Heat Shrink Tubing — How to Choose the Diameter and Install It Correctly

Heat shrink tubing is a tube made of a special polymer that shrinks when heated and grips the wire or joint underneath it tightly. It is the fastest and cleanest way to insulate a solder joint, protect a cable splice or mark a conductor in a panel.

What heat shrink tubing is and how it works

The polymer it is made from has a "shape memory". At the factory the tube is heated and expanded, then cooled in its expanded state. When you heat it again — usually to 90–120°C — it returns to its original, smaller diameter and fits tightly. Once it cools down, it keeps that shape permanently.

Shrink ratio

The ratio shows how many times the diameter decreases after heating:

• 2:1 — shrinks to half. The standard for most applications.
• 3:1 — shrinks to a third. For uneven surfaces, such as a cable lug on a wire.
• 4:1 and above — for places with a big difference in diameters, such as a splice on a thick cable.

The higher the ratio, the more flexibility you have when choosing a size.

Types of heat shrink tubing

Standard (thin-wall) — the most common type. Available in many colours. For insulating solder joints, marking and protection against abrasion.

Adhesive-lined — has an inner layer of hot-melt adhesive that melts when heated and fills every gap. This makes the joint sealed and moisture-resistant.

Heavy-wall, adhesive-lined — the same, but with a thicker wall. Withstands greater mechanical stress. For power cable splices, outdoors and underground.

PTFE (Teflon) — for very high temperatures and aggressive chemical environments. Used mainly in industry.

How to choose the right diameter

This is the most common mistake. The rule has two conditions and both must be met:

• Before shrinking the tubing must be wider than the thickest point it will cover — so it slides on freely.
• After shrinking the tubing must be narrower than the thinnest point — so it grips tightly.

Sizes are written as two diameters — for example 16/4 means 16 mm before shrinking and 4 mm after full recovery.

Example: you are splicing a 3×2.5 mm² cable. The outer diameter of the cable is about 10 mm, and about 13 mm at the joint. You need tubing wider than 13 mm that shrinks below 10 mm. Adhesive-lined heat shrink Ø16/4 mm fits perfectly: 16 mm is enough to slide over, and 4 mm is well below 10 mm.

Typical examples:

• Insulating a single 1.5 mm² wire (about 3 mm) — Ø5 mm
• Insulating a single 2.5 mm² wire (about 3.5 mm) — Ø6 mm
• Splicing a 3×1.5 or 3×2.5 mm² cable — Ø16/4 mm adhesive-lined
• Splicing a 5×2.5 mm² cable or thicker — Ø22/6 mm or Ø27/8 mm adhesive-lined

Cable diameters are approximate — they vary by manufacturer. If in doubt, measure with a caliper.

How to install it — step by step

Step 1 — Cut the length you need. The tubing should cover the joint and extend at least 10–15 mm on each side. Keep in mind it gets about 5–10% shorter when it shrinks.

Step 2 — Slide it onto the cable BEFORE making the connection. This is the most commonly forgotten step. Once you have soldered or crimped, there is no way to put it on.

Step 3 — Make the connection. Solder, crimp or connect the wires.

Step 4 — Centre the tubing over the joint.

Step 5 — Heat evenly. Use a heat gun. Start from the middle and work towards the ends, rotating the cable or moving the gun around it. This pushes the air out and leaves no bubbles.

Step 6 — With adhesive-lined tubing keep heating until you see a thin bead of adhesive coming out at both ends. That is the sign the joint is sealed.

Step 7 — Let it cool for 1–2 minutes without bending the cable.

What about a lighter? Only as a last resort and only for small pieces of standard tubing — keep the flame at a distance and constantly moving. An open flame easily scorches the tubing, makes it brittle and leaves soot. For adhesive-lined tubing and cable splices, use a heat gun only.

When you need adhesive-lined tubing

• For joints outdoors or in damp rooms
• For cables laid underground
• When connecting cables of different diameters
• In cars, boats and anywhere with vibration and moisture

For dry rooms and marking, standard tubing is perfectly sufficient.

What it is used for

• Insulating solder and crimp joints
• Protecting cable lugs and terminals
• Colour-marking conductors in a panel
• Repairing damaged insulation
• Protecting a cable where it passes over a sharp edge
• Bundling several wires together

Colours for marking

According to IEC 60445, conductor colours are:

• Brown, black, grey — phase (L1, L2, L3)
• Blue — neutral (N)
• Green-yellow — protective earth (PE)

When marking with heat shrink, follow these colours. That way any electrician after you will immediately know which conductor is which.

Related articles

• WAGO vs classic terminal strip — which one to choose?
• How to install a junction box — step by step
• IP ratings explained — which protection class for bathroom, terrace or garden

Browse all heat shrink tubing in our store — standard from Ø1 to Ø15 mm and adhesive-lined from 8/2 to 48/12 mm.

Important: Electrical installations must be carried out by a qualified electrician. The information in this article is for educational purposes only.