GUIDE
Low Voltage Wire Gauge Chart
Thermostats, doorbells, cameras and access control all ride low-voltage cable — tiny currents, long runs and their own class-of-service rules.
Common control/VDC sizes
18–12 AWG copper; amps column shows the 75 °C NEC rating where it exists.
| AWG | Diameter (mm) | Area (mm²) | Cu Ω/1000 ft | Typical amps |
|---|---|---|---|---|
| 12 | 2.05 | 3.31 | 1.588 | 25 |
| 14 | 1.63 | 2.08 | 2.525 | 20 |
| 16 | 1.29 | 1.31 | 4.015 | 18 |
| 18 | 1.02 | 0.82 | 6.385 | 14 |
You're sizing for a few mA — usually
Thermostats, bell buttons and access panels draw milliamps, so wire gauge barely matters electrically; what matters is insulation ratings, fire separation and mechanical durability behind walls. The exceptions are the DC power feeds: cameras, electric strikes and video doorbells with single-digit amps, where length quietly eats voltage at 12 V DC.
When the run gets long
A 12 V camera pulling 0.5 A at 150 ft loses 2.3 V round-trip on 18 AWG — 19% of the supply! — which is exactly why verified installs size those feeds like circuit wiring: compute drop with the calculator, and step to 14 AWG (or power locally at 24 V) on anything past ~100 ft.
NEC still applies
Class 2 wiring rules limit thickness/energy of low-voltage sources; conduit fill, plenum ratings (CMP) and fire-stop separation still govern installation. "Low voltage" is not "no code".
Low voltage wire gauge chart, application by application
Every Class 2/Class 3 application has a workhorse size, and they are all on the chart above: thermostat wire 18 AWG (4–8 conductor) for HVAC control loops up to ~50 ft; doorbell chimes 18 AWG with the transformer sized for multiple chimes; security keypads and sensors 22 AWG panel runs, 18 AWG for power-invasive devices; door strikes and access control 18 AWG; landscape lighting 12-10 AWG trunks on the 12 V side of the transformer; irrigation controllers 18 AWG valve wires over reasonable yard distances; intercoms and small motors 16-18 AWG with the drop math done before the staples go in. When a run involves speakers instead of controls, the speaker wire gauge chart takes over at 16-12 AWG.
Planning camera and long-loop runs
Distance kills low-voltage runs quietly: the load still works, it just runs weakly. A 12 V camera drawing 500 mA over 150 ft on 18 AWG loses about 0.61 V round trip — 5% of supply, exactly where sensors start rebooting. The fixes are classic: thicker wire (16 or 14 AWG halves the loss each step, see the copper resistance column), a power injector at the mount, or a 24 V-to-12 V step-down at the camera so the long stretch happens at efficient voltage. PoE cameras dodge the whole problem: data and power ship on Cat5e/6 at 48 V, where the same current costs only a twelfth as much in volts. For DC runs inside a vehicle instead of a building, the 12V DC wire gauge chart governs.
Cable classes and separation: CL2, CL3, CMR, CMP
The jacket letters describe fire behavior and where the cable may legally run, not the copper inside. CL2/CL3 class-rated cables are for in-wall runs of low-voltage power and control; CL3 handles the higher voltage (up to 300 V) that some access and audio systems carry. CMR riser versions resist vertical fire spread; CMP plenum versions survive air-handling spaces. Two rules don't budge: low-voltage cable stays out of any box that also contains line voltage unless a listed barrier separates them, and every run keeps an inch of clearance or a barrier from power in the same wall cavity — the electrical wire gauge chart explains the line-voltage side of that distance.
Frequently asked questions
What gauge wire for a thermostat?
Modern HVAC thermostats use 18-gauge 4–8 conductor cable. Long runs (over 60 ft) between air handler and thermostat move a size up only if transformer capacity is marginal.
What gauge for a doorbell transformer wire?
16 or 18 AWG. Doorbell transformers push under 1 A at 16–24 V, so the wire carries trivially low current — the frame matters more than the gauge.
What gauge wire for security cameras?
Power + video runs commonly use 18 AWG Siamese/RG59. 12 V DC camera power over long runs needs voltage-drop care: 50 ft is fine on 18 AWG, 150 ft prefers 14 AWG.
Can I run low-voltage and electrical wire in the same hole?
Keep them separated: Class 2/3 cable must stay out of boxes and raceways shared with line-voltage conductors unless a listed barrier divides them, and maintain distance in shared wall cavities.
Is low voltage wire dangerous?
It can still start fires if undersized or unprotected in contact with building wire. Follow Class 2/Class 3 separation rules and keep it out of boxes shared with line-voltage cable.