Voltage drop: interpreting the result
How This Calculation Works
This tool estimates voltage loss along the outgoing and returning conductors using the selected conductor size, current, one-way distance, and circuit voltage. The result is shown as volts and a percentage of source voltage. Longer runs, higher current, and smaller conductors create more resistance. A 50-foot measurement is one-way, but the current travels out and back, so the calculation accounts for the circuit loop. The result is a planning estimate using typical copper resistance; actual cable construction, temperature, connections, and load behaviour can change the measured value.
Applicable Code Rules & Standards
The CEC discusses voltage drop and conductor selection in the context of an installation, while the commonly used design target is about 3% on a branch circuit and 5% total feeder plus branch circuit. These are not a universal permission to ignore other requirements. Ontario installations must follow the current OESC and ESA direction. NEC designs in the United States commonly use the same 3%/5% guidance, but local amendments and equipment instructions control. Verify conductor ampacity separately with the Wire Size Calculator; voltage drop cannot justify exceeding ampacity or breaker ratings.
Common DIY/Installation Errors
- Entering one-way distance as the total circuit loop without understanding the tool's assumptions.
- Using a larger breaker to compensate for dim lights or motor starting problems.
- Ignoring aluminum conductor resistance, high ambient temperature, bundling, or loose terminations.
- Checking only the normal load and not the starting current of pumps, compressors, or motors.
- Forgetting that low-voltage equipment can be especially sensitive to small voltage losses.
Voltage drop is also a performance issue: excessive loss can make lamps dim, motors run hot, electronics reboot, and heating equipment fail to reach its rated output. Measure the actual one-way route rather than estimating from a straight line, include the route through the structure, and identify whether the circuit is two-wire, multi-wire, or three-phase. A professional may choose a larger conductor, a closer disconnect, a different voltage, or a new route rather than accepting a marginal result.
When to Hire a Licensed Professional
Have a professional design long feeders, detached-building supplies, EV chargers, pumps, HVAC circuits, and service conductors. A Licensed Electrical Contractor can measure loaded voltage at both ends, select conductor material and insulation, verify raceway fill and correction factors, and coordinate permits. Stop if cables or terminations are hot, lights flicker widely, or a motor will not start.