Recommended Wire Size & Voltage Drop for 15A 120V at 300ft (Copper)
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Use 4 AWG Copper Wire
Estimated Voltage Drop: 2.32% · Max Recommended Fuse/Breaker: 20A
Ohmla — Wire Sizing Spec Sheet
Generated July 27, 2026 · ohmla.com
| Load Current | 15 A |
| Source Voltage | 120 V |
| One-Way Distance | 300 ft |
| Conductor Material | Copper |
| Recommended AWG | 4 AWG |
| Circular Mils | 41,740 |
| Voltage Drop | 2.78 V |
| Percent Drop | 2.32% (limit 3%) |
| Power Lost | 41.7 W |
| Conductor Ampacity (75°C) | 85 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 Copper Wire
Estimated Voltage Drop: 2.32% · Limit: 3%
| Recommended AWG | 4 AWG Copper |
| Circular Mils (CM) | 41,740 |
| Voltage Lost | 2.78 V |
| Percent Drop | 2.32% |
| Power Lost | 41.7 W |
| Conductor Ampacity (75°C) | 85 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 | 20A | 28.25V | 23.54% | 423.7W | 20A |
| 12 AWG | 6,530 | 25A | 17.78V | 14.82% | 266.7W | 20A |
| 10 AWG | 10,380 | 35A | 11.18V | 9.32% | 167.8W | 20A |
| 8 AWG | 16,510 | 50A | 7.03V | 5.86% | 105.5W | 20A |
| 6 AWG | 26,240 | 65A | 4.42V | 3.69% | 66.4W | 20A |
| 4 AWG | 41,740 | 85A | 2.78V | 2.32% | 41.7W | 20A |
| 2 AWG | 66,360 | 115A | 1.75V | 1.46% | 26.2W | 20A |
| 1/0 AWG | 105,600 | 150A | 1.10V | 0.92% | 16.5W | 20A |
| 2/0 AWG | 133,100 | 175A | 0.87V | 0.73% | 13.1W | 20A |
| 3/0 AWG | 167,800 | 200A | 0.69V | 0.58% | 10.4W | 20A |
| 4/0 AWG | 211,600 | 230A | 0.55V | 0.46% | 8.2W | 20A |
NEC Guidance for 15A Circuits at 120V
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 120V, 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.