18 AWG Wire: Ampacity, Uses & Specs
NEC 402.5 fixture-wire ratings, Class 2/3 signal circuit sizing, and thermostat-lead applications for 18 AWG.
18 AWG wire is a small-gauge conductor used almost exclusively for control, signal, fixture, and low-voltage applications — including thermostat wire, doorbells, low-voltage landscape lighting, alarm and security systems, RS-485 data runs, thermocouples, and Class 2 remote-control circuits. It has a 10 A ampacity at 75°C for fixture-wire and control-circuit applications under NEC 402.5 and 725/760.
18 AWG wire has a cross-sectional area of 1.62 kcmil (0.823 mm²) with a conductor diameter of 0.0403 inches (1.024 mm). The copper weight is approximately 4.92 lbs per 1,000 feet, with a DC resistance of 7.95 Ω/1,000 ft for copper and 13.1 Ω/1,000 ft for aluminum at 75°C.
The physical dimensions of 18 AWG are defined by NEC Chapter 9, Table 8. These values are used for conduit fill calculations (NEC Chapter 9, Table 4), voltage-drop math, and mechanical connector sizing.
| Specification | Copper (Cu) | Aluminum (Al) |
|---|---|---|
| AWG Size | 18 | 18 |
| Area (kcmil) | 1.62 | 1.62 |
| Area (mm²) | 0.823 | 0.823 |
| Diameter (in) | 0.0403 | 0.0403 |
| Diameter (mm) | 1.024 | 1.024 |
| DC Resistance @ 75°C (Ω/1000ft) | 7.95 | 13.1 |
| Weight (lbs/1000 ft) | 4.92 | 1.49 |
| 60°C ampacity (A) | 7 | 5 |
| 75°C ampacity (A) | 10 | 8 |
| 90°C ampacity (A) | 14 | 11 |
NEC Table 310.16 ampacity for 18 AWG copper conductors in raceway, cable, or earth (not more than three current-carrying conductors), based on 30°C (86°F) ambient. Termination temperature ratings (NEC 110.14(C)) apply — most 100 A and smaller equipment is rated 60°C; most 100 A+ equipment is rated 75°C.
| Insulation Type | Temp Rating | Ampacity | Common Use |
|---|---|---|---|
| TW, UF | 60°C | 7 A | Direct-burial UF cable, wet locations, older wiring |
| RHW, THW, THWN, USE, XHHW (wet) | 75°C | 10 A | Panel feeders, service conductors, wet-rated raceway |
| THHN, XHHW-2, RHW-2, USE-2, RHH | 90°C | 14 A | Dry raceway, derating base ampacity, high-temp motors |
18 AWG Aluminum Ampacity
NEC Table 310.16 ampacity for 18 AWG aluminum and copper-clad aluminum conductors. Aluminum ampacity is roughly 78% of copper for the same size. Do not use aluminum smaller than 6 AWG on general-purpose branch circuits.
| Insulation Type | Temp Rating | Ampacity |
|---|---|---|
| TW, UF | 60°C | 5 A |
| RHW, THW, THWN, USE, XHHW (wet) | 75°C | 8 A |
| THHN, XHHW-2, RHW-2, USE-2, RHH | 90°C | 11 A |
In solar systems, 18 AWG is used for RS-485 monitoring buses to inverters, temperature-sensor leads on batteries, and combiner-box auxiliary contact wiring.
Typical applications for 18 AWG wire
| Application | Typical Load | Notes |
|---|---|---|
| Class 2 remote control (NEC 725.49) | Up to 30 V | Doorbells, thermostats, low-voltage lighting controllers |
| Fixture wire (NEC 402) | 10 A max | Chandelier and pendant lead connections to junction boxes |
| Alarm & signaling | ≤ 1 A typical | Fire alarm initiating loops, security zone wiring |
| Instrumentation | mA signals | 4-20 mA sensor loops, RS-485 daisy chains |
| Automotive / equipment control | 8-14 A | Cabinet wiring, PLC I/O to field terminals |
| Solar monitoring | < 1 A | Modbus/RS-485 to inverter data ports |
Installing 18 AWG wire correctly requires attention to termination temperature ratings, conduit fill, bending radius, and NEC 110.14 lug torque specifications. Miswiring at these points is the leading cause of thermal failures and permit rejections.
Confirm the circuit is Class 2 or fixture wire
verify total load stays under NEC 725.49 or 402.5 limits before pulling 18 AWG.
Strip 3/8-inch of insulation
using a manufacturer-matched stripper — do not nick the strands.
Terminate to correct connector type
spring-cage for control, screw-clamp for terminal blocks, or IDC for phone-style low voltage.
Torque to spec
4.5 in-lb for typical control terminal blocks (verify with equipment manual).
Route separately from power
NEC 725.136 requires physical separation from Class 1 or power circuits sharing the same raceway.
Label at both ends
heat-shrink or engraved tags with circuit ID.
Support at 4.5 ft or less
for open runs; secure at 12 in of every junction box (NEC 314.17).
NEC 210.19(A) FPN No. 4 recommends limiting branch-circuit voltage drop to 3% and total feeder + branch drop to 5%. Use the formula:
VD = (2 × K × I × L) / CM
- K = 12.9 for copper, 21.2 for aluminum (Ω-cmil/ft at 75°C)
- I = load current in amps
- L = one-way distance in feet
- CM = 1620 for 18 AWG
Voltage drop reference — 18 AWG copper at various loads/distances
| Load (A) | 50 ft | 100 ft | 200 ft | 300 ft |
|---|---|---|---|---|
| 2 A | 0.66% | 1.33% | 2.65% | 3.98% |
| 5 A | 1.66% | 3.32% | 6.64% | 9.95% |
| 7 A | 2.32% | 4.65% | 9.29% | 13.94% |
| 10 A | 3.32% | 6.64% | 13.27% | 19.91% |
Values shown for 240 V single-phase copper. Green = under 3% (branch circuit target). Yellow = between 3% and 5% (feeder acceptable but tight). Red = exceeds 5% total drop limit — upsize the conductor.
| NEC Article | What it Covers |
|---|---|
| NEC 110.14(C) | Termination temperature rating limitations |
| NEC 310.16 | Ampacity table — note: 18 AWG not listed for power branch circuits |
| NEC 402.5 | Fixture wire ampacity (18 AWG rated for lighting fixture leads) |
| NEC 725.49 | Class 2 and 3 remote control and signaling circuits |
| NEC 725.136 | Separation of Class 2 from power conductors |
| NEC 760 | Fire alarm systems |
| NEC Ch 9, Table 8 | Conductor properties: area, DC resistance, diameter |
PES Supply stocks 18 AWG in THHN, THWN-2, XHHW-2, and USE-2 insulations in both copper and aluminum. All conductors are UL Listed and manufactured to NEC and ASTM specifications. Explore our full wire and cable collection or view the master NEC ampacity chart for all wire gauges.
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What is 18 AWG wire used for?
18 AWG wire is used for low-voltage control and signal circuits including thermostat wire, doorbells, low-voltage landscape lighting, alarm and security systems, RS-485 data runs, thermocouples, and Class 2 remote-control circuits.. It is not permitted for general 120/240 V branch circuits under NEC 310.16 because its cross-section is too small for 15 A or 20 A branch protection.
What is the ampacity of 18 AWG copper wire?
Under NEC 402.5 for fixture wire and 725/760 for control and signal, 18 AWG copper carries 7 A at 60°C, 10 A at 75°C, and 14 A at 90°C. These ratings apply only to fixture wire and Class 2 or Class 3 circuits — never to branch-circuit power conductors.
Can I use 18 AWG for a 15 A outlet?
No. 18 AWG is not listed in NEC Table 310.16 for branch-circuit power conductors. NEC 240.4(D) requires 14 AWG copper minimum for a 15 A branch circuit and 12 AWG copper for a 20 A circuit. Using 18 AWG on a general receptacle circuit is a code violation and creates fire risk.
What is the diameter of 18 AWG wire?
18 AWG has a bare-conductor diameter of 0.0403 inches (1.024 mm) and a cross-sectional area of 1.62 kcmil (0.823 mm²), per NEC Chapter 9 Table 8. Insulated diameter varies by jacket type — THHN is roughly 0.09-0.11 inches OD.
Is 18 AWG solid or stranded?
18 AWG is available in both solid and stranded (typically 7-strand or 19-strand). Solid is used for permanent fixture leads and short push-in connections; stranded is used for control panels, flexible conduit runs, and any application with repeated movement.
What color is 18 AWG thermostat wire?
Thermostat cable using 18 AWG or 20 AWG conductors typically follows the HVAC industry convention: R=red (24 V hot), C=blue (common), Y=yellow (cool), G=green (fan), W=white (heat), O=orange (heat pump reversing). NEC does not mandate these colors — they are HVAC manufacturer conventions.
Can I splice 18 AWG to a larger wire?
Yes, at a listed splice inside an approved enclosure. Use a UL-Listed splice connector or terminal block rated for both wire sizes. For low-voltage circuits, small wire nuts (yellow, orange) or push-in Wago-style connectors are typical. Never splice inside a wall without an accessible junction box (NEC 300.15).
- Complete NEC Wire Ampacity Chart — full table for all AWG and MCM sizes
- Wire & Cable Collection — shop THHN, THWN, XHHW, USE, and PV wire
- 2/0 AWG Wire Specs
- 4/0 AWG Wire Specs
- 6 AWG Wire Specs
- 8 AWG Wire Specs
- 10 AWG Wire Specs
- 12 AWG Wire Specs
- 14 AWG Wire Specs
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