12 AWG, 20 AWG, and 22 AWG are not interchangeable simply because they can all appear in low-voltage systems. The usable wire size depends on circuit current, one-way cable length, allowable voltage drop, load minimum-voltage requirement, conductor material, insulation rating, and the installation method.
For a 24V control circuit, a few volts of drop can be significant. A load that operates normally at 24V may become unreliable if its actual terminal voltage falls below the manufacturer’s operating range. The calculation starts with conductor resistance, converts that to total loop resistance, and then calculates voltage drop at the actual load current.
AWG Conductor Resistance
American Wire Gauge is an inverse sizing system: a smaller AWG number identifies a larger conductor.
| Conductor size | Relative conductor area | Relative resistance | Typical low-voltage use |
|---|---|---|---|
| 12 AWG | Largest of the three | Lowest | Longer runs, higher-current 24V loads, power distribution |
| 20 AWG | Smaller than 12 AWG | Higher than 12 AWG | Controls, sensors, signaling, limited-power loads |
| 22 AWG | Smallest of the three | Highest | Low-current controls, communications, short sensor runs |
For the same conductor material and temperature, conductor resistance increases as AWG size becomes smaller. That resistance creates voltage drop and conductor heating when current flows.
Nominal copper conductor resistance values commonly used for comparison are approximately:
| Conductor size | Resistance per 1,000 ft |
|---|---|
| 12 AWG copper | 1.59 ohms |
| 20 AWG copper | 10.15 ohms |
| 22 AWG copper | 16.14 ohms |
These values are useful for voltage-drop arithmetic, but the final circuit review must use the actual cable construction, conductor material, temperature condition, and equipment requirements.
Use the AWG Wire Resistance Calculator to check resistance for the selected conductor size and length.
Total Loop Resistance
Low-voltage voltage drop is based on the resistance of the complete current path, not just the outgoing conductor.
For a two-conductor DC circuit:
\(\displaystyle R_{\text{loop}} = R_{\text{outgoing}} + R_{\text{return}}\)
When both conductors are the same AWG and the same one-way length:
\(\displaystyle R_{\text{loop}} = 2 \times L \times R_{\text{per foot}}\)
Where:
- \(R_{\text{loop}}\) = total circuit resistance in ohms
- (L) = one-way conductor length in feet
- \(R_{\text{per foot}}\) = conductor resistance in ohms per foot
Do not use only the panel-to-load distance unless the return path has separately been accounted for. A 100-foot run to a 24V device normally means approximately 200 feet of conductor in the voltage-drop calculation.
24V Voltage-Drop Calculation
Voltage drop follows Ohm’s law:
\(\displaystyle V_{\text{drop}} = I \times R_{\text{loop}}\)
The load voltage is then:
\(\displaystyle V_{\text{load}} = V_{\text{supply}} - V_{\text{drop}}\)
Voltage-drop percentage is:
\(\displaystyle \text{Voltage Drop \%} = \frac{V_{\text{drop}}}{V_{\text{supply}}} \times 100\)
The primary output is not merely the voltage-drop percentage. For control circuits, the more useful number is often the actual voltage available at the load. That value can be compared with the coil, actuator, controller, relay, LED driver, sensor, or other equipment minimum-voltage specification.
Use the Voltage Drop Calculator for general circuit voltage-drop review or the 24 VDC Control Voltage Drop Calculator when reviewing a 24VDC control circuit.
Calculation Example
Assume the same conditions for all three conductor sizes:
- Supply voltage: 24 VDC
- Load current: 0.50 A
- One-way cable length: 100 ft
- Circuit type: two-conductor copper DC circuit
- Total loop length: 200 ft
| Conductor size | Loop resistance | Voltage drop | Voltage drop | Voltage at load |
|---|---|---|---|---|
| 12 AWG | 0.318 ohms | 0.159 V | 0.7% | 23.84 V |
| 20 AWG | 2.030 ohms | 1.015 V | 4.2% | 22.99 V |
| 22 AWG | 3.228 ohms | 1.614 V | 6.7% | 22.39 V |
For 12 AWG: \[R_{\text{loop}} = 200 \text{ ft} \times \frac{1.59 \Omega}{1{,}000\text{ ft}} = 0.318 \Omega\] \[V_{\text{drop}} = 0.50\text{ A} \times 0.318 \Omega = 0.159\text{ V}\] \[V_{\text{load}} = 24\text{ V} – 0.159\text{ V} = 23.84\text{ V}\]
At the same 0.50 A load and 100-foot one-way distance, 22 AWG produces about ten times the voltage drop of 12 AWG. The difference is caused by conductor resistance, not by the supply voltage itself.
Load-Voltage Margin
The circuit may still function with 20 AWG or 22 AWG under the example conditions. Acceptance depends on the minimum voltage required by the connected equipment.
For example, if a 24VDC device requires at least 21.6V to operate, all three wire sizes in the example remain above that threshold. If the load requires at least 23V, 20 AWG is already below the required operating voltage, while 22 AWG has substantially less margin.
Current changes the result linearly:
- Doubling circuit current doubles voltage drop.
- Doubling one-way cable length doubles total loop resistance and voltage drop.
- Using a larger conductor reduces resistance and voltage drop.
- Increasing supply voltage reduces voltage-drop percentage, but the voltage-drop value in volts still depends on current and resistance.
A control circuit with a small steady-state current can also have a higher startup, inrush, pull-in, or actuator current. Review the maximum expected circuit current where equipment performance depends on load voltage.
Wire Size and Installation Review
Voltage drop is one conductor-sizing check. It does not establish that a cable is suitable for the installation.
Confirm these items separately:
- The cable’s conductor material, insulation temperature rating, voltage rating, and listed application.
- The circuit overcurrent protection and equipment instructions.
- The conductor ampacity, including any applicable correction factor or adjustment factor.
- The number of current-carrying conductors in a raceway or cable assembly.
- Raceway fill, cable routing, pulling tension, bending radius, and termination space.
- Terminal ratings and the equipment’s specified conductor-size range.
- Separation requirements for power-limited, control, communications, or Class 2 and Class 3 wiring where applicable.
- AHJ requirements and manufacturer instructions for the specific system.
A larger conductor can reduce voltage drop, but it can also affect raceway fill, bend space, termination compatibility, connector selection, and cable management. A 12 AWG conductor may be electrically effective for a long 24V run while being impractical or incompatible with a small control-device terminal intended for 20 AWG or 22 AWG conductors.