Wire Size Calculator for 12 V, 24 V and 48 V Automotive Circuits
Automotive Wire Gauge Calculator
Enter the circuit's voltage, load and length. The calculator finds the smallest copper AWG that keeps voltage drop under your limit, using the NBS copper wire table and the ABYC E-11 3% and 10% rules, then checks that size against the ABYC ampacity table.
20 A over 30 ft of wire (15 ft each way) at 6 AWG (0.3952 Ω per 1000 ft) drops 0.237 V, 1.98% of 12 V, within the 0.36 V allowed at 3%. 8 AWG would drop 0.377 V (3.14%), just over the limit.
Next size up, 4 AWG, drops 0.149 V (1.24%).
Ampacity check: ABYC E-11 Table 6A rates 6 AWG (105 °C insulation, outside engine spaces) at 120 A, above your 20 A load.
At the 10% non-critical limit the same run needs 12 AWG.
Formula
Voltage drop = current × wire length × resistance per foot, where the length is the full round trip when the return runs through a wire (2 × one-way) and the one-way length when the chassis is the return. The gauge shown is the smallest listed size whose drop is at or under the allowed volts (3% or 10% of system voltage). Resistance is taken at 20 °C from the NBS table; the 75 °C option multiplies it by 1 + 0.00393 × 55 = 1.216 using the NBS temperature coefficient for standard annealed copper.
Copper wire table used (AWG 22 to 4/0)
NBS Handbook 100, Table 5: standard annealed copper, American Wire Gage, values at 20 °C. Ampacity from ABYC E-11 Table 6A, 105 °C insulation.
| AWG | Diameter (mils) | Circular mils | Ω per 1000 ft at 20 °C | Amps, outside engine space | Amps, inside engine space |
|---|---|---|---|---|---|
| 22 | 25.3 | 640 | 16.2 | — | — |
| 20 | 32.0 | 1,020 | 10.1 | — | — |
| 18 | 40.3 | 1,620 | 6.39 | 20 | 17 |
| 16 | 50.8 | 2,580 | 4.02 | 25 | 21.3 |
| 14 | 64.1 | 4,110 | 2.52 | 35 | 29.8 |
| 12 | 80.8 | 6,530 | 1.59 | 45 | 38.3 |
| 10 | 101.9 | 10,380 | 0.9988 | 60 | 51 |
| 8 | 128.5 | 16,510 | 0.6281 | 80 | 68 |
| 6 | 162.0 | 26,240 | 0.3952 | 120 | 102 |
| 4 | 204.3 | 41,740 | 0.2485 | 160 | 136 |
| 3 | 229.4 | 52,620 | 0.1971 | 180 | 153 |
| 2 | 257.6 | 66,360 | 0.1563 | 210 | 178.5 |
| 1 | 289.3 | 83,690 | 0.1239 | 245 | 208.3 |
| 1/0 | 324.9 | 105,600 | 0.09825 | 285 | 242.3 |
| 2/0 | 364.8 | 133,100 | 0.07793 | 330 | 280.5 |
| 3/0 | 409.6 | 167,800 | 0.06182 | 385 | 327.3 |
| 4/0 | 460.0 | 211,600 | 0.04901 | 445 | 378.3 |
The calculator picks from the sizes above (the ABYC E-11 Table 6A sizes plus 22 and 20 AWG). Odd gauges between them exist in the NBS table but are not stocked as automotive primary wire. Ampacity is for a single conductor not bundled, with 105 °C insulation; ABYC Table 6A does not list 22 or 20 AWG, shown as a dash. Bundled wires are derated (0.7 for 2 to 3 conductors, down to 0.4 for 25 or more). This tool sizes for voltage drop and checks ampacity; it does not replace the circuit fuse, which must protect the wire.
Sources: NBS Handbook 100, Copper Wire Tables (1966), Table 5 and the 0.00393 per °C temperature coefficient · ABYC E-11 Table 6A ampacity and the 3% / 10% voltage drop tables, reproduced by BoatHowTo courtesy of ABYC · Blue Sea Systems: round-trip length and ABYC E-11 3% critical / 10% non-critical rule
How It Works
A wire is a resistor. Every foot of copper drops a little voltage in proportion to the current through it, so a long run to a big load can starve the device even though the battery reads fine. The calculator multiplies your current by the wire length and by the resistance per foot of each gauge, then reports the smallest size whose drop stays within your limit.
Length is the full path the current takes. If the load’s negative wire runs back to the battery, the current crosses the copper twice, so the calculator doubles your one-way length. If the return is through the chassis or frame, untick the box and only the one-way wire counts. Resistance per foot comes from the National Bureau of Standards copper wire table, stated at 20 °C; the 75 °C option scales it by 1.216 using the same handbook’s temperature coefficient, since copper in a hot engine bay resists more.
The 3% and 10% limits follow ABYC E-11: 3% for critical circuits such as panel feeders, electronics and navigation lights, 10% for non-critical loads such as general lighting, winches and pumps. Voltage drop is only half of sizing. The wire must also carry the current without overheating, so the result is checked against ABYC E-11 Table 6A (105 °C insulation, single conductor, outside or inside engine spaces) and stepped up if the drop-based size is not rated for your load.
Worked Example
A 20 A load sits 15 ft from a 12 V battery with its own return wire, so the current crosses 30 ft of copper. At 3% the allowed drop is 0.36 V. 8 AWG is 0.6281 Ω per 1000 ft: 2 × 20 × 15 × 0.0006281 = 0.377 V, or 3.14%, just over. 6 AWG is 0.3952 Ω per 1000 ft: 2 × 20 × 15 × 0.0003952 = 0.237 V, or 1.98%, so 6 AWG is the answer. The ABYC 3% table for 12 V gives the same 6 AWG for 20 A at 30 ft, and Table 6A rates 6 AWG at 120 A outside engine spaces, far above the load. At the 10% limit the same run only needs 12 AWG.
FAQ
Why does the calculator disagree with a chart I found?
Most online charts use one fixed resistance and one drop limit. This one shows its inputs: the NBS 20 °C table, your exact length, and your chosen limit. The printed ABYC tables assume slightly warmer copper and occasionally land one size larger on borderline runs; when a run is close to the limit, take the larger size.
Does the fuse size matter here?
The fuse protects the wire, not the load, so it must be at or under the wire’s ampacity. This tool does not pick fuses. Size the wire first, then choose a fuse that the wire can carry.
Can I use the one-way length if my load grounds to the frame?
Yes. Untick the return-wire box. The frame has far more copper-equivalent cross-section than any wire, so only the positive run counts. Trailers are the usual exception: a corroded ball-and-coupler ground is why trailer harnesses need their own white return wire.
What about 22 and 20 AWG?
They are in the resistance table, so the calculator will pick them for tiny loads, but ABYC Table 6A starts at 18 AWG. When the result is 22 or 20 AWG, the ampacity line tells you to confirm the wire maker’s own rating.
Is this for house wiring too?
No. It is a DC low-voltage tool built on marine and automotive standards. Household AC circuits are sized under the NEC with different tables and rules.
Sources
- NBS Handbook 100: Copper Wire Tables (1966) - Table 5, standard annealed copper, American Wire Gage, ohms per 1000 ft at 20 °C, and the 0.00393 per °C temperature coefficient of resistance
- ABYC E-11 tables reproduced by BoatHowTo, courtesy of ABYC - Table 6A allowable amperage by insulation rating inside and outside engine spaces, bundle derating factors, and the 3% and 10% conductor-size tables for 12 V and 24 V used as a cross-check
- Blue Sea Systems: Choosing the Correct Wire Size for a DC Circuit - circuit length is the round trip from source to load and back; ABYC E-11 3% critical and 10% non-critical circuit lists; the 80 A windlass at 25 ft example