Electrical
Calculate voltage loss in electrical conductors to ensure circuit adequacy and safety compliance.
Voltage Drop
5.79V
✗ Exceeds 3%
Most electrical codes require voltage drop to remain under 3% on branch circuits and 5% combined with feeder circuits.
Voltage drop is the reduction in voltage as electricity travels through a wire, caused by the wire's resistance. High voltage drop reduces power available at the load (appliance or equipment), causing dimmer lights, weaker motors, and wasted energy. Codes like the NEC (National Electrical Code) mandate drop remain under 3% on branch circuits and 5% total across the entire run. This calculator applies Ohm's Law to predict drop before installation.
Enter system voltage, current in amperes, one-way wire length in feet, and select wire gauge. The calculator uses the formula Vdrop = 2 × length × current × R / 1000, then divides by voltage to express drop as a percentage and shows voltage reaching the load. Color-coded status indicates pass (under 3%) or fail.
Vdrop = (2 × L × I × R) / 1000 where L = one-way length (ft), I = current (A), R = resistance per 1000 ft (Ω)The factor 2 accounts for current flowing down and returning through the neutral. Length is one-way distance from source to load. Resistance per 1000 feet varies by wire gauge (smaller AWG = larger wire = lower resistance). Divide by 1000 to convert the per-1000-ft rating to per-actual-length. The result is volts lost.
| Input | Result | Notes |
|---|---|---|
| 120V, 15A, 100ft, AWG 12 | Voltage drop: 5.79V (4.8% of 120V) | Exceeds 3% code threshold; unacceptable for branch circuit. Upgrade to AWG 10 or shorten the run. |
| 120V, 15A, 50ft, AWG 12 | Voltage drop: 1.93V (1.6% of 120V) | Well within 3% limit; safe for residential branch circuits. |
| 240V, 30A, 150ft, AWG 6 | Voltage drop: 4.41V (1.8% of 240V) | Larger wire diameter AWG 6 keeps drop low even over longer runs typical in well systems and submersibles. |
Voltage drop is the loss of electrical potential (measured in volts) as current flows through wire resistance. Long runs, high current, or thin wire cause larger drops. If 120V is supplied but 8V is lost in the wire, only 112V reaches the device.
The NEC (National Electrical Code) adopted 3% for branch circuits and 5% combined to protect appliance motors and lighting. Below 3%, power delivered is nearly constant; above 3%, voltage-sensitive equipment like motors draw excess current to compensate, causing overheating and shortened lifespan.
One-way length is the distance from the breaker (or source) to the load (appliance). The formula multiplies by 2 because current travels down the hot wire and returns on the neutral, so total wire is 2 × one-way distance.
Start with current capacity (NEC Table 310); e.g. AWG 12 handles 20A. Then use this calculator to verify voltage drop. If drop exceeds 3%, step up to the next gauge (10, 8, 6, etc.) and recalculate.
Yes. Resistance increases ~0.4% per °C above 20°C. This calculator uses standard 20°C copper resistance; in hot conduit, actual drop may be slightly higher. For critical circuits, apply a safety factor.
No, this calculator is for single-phase (120/240V residential and light commercial). For 3-phase, the formula uses a different factor (~√3) and requires line-to-line voltage. Consult an electrician or 3-phase calculator.