3/8 inch is the 6–8 AWG size — the gauge of alternator output leads, winch motor feeds, 50-amp shore power, and most marine and off-road battery cable. It expands to 9.5mm so it clears the conductor and its crimp barrel, then recovers to about 3.2mm, drawing the hot-melt adhesive liner tight against the jacket. That liner is the whole point of the 3:1 dual-wall build: at 6 AWG you are running roughly 55 amps through a connection that lives in heat, salt, and vibration, and the adhesive both blocks the corrosion path a single-wall tube leaves open and bonds the sleeve down as strain relief. One sizing note worth knowing before you order — 3/8" fits a bare 6–8 AWG splice or butt connector cleanly; if you are sleeving over a heavy battery lug, measure the barrel first and step up to 1/2" if it will not slide on with clearance. Sold cut-to-length on a 60ft roll. UL 224, 600V, ships direct from the USA.
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The three sizes most jobs actually need — 1/4", 3/8" and 1/2", 60ft each.
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SKU: B0D63ZBT1B · 3/8" (9.5mm) · 60 ft
Complete dual-wall adhesive performance in a bulk roll format. Permanently waterproof, 600V rated, and rated for the harshest environments.
Sizing over a connector? The Wire Gauge row above is for bare conductor. Heat shrink must be sized to the widest point it passes over, so a crimped butt splice or lug barrel needs a larger tube than the wire alone — a 22–18 AWG splice barrel measures 5–6mm, a 16–14 AWG barrel 7–8mm, and a 12–10 AWG barrel 9–11mm. Measure the barrel, allow about 20% clearance, and confirm the recovered diameter is smaller than the narrowest thing the tube must grip. Use the size calculator.
High quality and amazing value for the price
I'll never buy individual bags of heat shrink tubing again. This is very high quality with a shrink ratio that is truly amazing (all of the tubing from this company is). I wonder if I'll be able to use this entire roll in 10 years!
Good quality and sealing heat shrink
Heat shrink works good. Bought this for work. Makes a nice sealed wire connection. Good quality, better than pre cut heat shrink, you can make whatever length you want.
I LIKE THIS!
I solder, and heat shrink, all of my automotive electrical connections. This is a great product, and I can cut it to any length. The price is great, and the shipping was fast!
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3/8 inch (9.5mm) is the correct heat shrink for sealing 6 and 8 AWG cable — alternator output leads, winch motors, 50-amp shore power, and most marine and off-road battery feeds. It expands to clear the conductor and its crimp, then recovers to roughly 3.2mm onto the adhesive liner. One question decides whether 3/8" is actually your size: are you covering the wire, or the lug?
Search around and you will find the advice split — some sources say 3/8" for 6 AWG, others insist on 1/2". Both are right, because they are measuring two different things. 3/8" fits 6–8 AWG conductor plus a standard butt splice or heat-shrink butt connector. But if you are sleeving over a crimped ring or lug terminal — a battery lug, a 5/16" or 3/8" stud ring — the barrel and the shoulder where it meets the insulation can run wider than 9.5mm.
The fix is a habit, not a chart: measure the largest outside diameter the tube has to slide over, add 20–30% clearance, and size to that — never to the bare wire. For a bare 6 AWG splice, 3/8" is the size. For a 6 AWG cable terminating in a heavy lug, step up to 1/2" (12.7mm) so the tube clears the barrel on the way on and still recovers down onto the jacket once heated.
| Conductor | ABYC ampacity | Bare wire / splice | Over a crimped lug |
|---|---|---|---|
| 8 AWG | 40 A | 3/8" — ideal | 3/8" if barrel ≤9mm, else 1/2" |
| 6 AWG | 55 A | 3/8" — ideal | Step up to 1/2" |
| 4 AWG | 70 A | Tight — use 1/2" | 1/2" |
| 10 AWG | 30 A | Loose — use 5/16" | 3/8" |
Expanded ID 9.5mm · recovered ~3.2mm · 3:1 ratio. Ampacity per ABYC E-11 Table XI, conductor bundled in an engine space.
At 6–8 AWG you are almost always in an engine bay, a bilge, a wheel well, or a battery box — heat, vibration, salt, and standing water, often all four. A 6 AWG alternator lead carries about 55 amps; a crimp that starts to corrode under that load does not just add resistance, it concentrates heat at the exact spot a single-wall sleeve leaves open to begin with.
The hot-melt liner in Helixal’s dual-wall flows at 120°C and fills the void between the cut insulation, the crimp barrel, and the tube — the capillary path corrosion actually travels. It also bonds the sleeve to the cable jacket, so the same tube doubles as strain relief where the cable flexes off a vibrating alternator or windlass. A 2:1 single-wall tube insulates; it does not seal and it does not hold. On a 55-amp connection that sees water, that is the difference between a termination that lasts the life of the boat and one you redo in two seasons.
A 6–8 AWG lug is a heat sink. Warm the metal first, then bring the heat back to the tube — otherwise the adhesive sets on the thin jacket side before it ever bonds to the heavy barrel.
Run the tube at least 10mm past the barrel onto the jacket on each side. A short sleeve seals the crimp but skips the flex point — which is where these cables actually fail.
Stop when a thin bead of clear adhesive appears at both ends. No bead means no seal; a flood of it means you have overheated the polyolefin and started to scorch the wall.
Start mid-tube and work toward the ends to push trapped air out — especially over a fat lug, where a pocket of air leaves an open channel straight to the crimp.
The 3/8" roll is 60 continuous feet and you cut it to length, so there is no offcut waste the way there is with fixed pre-cut pieces. At a typical 5-inch finished termination that is about 144 sealed connections; at 2.5-inch battery-lug caps, closer to 288. A single dual-battery or alternator upgrade burns a few feet — the roll is sized for a shop, a fleet, or a builder who would rather not reorder in the middle of a job.
Keep reading →Heat Shrink Size Calculator · Best Heat Shrink for Marine Wiring · Heat Shrink for Battery Terminals · Size Chart & Selection Guide
Verify the expanded diameter is 20–30% larger than your wire or connector. The tubing must slide freely before heat is applied — if it drags, go one size up.
Cut to length and thread onto the wire or cable before making the splice or crimp. Overlap each terminal or connector end by at least 10mm for a proper seal.
The tubing starts shrinking at 100°C, but the adhesive liner only flows at 120°C — so keep the heat gun moving past the point where it looks shrunk. Start at the center and work outward to push air toward the ends. Adhesive will appear at the tube ends — that is your visual seal indicator.
The tubing should be tight and uniform with adhesive visible at both ends. No gaps, lifting, or cold spots. Pull-test the finished connection to confirm the adhesive bond.
Actual 3/8" tube at full expanded diameter (9.5mm) and after heat recovery. Use this as your fit reference before cutting.
Seals bilge pump leads, navigation light splices, and sonar cable entries against continuous moisture and saltwater exposure.
Protects harness connectors, oxygen sensor leads, and battery tender connections in high-temperature, high-humidity engine compartments.
Seals outdoor junction connections, landscape lighting harness splices, and irrigation system wiring from water ingress.
Seals battery tender leads, solar charge controller wiring, and inverter connections against corrosion and moisture.
Used on fresh water pump wiring, slide-out harnesses, and underbody electrical runs on recreational vehicles.
Seals cable entries and harness connections in outdoor and washdown-rated control enclosures where IP protection is required.
Recovered diameter is approx. 1/3 of the expanded size shown · All sizes ship to USA & Canada
