.3mm |
| Ring terminal | Bolt onto a stud, battery post, bus bar or earth point | Wire size and stud size, for example M4 or M5 |
| Bullet connector | Make a joint you can pull apart later | Wire size, plus a matching male half |
| Bootlace ferrule | Finish stranded wire before it goes into a screw or spring terminal | Printed mm² size |
| Solder seal connector | Join two wires with solder and adhesive heat shrink in one step | Wire size, using the colour chart for that product |
A ring terminal is the most secure choice anywhere a joint will be bolted down. A spade is convenient for parts that need to be removed. A ring terminal cannot slide off a loosened screw, so choose it over an open fork or spade near a battery.
The right crimping tool
A ratcheting crimper for insulated terminals has three colour-marked jaw positions: red, blue and yellow. The ratchet will not release until the jaws have closed fully, so every crimp gets the same pressure. It is the single biggest improvement you can make to your wiring.
- Insulated terminals: use a ratcheting crimper with red, blue and yellow jaw positions.
- Bootlace ferrules: use a ferrule crimper with a square or hexagonal die. An insulated-terminal crimper squashes ferrules flat and does not grip evenly.
- Solder seal connectors: no crimper is needed. Use a heat gun.
- Wire strippers: use strippers sized for the wire so you remove insulation without nicking strands.
Pliers are not a crimping tool. Squashing a terminal with pliers or the crimp jaws of a cheap stripper often looks fine but leaves the wire loose inside the barrel. A loose crimp can pull out, or heat up under load.
How to crimp a terminal properly
- Isolate the circuit. Disconnect the battery negative or remove the fuse before cutting into any vehicle, boat or solar wiring.
- Slide on any heat shrink first. If the joint needs extra sealing, put the heat shrink on the wire before crimping. It cannot go on afterwards.
- Strip the right length. Strip enough for the conductor to reach the end of the metal barrel without bare copper showing past the sleeve. For our blue butt connectors that is about 6 to 8mm per wire.
- Twist the strands. Lightly twist them in their original direction so none splay out. Do not tin the wire with solder before crimping, because solder settles under pressure and the crimp loosens.
- Insert the wire fully. Push it in until the conductor reaches the end of the barrel, or the centre stop on a butt connector. The insulation should sit just inside the sleeve.
- Use the matching jaw. Put the terminal in the jaw marked with the same colour as its sleeve, with the barrel centred in the jaw. For a butt connector, crimp one half at a time.
- Squeeze until the ratchet releases. Do not stop part way. The release is your sign that the crimp is complete.
Check the crimp
A quick inspection catches almost every bad joint before it causes trouble.
- Pull test. Hold the terminal and pull the wire firmly. It should not move at all. If it slides, cut the terminal off and start again.
- No stray strands. No copper should poke out of the sleeve or sit outside the barrel.
- Sleeve intact. The insulation may be dented but should not be split or cut through.
- Crimp in the right place. The squeeze mark should be on the barrel, not on the spade, ring or bullet end.
- Fits its mate. A spade should push onto its tab firmly, and a ring should sit flat under the screw head or nut.
Practise on offcuts. Make two or three crimps on scrap wire and pull them apart before you work on the real job. It is the quickest way to learn how your tool feels.
Crimp or solder seal?
| Insulated crimp terminal | Solder seal connector | |
|---|---|---|
| Tool | Ratcheting crimper | Heat gun |
| Speed | Very fast once set up | Slower, waits for the solder to flow |
| Moisture | Standard nylon or PVC sleeves are not sealed | Adhesive-lined heat shrink seals the joint when properly shrunk |
| Vibration | Handles vibration well when crimped correctly and supported | Support the wire either side so it does not flex at the solder |
| Can be undone | Spades, rings and bullets can be disconnected | Permanent |
| Best for | Connections to components, studs and terminals | Inline joins in damp places such as trailers, boats and outdoor lighting |
For a waterproof inline join, a solder seal connector is usually the simplest option. For everything that connects to a switch, stud or terminal, crimp the right terminal and add adhesive heat shrink if it is exposed to weather.
Common mistakes and fixes
| Problem | Cause | Fix |
|---|---|---|
| Wire pulls out easily | Terminal too big for the wire, wrong jaw position or pliers used | Use the colour whose range suits the wire, the matching jaw and a ratcheting crimper. |
| Wire will not fit in the barrel | Terminal too small | Move up a colour. Never cut strands off to make it fit. |
| Joint gets warm | Loose crimp, damaged strands or undersized wire | Remake the crimp, and check that the wire size suits the current and fuse. |
| Sleeve split during crimping | Wrong jaw position or a worn tool | Use the jaw that matches the sleeve colour. Replace the terminal. |
| Spade falls off the tab | Wrong tab size or a stretched receptacle | Check the tab width, commonly 6.3mm, and fit a new terminal. |
| Intermittent fault in a vehicle | Corrosion or a partly broken wire at the crimp | Cut back to clean wire, remake the joint and seal it with adhesive heat shrink. |
Current, fuses and mains wiring
A terminal does not have one fixed amp rating that applies everywhere. The safe current depends on the wire size, cable length, temperature, how the joint is made and the rest of the installation. Size the wire for the load, and protect the circuit with a fuse close to the battery or supply.
This guide is for low-voltage DC wiring such as vehicles, trailers, boats, caravans, solar setups and hobby projects. Household mains wiring in New Zealand is regulated, so use a registered electrician for fixed mains work.
Blue butt connectorsView connectors
Red 6.3mm female spadesView terminals
Blue M5 ring terminalsView terminals
Bootlace ferrulesView ferrules
Frequently asked questions
What size wire does a blue crimp terminal take?
Blue insulated terminals take 1.5 to 2.5mm² wire, which is 16 to 14 AWG. That covers a lot of general 12V vehicle and boat wiring.
Can I use a red terminal on 2.5mm² wire?
No. Red terminals are made for 0.5 to 1.5mm². Thicker wire will not fit the barrel properly, and trimming strands to force it in reduces the wire's capacity. Use blue.
Do I need to solder a crimped terminal as well?
No. A correct crimp is a complete joint. Adding solder can make the wire stiff and likely to break where the solder stops, especially where there is vibration.
Are insulated crimp terminals waterproof?
Standard PVC or nylon sleeves are not sealed. For a wet location, cover the joint with adhesive-lined heat shrink, or use a solder seal connector for inline joins.
What is the difference between a spade and a quick disconnect?
They are the same thing. A female spade, quick disconnect or push-on terminal slides onto a flat male tab. A very common size is 6.3mm, also called 1/4 inch or 250 series. Check the tab before you buy.
Why use ferrules on stranded wire?
A screw terminal can cut or spread loose strands, which leaves a weak connection that loosens over time. A ferrule holds every strand together so the screw clamps a solid tube.
Reference material checked: terminal colour ranges were cross-checked against published insulated terminal listings from RS Components, which use the same red 0.5 to 1.5mm², blue 1.5 to 2.5mm² and wider yellow ranges. NZN wire ranges, strip lengths and tab and stud sizes come from our own product records. AWG to mm² values are standard conversions, rounded.
RS PRO blue insulated blade terminal | RS PRO yellow insulated blade terminal
Match the colour, use the matching jaw, then pull test
Get those three things right and your crimped joints will stay tight for years. Our terminals are sold in small quantities, so you can buy exactly what the job needs.
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