Insulated wire prevents leakage and short circuits through a layer of insulation. The type must match the environment — THW for conduit indoors, VAF for cleats, NYY/CV for buried work. Don’t use aluminum wire inside homes, and use an RCD (residual-current device) per TIS and EIT standards.
What Is Insulated Wire?
Insulated wire consists of a conductor (copper or aluminum) covered with insulation (PVC or XLPE) to prevent leakage and short circuits.
Bare wire has no covering and is used only on high-voltage transmission lines on poles.
The 3-Layer Structure of Insulated Wire
- Conductor — copper or aluminum.
- Insulation — PVC or XLPE covering the conductor.
- Outer sheath — adds strength and protects against the environment.
Functions of Electrical Insulation
- Prevents leakage — keeps current inside the conductor.
- Prevents short circuits — separates each conductor.
- Improves safety — prevents direct contact with the conductor.
- Protects from the environment — moisture, heat, chemicals.
Classifying Insulated Wire
By Insulation Type
- PVC — flexible, withstands 70°C, general use.
- XLPE — stronger, withstands 90°C, for heavy-duty work.
- PE — good moisture resistance.
- Rubber — highly flexible, for machinery.
By Special Property
- Heat-resistant — for high-temperature areas.
- Moisture-resistant — for outdoors and burial.
- Chemical-resistant — for factories.
- Fire-resistant — for safety systems.
- Flame-retardant — flame doesn’t spread along the wire.
- Low-smoke, halogen-free — emits little toxic gas.
7 Popular Wire Types in Thailand
For Indoor Use
| Type | Voltage | Use | Cautions |
|---|---|---|---|
| THW | 450/750V | In conduit, main lines | Do not run exposed or bury |
| VAF | 300/500V | Cleated on walls | Do not run in conduit; may build heat |
| VCT | 450/750V | Portable cable, tools | Do not use as a permanent main |
| CVV | 600V | Control/signal cable | Not for feeding appliances |
For Outdoor and Buried Use
| Type | Voltage | Use | Cautions |
|---|---|---|---|
| NYY | 450/750V | Direct burial | Resists moisture and insects |
| CV (XLPE) | 600V–115kV | Mains, factories | Withstands 90°C |
| THW-A | 750V | Utility feed into the home | Do not use indoors; builds heat |
Guidelines for Choosing Safe Wire
- Check voltage and temperature ratings — must handle the system’s current.
- Consider the environment — buried, conduit, exposed, outdoors.
- Check the TIS standard — guarantees quality and safety.
Frequently Asked Questions
Q: What wire do ordinary homes use?
A: VAF for traditional cleated wiring, or THW when run in conduit per current standards.
Q: Why can’t THW be buried in the ground?
A: Because it has only a single PVC insulation layer, lacking protection against moisture and rodents — it can cause dangerous leakage.
Q: What do the insulation colors mean?
- Brown — line (L)
- Blue — neutral (N)
- Green with yellow stripe — ground (G)
- Black/gray — 3-phase lines (L1, L2, L3)
