24V Wire Size Calculator
Enter your circuit current, one-way run length, and the maximum voltage drop you can tolerate. The calculator finds the smallest copper AWG gauge that keeps your 24V circuit within spec, then shows the actual voltage drop, power loss, system efficiency, and wire resistance - all updated as you type. Switch between feet and metres for distance.
Why wire sizing matters on a 24V circuit
A 24-volt system operates at less than a tenth the voltage of a standard 240V mains circuit. Because power loss in a wire scales with resistance, and because voltage drop is a larger fraction of a low-voltage supply, undersized wire in a 24V system causes far more noticeable performance problems than it would at mains voltage. A 1-volt drop on a 24V circuit is a 4.2% loss; the same drop on a 240V circuit is only 0.4%. This is why wire sizing on 24V DC systems - solar panels, RV systems, forklift charging, industrial control panels, lithium battery banks, and 24V LED lighting - requires careful calculation rather than a rule of thumb.
How the calculation works: resistivity, cross-section, and AWG
The calculator uses the DC resistance formula: A = rho x I x L / Vdrop, where A is the conductor cross-section in square metres, rho is the temperature-corrected resistivity of copper (1.68 x 10^-8 ohm-m at 20 C, adjusted upward at higher temperatures using a coefficient of 0.00393/C), I is the circuit current in amperes, L is the total round-trip length in metres, and Vdrop is the maximum allowable voltage drop in volts. The resulting minimum cross-section is matched to the next larger standard AWG size. A second check is then run against NEC Table 310.16 ampacity limits for the selected insulation temperature rating - if the ampacity-constrained gauge is larger, that one is used instead, ensuring the wire is safe from both a voltage-drop and a thermal perspective.
Voltage drop guidelines for 24V DC systems
Different applications have different tolerances for voltage drop. Battery chargers and sensitive electronics (PLCs, sensors, communication equipment) should keep total drop at or below 2% to protect against under-voltage faults. General-purpose 24V circuits and DC motor drives are typically fine at 3%, which is the value used in the quick-reference table above. Resistive heating elements and incandescent lighting can tolerate up to 5%. Exceeding these thresholds leads to poor performance, overheating, nuisance trips, and reduced equipment life. The calculator lets you set your own limit so you can match the requirement of the specific load.
Temperature effects and NEC insulation ratings
Copper resistance rises with temperature. At 90 C (the upper limit for THHN/THWN-2 insulation), the resistivity is about 27% higher than at 20 C, which noticeably increases voltage drop and power loss on long runs. The NEC assigns ampacity ratings to conductor types based on their insulation temperature limit: TW and THW are rated at 60 C; THWN and XHHW at 75 C; THHN and THWN-2 at 90 C. Using higher-rated insulation means the wire can carry more current before its insulation degrades, but the conductor still gets hotter and its resistance still rises. The calculator applies temperature correction to resistance automatically when you change the conductor temperature slider, and it selects the ampacity column from the table matching the insulation rating you choose.
24V DC wire size quick-reference (3% drop, 75 C copper)
| Current (A) | 10 ft (3 m) | 25 ft (7.6 m) | 50 ft (15.2 m) | 75 ft (22.9 m) | 100 ft (30.5 m) | 150 ft (45.7 m) |
|---|---|---|---|---|---|---|
| 5 | 18 AWG | 18 AWG | 16 AWG | 14 AWG | 14 AWG | 12 AWG |
| 10 | 18 AWG | 16 AWG | 14 AWG | 12 AWG | 12 AWG | 10 AWG |
| 15 | 18 AWG | 14 AWG | 12 AWG | 10 AWG | 10 AWG | 8 AWG |
| 20 | 16 AWG | 14 AWG | 10 AWG | 10 AWG | 8 AWG | 8 AWG |
| 30 | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 8 AWG | 6 AWG |
| 40 | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 6 AWG | 4 AWG |
| 50 | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 6 AWG | 4 AWG |
| 60 | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 4 AWG | 2 AWG |
| 80 | 8 AWG | 6 AWG | 6 AWG | 4 AWG | 2 AWG | 1 AWG |
| 100 | 8 AWG | 6 AWG | 4 AWG | 2 AWG | 2 AWG | 1/0 AWG |
Recommended minimum AWG gauge for common current and one-way run-length combinations at 3% max voltage drop on a 24V DC copper circuit.
Frequently asked questions
What wire size do I need for 20 amps at 24V over 50 feet?
Using a 3% voltage drop limit, a 20-amp load over a 50-foot one-way run on a 24V circuit needs roughly 10 AWG copper wire. At 10 AWG, the round-trip resistance is about 0.034 ohms, producing a voltage drop near 0.68 V (2.8%), which falls inside the acceptable range. Enter your exact values in the calculator above to get the precise answer for your temperature and insulation choice.
Can I use aluminum wire for a 24V system?
Aluminum is legal for branch circuits but has about 60% the conductivity of copper, so you need one to two sizes larger for the same current and drop performance. Aluminum also requires anti-oxidant compound at connections and aluminum-rated connectors. For low-voltage DC systems the extra connection care is usually not worth the cost saving unless you are running very large cables (2/0 AWG and above). This calculator covers copper only - to size aluminum, multiply the required copper cross-section by 1.64.
What does one-way vs. round-trip length mean?
In a DC circuit, current flows out through the positive wire and returns through the negative wire, so the total conductor length that the current must travel is twice the physical distance from source to load. The calculator asks for the one-way (physical) run length and doubles it internally. If your run is 25 feet from the battery to the load, enter 25 feet; the total effective length used in the formula is 50 feet.
How do I account for multiple loads on the same wire?
Add all load currents together and use the total as your circuit current input. This gives the worst-case wire requirement for the entire run from the source. If individual branch legs branch off at intermediate points, size each leg separately for its own current and length.
What fuse or breaker should I use with the recommended wire?
The fuse or breaker should be rated at or below the NEC ampacity of the wire at its insulation temperature rating, never above it. For example, 12 AWG copper THWN-rated wire has a 75 C ampacity of 25 A - protect it with a 25 A or smaller fuse. Do not size protection to the load if the load current is higher than the wire can handle; always protect the wire.
Why does voltage drop matter more on 24V than on 120V systems?
Voltage drop is the same absolute number of volts regardless of supply voltage, but it is a larger fraction of a low-voltage supply. A 1-volt drop is 0.8% of 120 V but 4.2% of 24 V. Most 24V equipment has a narrower tolerance for under-voltage than mains equipment, so the impact on performance and reliability is proportionally greater.