3000W inverter wire size at 12V
A 3000 W inverter running flat out on 12 V draws about 294.2 A from the battery at 85% efficiency, or 277.8 A at 90%:
I = P / (V x efficiency) = 3000 / (12 x 0.85) = 294.2 A, rounded up. Over 5 ft one way at 3% drop that needs more than a single 4/0 copper.
Size the cable for the inverter's continuous rating, and check the cable and fuse sizes its manual gives.
No single copper conductor up to 4/0 AWG passes.
294.2 A is more than the 230 A rating of 4/0. Split the load into parallel runs, or use a higher system voltage to cut the current.
Check: 4/0 AWG (107 mm²)
| Load current | 294.2 A |
| Wire in the circuit (out and back) | 2 x 5 ft = 10 ft |
| Resistance of 4/0 AWG | 0.04901 ohm per 1000 ft; 0.00049 ohm for this circuit |
| Voltage drop | 0.144 V = 1.2% within 3% |
| Voltage at the load | 11.86 V |
| Power lost in the wire | 42.4 W |
| Ampacity of 4/0 AWG | 230 A (NEC 310.16, 75 C) too small |
Vd = I x 2L x R / 1000 = 294.2 x 2 x 5 x 0.04901 / 1000 = 0.144 V, and 0.144 / 12 = 1.2%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 18.785 | 156.54 | -6.78 | 10* | drop and current |
| 16 | 1.31 | 11.815 | 98.46 | 0.18 | 13* | drop and current |
| 14 | 2.08 | 7.429 | 61.9 | 4.57 | 20 | drop and current |
| 12 | 3.31 | 4.672 | 38.93 | 7.33 | 25 | drop and current |
| 10 | 5.26 | 2.939 | 24.49 | 9.06 | 35 | drop and current |
| 8 | 8.37 | 1.848 | 15.4 | 10.15 | 50 | drop and current |
| 6 | 13.3 | 1.162 | 9.69 | 10.84 | 65 | drop and current |
| 4 | 21.2 | 0.731 | 6.09 | 11.27 | 85 | drop and current |
| 3 | 26.7 | 0.58 | 4.83 | 11.42 | 100 | drop and current |
| 2 | 33.6 | 0.46 | 3.83 | 11.54 | 115 | drop and current |
| 1 | 42.4 | 0.365 | 3.04 | 11.64 | 130 | drop and current |
| 1/0 | 53.5 | 0.289 | 2.41 | 11.71 | 150 | current too high |
| 2/0 | 67.4 | 0.229 | 1.91 | 11.77 | 175 | current too high |
| 3/0 | 85 | 0.182 | 1.52 | 11.82 | 200 | current too high |
| 4/0 | 107 | 0.144 | 1.2 | 11.86 | 230 | current too high |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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3000W inverter cable size by length at 12V
| One-way length | 85%: 294.2 A, 3% | 90%: 277.8 A, 3% | 85%: 294.2 A, 10% |
|---|---|---|---|
| 3 ft (0.9 m) | over 4/0 | over 4/0 | over 4/0 |
| 5 ft (1.5 m) | over 4/0 | over 4/0 | over 4/0 |
| 6 ft (1.8 m) | over 4/0 | over 4/0 | over 4/0 |
| 8 ft (2.4 m) | over 4/0 | over 4/0 | over 4/0 |
| 10 ft (3 m) | over 4/0 | over 4/0 | over 4/0 |
| 12 ft (3.7 m) | over 4/0 | over 4/0 | over 4/0 |
| 15 ft (4.6 m) | over 4/0 | over 4/0 | over 4/0 |
Copper, minimum AWG. * ampacity, not voltage drop, sets this size. "over 4/0" means no single copper conductor up to 4/0 passes: shorten the run, use a higher system voltage or run conductors in parallel. Length is one way; the math uses twice that, out and back.
Worked example
294.2 A at 12 V, 5 ft one way, 3% drop allowed. The current goes out and comes back,
so the circuit is 2 x 5 = 10 ft of wire, and the allowed drop is 12 x 3 / 100 =
0.36 V. Voltage drop is Vd = I x 2L x R / 1000, with R in ohms per 1000 ft:
- 4/0 AWG:
294.2 x 10 x 0.04901 / 1000 = 0.144 V, which is 1.2% of 12 V; drop is fine, but it is rated only 230 A.
No single copper conductor up to 4/0 passes. 294.2 A is above the 230 A that 4/0 is rated for. Common answers are conductors in parallel (the NEC allows paralleling in 1/0 and larger, with matching length, material and terminations) or a higher system voltage, which cuts the current.
Why no single cable is listed for 3000W at 12V
294.2 A is more than the 230 A that NEC Table 310.16 gives 4/0 copper in the 75 C column, so no single conductor in this calculator is rated for it. The usual answers are a higher battery voltage (the 3000W inverter at 24V draws 147.1 A) or two conductors in parallel per pole; the NEC allows paralleling in 1/0 and larger, with matching length, material and terminations. Battery and welding cable with a higher temperature rating, sized to ABYC E-11 or the cable maker's own table, is allowed more current than the NEC 75 C column, so some installers use a single larger cable here. This calculator stays with the conservative NEC figures.
Before you buy wire
- Ampacity figures for 14 AWG and larger are from NEC (NFPA 70) Table 310.16, copper, 75 C column, which assumes 30 C ambient and no more than three current-carrying conductors bundled together. Hot spaces, bundles and conduit lower them. 18 and 16 AWG figures are chassis-wiring guidance, not NEC ratings.
- On boats, follow ABYC E-11 for DC wiring. Engine rooms are hot, and conductors there are allowed less current than the same wire elsewhere.
- Use wire rated for the location: heat, oil, water, sunlight and abrasion all matter. Stranded copper is the norm in vehicles and boats, where wire vibrates.
- Fuse every positive run close to the source, sized to protect the wire. NEC 240.4(D) limits the overcurrent device on 14, 12 and 10 AWG to 15, 20 and 30 A.
- This calculator does arithmetic. It does not certify an installation; check your work against the standards and the equipment makers' instructions, or have it checked.
Questions
How many amps does a 3000W inverter draw at 12V?
About 294.2 A at full load and 85% efficiency (3000 / (12 x 0.85)), or 277.8 A at 90%. At light load it draws far less.
What size cable for a 3000W inverter at 12V over 10 feet?
No single copper conductor up to 4/0 passes at 3% drop, sized for 294.2 A.