12V LED light wire size
Lighting is the usual example of a non-critical load, so the calculator is set to 10% here. Add up the watts of every light on the circuit; the calculator converts watts to amps at your voltage. If the lights dim or flicker at 10%, or the driver needs a steady voltage, use 3%.
Use 18 AWG (0.82 mm²) copper or larger.
Voltage drop and ampacity both land on this size. The 18 AWG current figure is chassis-wiring guidance, not an NEC rating.
Check: 18 AWG (0.82 mm²)
| Load current | 36 W / 12 V = 3 A |
| Wire in the circuit (out and back) | 2 x 20 ft = 40 ft |
| Resistance of 18 AWG | 6.385 ohm per 1000 ft; 0.2554 ohm for this circuit |
| Voltage drop | 0.766 V = 6.38% within 10% |
| Voltage at the load | 11.23 V |
| Power lost in the wire | 2.3 W |
| Ampacity of 18 AWG | 10 A (chassis-wiring guidance) ok |
Vd = I x 2L x R / 1000 = 3 x 2 x 20 x 6.385 / 1000 = 0.766 V, and 0.766 / 12 = 6.38%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 0.766 | 6.38 | 11.23 | 10* | smallest that passes |
| 16 | 1.31 | 0.482 | 4.02 | 11.52 | 13* | passes |
| 14 | 2.08 | 0.303 | 2.53 | 11.7 | 20 | passes |
| 12 | 3.31 | 0.191 | 1.59 | 11.81 | 25 | passes |
| 10 | 5.26 | 0.12 | 1 | 11.88 | 35 | passes |
| 8 | 8.37 | 0.075 | 0.63 | 11.92 | 50 | passes |
| 6 | 13.3 | 0.047 | 0.4 | 11.95 | 65 | passes |
| 4 | 21.2 | 0.03 | 0.25 | 11.97 | 85 | passes |
| 3 | 26.7 | 0.024 | 0.2 | 11.98 | 100 | passes |
| 2 | 33.6 | 0.019 | 0.16 | 11.98 | 115 | passes |
| 1 | 42.4 | 0.015 | 0.12 | 11.99 | 130 | passes |
| 1/0 | 53.5 | 0.012 | 0.1 | 11.99 | 150 | passes |
| 2/0 | 67.4 | 0.009 | 0.08 | 11.99 | 175 | passes |
| 3/0 | 85 | 0.007 | 0.06 | 11.99 | 200 | passes |
| 4/0 | 107 | 0.006 | 0.05 | 11.99 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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LED lighting wire size by length at 12V
| One-way length | 1 A, 10% | 3 A, 10% | 5 A, 10% | 8 A, 10% | 5 A, 3% |
|---|---|---|---|---|---|
| 5 ft (1.5 m) | 18 | 18 | 18 | 18 | 18 |
| 10 ft (3 m) | 18 | 18 | 18 | 18 | 14 |
| 15 ft (4.6 m) | 18 | 18 | 18 | 16 | 12 |
| 20 ft (6.1 m) | 18 | 18 | 16 | 14 | 12 |
| 25 ft (7.6 m) | 18 | 18 | 16 | 14 | 10 |
| 30 ft (9.1 m) | 18 | 18 | 14 | 12 | 10 |
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
36 W (3 A) at 12 V, 20 ft one way, 10% drop allowed. The current goes out and comes back,
so the circuit is 2 x 20 = 40 ft of wire, and the allowed drop is 12 x 10 / 100 =
1.2 V. Voltage drop is Vd = I x 2L x R / 1000, with R in ohms per 1000 ft:
- 18 AWG:
3 x 40 x 6.385 / 1000 = 0.766 V, which is 6.38% of 12 V; passes both checks.
Answer: 18 AWG (0.82 mm²), rated 10 A as chassis-wiring guidance, the smallest size that passes both.
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
Can I use very thin wire for LED strips?
Short runs at low current often pass on drop with 18 AWG, but check the current rating too, and the fuse protecting the circuit must suit the wire.