Recommended Wire Size & Voltage Drop for 15A 12V at 100ft (Aluminum)
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Use 4/0 AWG Aluminum Wire
Estimated Voltage Drop: 2.50% · Max Recommended Fuse/Breaker: 20A
Ohmla — Wire Sizing Spec Sheet
Generated July 27, 2026 · ohmla.com
| Load Current | 15 A |
| Source Voltage | 12 V |
| One-Way Distance | 100 ft |
| Conductor Material | Aluminum |
| Recommended AWG | 4/0 AWG |
| Circular Mils | 211,600 |
| Voltage Drop | 0.30 V |
| Percent Drop | 2.50% (limit 2%) |
| Power Lost | 4.5 W |
| Conductor Ampacity (75°C) | 180 A |
| Max Recommended Fuse/Breaker | 20 A |
This spec sheet is an engineering estimate based on NEC Table 310.16 (75°C ampacity) and NEC Chapter 9, Table 8 (circular mils), assuming a 30°C ambient and standard conductor bundling. It is provided for reference only and does not replace a code-compliant load calculation or inspection performed by a licensed electrician. Verify against the current NEC/CEC edition adopted in your jurisdiction.
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Instant Answer
Use 4/0 AWG Aluminum Wire
Estimated Voltage Drop: 2.50% · Limit: 2%
| Recommended AWG | 4/0 AWG Aluminum |
| Circular Mils (CM) | 211,600 |
| Voltage Lost | 0.30 V |
| Percent Drop | 2.50% |
| Power Lost | 4.5 W |
| Conductor Ampacity (75°C) | 180 A |
| Max Recommended Fuse/Breaker | 20 A |
Based on NEC Table 310.16 (75°C ampacity) and Chapter 9, Table 8 (circular mils). Assumes a 30°C (86°F) ambient, ≤3 current-carrying conductors. Always verify against your local jurisdiction's current NEC/CEC adoption before installation.
Full Technical Spec Table
| AWG | Circular Mils | Ampacity (75°C) | Voltage Lost | % Drop | Power Lost | Max Fuse |
|---|---|---|---|---|---|---|
| 14 AWG | 4,110 | N/A | 15.47V | 128.95% | 232.1W | 15A |
| 12 AWG | 6,530 | 20A | 9.74V | 81.16% | 146.1W | 20A |
| 10 AWG | 10,380 | 30A | 6.13V | 51.06% | 91.9W | 20A |
| 8 AWG | 16,510 | 40A | 3.85V | 32.10% | 57.8W | 20A |
| 6 AWG | 26,240 | 50A | 2.42V | 20.20% | 36.4W | 20A |
| 4 AWG | 41,740 | 65A | 1.52V | 12.70% | 22.9W | 20A |
| 2 AWG | 66,360 | 90A | 0.96V | 7.99% | 14.4W | 20A |
| 1/0 AWG | 105,600 | 120A | 0.60V | 5.02% | 9.0W | 20A |
| 2/0 AWG | 133,100 | 135A | 0.48V | 3.98% | 7.2W | 20A |
| 3/0 AWG | 167,800 | 155A | 0.38V | 3.16% | 5.7W | 20A |
| 4/0 AWG | 211,600 | 180A | 0.30V | 2.50% | 4.5W | 20A |
NEC Guidance for 15A Circuits at 12V
Under NEC Table 310.16, conductor ampacity is rated by insulation temperature class and ambient conditions. For a 15A load, the 75°C column is the standard reference used by most residential and commercial terminations, since the vast majority of circuit breakers and equipment terminals are only rated for 60°C or 75°C. At 12V, voltage drop becomes a more significant design constraint because low-voltage DC and solar systems are far more sensitive to line losses.
This calculation assumes a 100ft one-way run, which is doubled in the voltage drop formula to account for both the outgoing and return conductors. The NEC recommends keeping voltage drop under 3% for branch circuits and under 5% combined for feeders and branch circuits together, though solar and off-grid DC installers typically target a stricter 2% limit to preserve panel output efficiency.
Ambient temperature also affects the usable ampacity of a conductor. Table 310.15(B)(1) derating factors reduce the rated ampacity as ambient temperature rises above 30°C (86°F):
- 26-30°C (78-86°F): multiply ampacity by 1
- 31-35°C (87-95°F): multiply ampacity by 0.94
- 36-40°C (96-104°F): multiply ampacity by 0.88
- 41-45°C (105-113°F): multiply ampacity by 0.82
- 46-50°C (114-122°F): multiply ampacity by 0.75
- 51-55°C (123-131°F): multiply ampacity by 0.67
For installations in attics, near rooftops, or in high-heat industrial environments, always apply the appropriate derating factor before finalizing wire gauge, and consult a licensed electrician for any circuit exceeding 60A or involving service entrance conductors. This tool provides an engineering estimate only and does not replace a code-compliant load calculation performed by a qualified professional.