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Category Electronics

Input Mode

Current (Amps) Total circuit current draw

Power (Watts) Total load wattage at 24V

One-Way Distance (ft) Wire run from source to load (not round-trip)

System Voltage (V) Nominal DC voltage (default 24V)

Max Voltage Drop (%)

NEC recommends ≤3% for branch circuits

Wire Material

Ambient Temperature

Conductors in Conduit

Enter values and press Calculate to see results

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Recommended AWG

Voltage Drop

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Drop Percentage

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Voltage at Load

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Current

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Derated Ampacity

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Wire Diameter

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Wire Area

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Power Loss in Wire

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AWG	V-Drop	Drop %	Ampacity	Status

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About

Low-voltage 24V DC systems are unforgiving. A wire gauge that works at 120V AC fails catastrophically at 24V because the same current produces the same resistive loss, but that loss now represents a far larger fraction of your supply voltage. A 3% drop at 120V is 3.6V. At 24V, it is 0.72V - which demands conductor cross-sections roughly five times larger for the same load and distance. Undersized wire causes dimming LEDs, malfunctioning controllers, solenoid chatter, and in worst cases, insulation melt and fire. This calculator sizes conductors using NEC Chapter 9 Table 8 resistance values and ampacity tables from NEC 310.15, checking both voltage drop and current-carrying capacity to return the smallest compliant AWG gauge.

This tool approximates conditions for standard insulation types (THHN/THWN) in dry locations. For wet, underground, or high-ambient-temperature environments, apply additional derating factors per NEC 310.15(B). Results assume a two-conductor circuit (supply and return). Pro tip: always round up one gauge from the calculated minimum when runs exceed 50ft - real-world connections, splices, and terminal resistance add loss the formula does not capture.

Formulas

The calculator determines wire size by computing the voltage drop across a two-conductor run and comparing it against the user-specified maximum allowable drop. It simultaneously checks that the chosen gauge can safely carry the required current (ampacity).

V_drop = 2 × L × I × R1000

Where V_drop = voltage lost in conductors V, L = one-way wire length ft, I = circuit current A, R = conductor resistance per 1000ft at rated temperature Ω/kft. The factor 2 accounts for the round-trip (supply + return conductors).

V_drop% = V_dropV_source × 100

If current is unknown but wattage is given, current is derived from power:

I = PV

Where P = power in W and V = 24V. The algorithm iterates AWG sizes from 18 to 4/0, selects the smallest gauge where V_drop% ≤ tolerance AND I ≤ ampacity.

Reference Data

AWG	Diameter mm	Area mm²	Area kcmil	Cu Resistance Ω/1000ft @75°C	Al Resistance Ω/1000ft @75°C	Cu Ampacity A (THHN 90°C)	Al Ampacity A (THHN 90°C)	Typical 24V Use
18	1.024	0.823	1.62	7.77	12.8	14	-	Thermostats, doorbells
16	1.291	1.309	2.58	4.89	8.05	18	-	LED strips, sensors
14	1.628	2.081	4.11	3.07	5.06	25	-	Small solenoids, relays
12	2.053	3.309	6.53	1.93	3.18	30	25	Solar charge controllers
10	2.588	5.261	10.38	1.21	2.00	40	35	Bilge pumps, trolling motors
8	3.264	8.366	16.51	0.764	1.26	55	45	Winches, large pumps
6	4.115	13.30	26.24	0.491	0.808	75	60	Inverter feed lines
4	5.189	21.15	41.74	0.308	0.508	95	75	Battery bank connections
3	5.827	26.67	52.62	0.245	0.403	115	85	High-current DC distribution
2	6.544	33.62	66.36	0.194	0.319	130	100	Large inverters, solar arrays
1	7.348	42.41	83.69	0.154	0.253	150	115	High-power DC motors
1/0	8.252	53.49	105.6	0.122	0.201	170	135	Battery-to-inverter runs
2/0	9.266	67.43	133.1	0.0967	0.159	195	150	Large solar systems
3/0	10.404	85.03	167.8	0.0766	0.126	225	175	Commercial DC bus bars
4/0	11.684	107.2	211.6	0.0608	0.100	260	205	Main battery cables, EV systems

Frequently Asked Questions

Voltage drop is proportional to current, not voltage. A 100W load draws 4.17A at 24V versus 0.83A at 120V - five times the current. The resistive loss (V_drop = I × R) scales linearly with current, and that loss is a larger percentage of the lower source voltage. You need approximately 5× the conductor cross-sectional area to maintain the same percentage voltage drop.

NEC recommends no more than 3% for branch circuits and 5% total (feeder plus branch). For sensitive electronics (PLCs, LED drivers, communication equipment), many engineers design to 2% or less. At 24V, a 3% drop is only 0.72V - many devices have a minimum operating voltage of 22V, leaving very little margin.

Higher ambient temperatures reduce insulation ampacity. NEC 310.15(B)(1) provides correction factors: at 40°C ambient (vs. the 30°C base), THHN-rated copper ampacity drops by about 12%. At 50°C, the derating factor is 0.82, meaning a wire rated for 30A can only safely carry 24.6A. Additionally, resistance increases with temperature, worsening voltage drop. This calculator applies the NEC temperature correction factor to ampacity.

Copper is standard for low-voltage DC because its resistivity (10.4 Ω⋅cmil/ft) is about 61% that of aluminum (17.0 Ω⋅cmil/ft). Aluminum requires roughly 1.6× the cross-sectional area for equivalent performance, typically bumping up two AWG sizes. Aluminum is viable for long runs above 4 AWG where cost savings outweigh the size increase, but requires anti-oxidant compound and rated connectors to prevent galvanic corrosion at terminations.

The factor of 2 accounts for the round-trip current path - out on the positive conductor and back on the negative (or ground return). This applies to standard two-wire DC circuits. For systems with a chassis/hull ground return (common in marine and automotive), use a factor of 1 for the grounded conductor if the chassis impedance is negligible. However, this is not recommended for permanent installations due to corrosion risk and unreliable ground paths.

NEC 310.15(C)(1) requires ampacity derating when more than 3 current-carrying conductors occupy a single raceway. For 4 - 6 conductors, derate to 80%. For 7 - 9, derate to 70%. This calculator provides conduit fill adjustment. The derating stacks with temperature correction - multiply both factors against the base ampacity to find the effective current limit.