Choosing the right THW wire size is essential for a safe, efficient electrical system in both homes and factories. Wire that’s too small builds up heat, causes voltage drop, and can damage equipment over time. This guide explains THW wire, the common sizes for each use, and how to select the correct size to standard.
What Is THW Wire?
THW is a single-core copper conductor with PVC insulation, rated 450/750V at a maximum operating temperature of 70°C. It is designed to be installed in a conduit system and complies with the Thai standard (TIS). It’s one of the most widely used wire types for building and industrial wiring.
Common THW Wire Sizes for Homes
| Size | Typical use | Current range |
|---|---|---|
| 1.5 mm² | Lighting circuits | under 10 A |
| 2.5 mm² | General outlets | 10–20 A |
| 4 mm² | High-power appliances (e.g. air conditioning) | 20–40 A |
| 6 mm² | Water heaters, subpanel feeders | 40–60 A |
THW Wire Sizes for Industrial Use
In factories, THW is used across a much wider range — from 10 mm² up to 240 mm² — depending on the load, from feeder lines to main distribution panels. The larger the load and the main breaker, the larger the wire required.
How to Select the Correct THW Wire Size
- Calculate the current from the equipment’s power rating (Amps = Watts ÷ Volts).
- Apply a 1.25 safety factor to the calculated current, so the wire isn’t run at its absolute limit.
- Cross-reference the ampacity against the installation conditions — conduit density, ambient temperature, and the number of conductors in the conduit — and pick the wire whose rated capacity meets or exceeds the adjusted current.
Common Mistakes to Avoid
- Undersizing the wire for the load — the most common and most dangerous error.
- Overloading the conduit with too many conductors, which traps heat.
- Improper installation methods that don’t match the wire’s rating.
- Skipping standard compliance — always verify against the Thai standard (TIS 11-2553).
Correct wire selection prevents heat build-up, voltage drop, and long-term equipment damage.
Buy Standard THW Wire from TETA Intertrade
TETA Intertrade is a wire-wholesale specialist supplying TIS-standard THW copper wire in every common size for residential and industrial installation. Choosing correctly-sized, standard-compliant wire from the start keeps your system safe, efficient, and durable.
Frequently Asked Questions (FAQs)
What voltage and temperature is THW wire rated for?
THW is rated 450/750V with a maximum operating temperature of 70°C, and is designed for installation inside conduit.
What size THW wire should I use for a home air conditioner?
High-power appliances such as air conditioners typically use 4 mm² (for roughly 20–40 A), but always confirm against the unit’s actual current draw and installation conditions.
Why apply a 1.25 safety factor when sizing wire?
Multiplying the load current by 1.25 gives headroom so the wire isn’t run continuously at its maximum rating, which reduces heat build-up and extends the wire’s life.
Does the number of wires in a conduit affect the size I need?
Yes. More conductors in one conduit, higher ambient temperature, and denser conduit runs all reduce a wire’s effective ampacity, so you may need to upsize to stay within the standard.
