Recommended Wire Size & Voltage Drop for 15A 240V at 300ft (Aluminum)
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Use 4 AWG Aluminum Wire
Estimated Voltage Drop: 1.90% · Max Recommended Fuse/Breaker: 20A
Ohmla — Wire Sizing Spec Sheet
Generated July 27, 2026 · ohmla.com
| Load Current | 15 A |
| Source Voltage | 240 V |
| One-Way Distance | 300 ft |
| Conductor Material | Aluminum |
| Recommended AWG | 4 AWG |
| Circular Mils | 41,740 |
| Voltage Drop | 4.57 V |
| Percent Drop | 1.90% (limit 3%) |
| Power Lost | 68.6 W |
| Conductor Ampacity (75°C) | 65 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 AWG Aluminum Wire
Estimated Voltage Drop: 1.90% · Limit: 3%
| Recommended AWG | 4 AWG Aluminum |
| Circular Mils (CM) | 41,740 |
| Voltage Lost | 4.57 V |
| Percent Drop | 1.90% |
| Power Lost | 68.6 W |
| Conductor Ampacity (75°C) | 65 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 | 46.42V | 19.34% | 696.4W | 15A |
| 12 AWG | 6,530 | 20A | 29.22V | 12.17% | 438.3W | 20A |
| 10 AWG | 10,380 | 30A | 18.38V | 7.66% | 275.7W | 20A |
| 8 AWG | 16,510 | 40A | 11.56V | 4.82% | 173.3W | 20A |
| 6 AWG | 26,240 | 50A | 7.27V | 3.03% | 109.1W | 20A |
| 4 AWG | 41,740 | 65A | 4.57V | 1.90% | 68.6W | 20A |
| 2 AWG | 66,360 | 90A | 2.88V | 1.20% | 43.1W | 20A |
| 1/0 AWG | 105,600 | 120A | 1.81V | 0.75% | 27.1W | 20A |
| 2/0 AWG | 133,100 | 135A | 1.43V | 0.60% | 21.5W | 20A |
| 3/0 AWG | 167,800 | 155A | 1.14V | 0.47% | 17.1W | 20A |
| 4/0 AWG | 211,600 | 180A | 0.90V | 0.38% | 13.5W | 20A |
NEC Guidance for 15A Circuits at 240V
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 240V, voltage drop becomes a more significant design constraint over longer conductor runs, since even a small resistive loss represents a larger percentage of the source voltage.
This calculation assumes a 300ft 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.
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.