20 AWG wire has relatively high resistance for a small conductor, so the resistance result becomes a design input as cable length increases. For copper at 20°C, 20 AWG is commonly listed at approximately 10.15 Ω per 1,000 ft; a smaller 22 AWG copper conductor is approximately 16.14 Ω per 1,000 ft.
That value is not an ampacity rating. It is the conductor resistance used to estimate circuit loop resistance, voltage drop, power loss, and the voltage actually available at a low-voltage load, relay, solenoid, sensor, controller, or other field device.
Use the AWG Wire Resistance Calculator to select the conductor material and AWG size, then scale the resistance value to the installed conductor length. For a two-wire DC or single-phase control circuit, include both the outgoing and return conductors.
20 AWG Resistance Calculation
The basic resistance calculation is:
\(\displaystyle R = R_{1000} \times \frac{L}{1000}\)
Where:
- (R) = conductor resistance in ohms
- \(R_{1000}\) = resistance in ohms per 1,000 ft for the selected AWG and material
- (L) = conductor length in feet
For a two-conductor circuit with equal-length supply and return conductors:
\(\displaystyle R_{loop} = R_{1000} \times \frac{2L_{one-way}}{1000}\)
The one-way cable run is not the same as the electrical path length. A controller located 100 ft from a 24V DC power supply normally has approximately 200 ft of active conductor in the DC loop: 100 ft outbound and 100 ft returning.
With 20 AWG copper:
\(\displaystyle R_{loop} = 10.15 \times \frac{2L_{one-way}}{1000}\)
For a 100 ft one-way run:
\(\displaystyle R_{loop} = 10.15 \times \frac{200}{1000} = 2.03\ \Omega\)
The calculator’s resistance output can then be carried into a voltage-drop calculation.
Voltage Drop in Control Circuits
Voltage drop is calculated from circuit current and loop resistance:
\(\displaystyle V_{drop} = I \times R_{loop}\)
For DC control circuits, this result directly reduces the voltage delivered to the load:
\(\displaystyle V_{load} = V_{source} - V_{drop}\)
A 20 AWG conductor may be entirely appropriate for a low-current signal, dry-contact input, communication circuit, or short control-circuit run. It can become less suitable when the run is long, the load current is higher, or the connected device has a narrow operating-voltage range.
A 24V DC device supplied from a 24V source does not necessarily receive 24V. The actual terminal voltage depends on the source voltage under load, the total circuit resistance, conductor temperature, terminal connections, and the current drawn by the load.
Use the Voltage Drop Calculator for general conductor voltage-drop review, or use the 24 VDC Control Voltage Drop Calculator when checking a 24V DC control loop.
20 AWG vs. 22 AWG Example
Assume a copper control cable has a 100 ft one-way route from a 24V DC power supply to a 1A load. The circuit uses one conductor for positive and one conductor for negative, so the electrical loop length is 200 ft.
| Conductor size | Resistance per 1,000 ft | Loop length | Loop resistance | Voltage drop at 1A | Voltage at 24V load |
|---|---|---|---|---|---|
| 20 AWG copper | 10.15 Ω | 200 ft | 2.03 Ω | 2.03 V | 21.97 V |
| 22 AWG copper | 16.14 Ω | 200 ft | 3.23 Ω | 3.23 V | 20.77 V |
The 20 AWG circuit drops about 8.5% of a nominal 24V supply in this example. The 22 AWG circuit drops about 13.5%. The 20 AWG conductor therefore leaves approximately 1.20V more at the load than 22 AWG at the same route length and current.
The calculation does not automatically establish whether 21.97V or 20.77V is acceptable. Check the device’s operating-voltage specification, pickup and hold-in requirements where applicable, power-supply output tolerance, inrush current, and any other loads sharing the supply.
Resistance and Conductor Selection
Resistance often drives the conductor-size decision in low-voltage wiring even where conductor heating is not the limiting condition. Moving from 22 AWG to 20 AWG reduces resistance, but it also affects cable cost, termination compatibility, cable outside diameter, tray or raceway fill, bending space, and available connector sizes.
For a longer control route, the practical choices may include:
- Use 20 AWG instead of 22 AWG to reduce loop resistance.
- Increase to a larger conductor when voltage-drop margin remains insufficient.
- Shorten the route or relocate the power supply closer to the load.
- Use a higher distribution voltage only where the equipment and system design permit it.
- Separate high-current loads from sensitive control or signal circuits.
- Verify that the selected cable insulation, conductor construction, terminations, and equipment listings match the installation.
Stranded conductors can have different resistance than a solid-conductor reference because of strand construction and finished conductor area. Temperature also increases copper resistance. Use the resistance value for the actual material and construction when manufacturer data is available.
Field Verification
A calculated 20 AWG resistance value is useful for cable sizing and voltage-drop review, but it is not a final control-cable, insulation, ampacity, overcurrent-protection, or equipment-selection decision. Confirm the installed cable type, conductor temperature rating, terminal rating, routing method, current-carrying conductors, environmental conditions, equipment instructions, and AHJ requirements separately.
For a field check, measure voltage at the load terminals while the equipment is operating. A no-load reading can look acceptable even when a solenoid, relay bank, actuator, or powered controller produces excessive voltage drop under normal or peak load.