Voltage Drop at 120V (Single-Phase AC)
120V single-phase is the standard voltage for most receptacle circuits in North American homes and offices. Because 120V is relatively low, voltage drop matters more on longer runs.
Why 120V Systems Need Attention
At 120V, voltage drop matters on longer runs. A 3% drop on a 120V circuit is only 3.6V, but a load expecting 120V may run noticeably hotter or dimmer at 116V.
Example Calculation
For a 120V Single-Phase AC circuit at 20 A over 100 ft using 12 AWG copper:
Voltage drop = 2 x I x R = 2 x 20 x (ohms per ft for 12 AWG x 100 ft)
Common Applications at 120V
- Residential outlets
- Lighting circuits
- Small appliances
- Home office equipment
Common Questions
What is the acceptable voltage drop for a 120V circuit?
Most engineers target 3% or less on branch circuits and 5% total (feeder plus branch). These are general guidelines, not hard NEC requirements. Check your local code.
What wire gauge should I use for a 120V circuit?
It depends on load current and run length. Use the voltage drop calculator with your specific values. A longer run or heavier load requires a larger gauge.
Does voltage drop affect safety?
Excessive voltage drop can cause motors to overheat, breakers to nuisance-trip, and equipment to malfunction. It is not just an efficiency issue.
Can I use aluminum wire on a 120V circuit?
Aluminum is commonly used for larger feeders (service entrances, subpanel feeds). For small branch circuits, copper is generally required. Aluminum has higher resistance than copper for the same gauge, so voltage drop is worse for the same wire size.