Voltage drop in a single-phase circuit: formula and examples
Calculate loop resistance, voltage drop, percent drop and load voltage for a single-phase circuit from current, length and conductor resistance.
Voltage drop in a two-wire circuit is the load current times the resistance of the whole loop, meaning the outgoing and the returning conductor. Percent drop divides that by the supply voltage. The method below treats the circuit as resistive.
How do you calculate loop resistance?
Cable data sheets give resistance in ohms per kilometer. The loop is twice the one-way length. For a fictional feeder with a 40 m one-way run at 4.5 ohms per kilometer:
=2*D11/1000*E11 gives 2*40/1000*4.5 = 0.36 ohms.
How do you calculate the drop and the load voltage?
At 16 amperes on a 240 V supply:
| Item | Formula | Result |
|---|---|---|
| Voltage drop | =C11*F11 |
5.76 V |
| Drop share | =G11/B11 |
2.4% |
| Load voltage | =B11-G11 |
234.24 V |
How do different circuits compare?
| Circuit | Supply | Current | Length | Ohm/km | Drop | Share |
|---|---|---|---|---|---|---|
| Feeder A | 240 V | 16 A | 40 m | 4.5 | 5.76 V | 2.4% |
| Lighting B | 120 V | 10 A | 25 m | 7.2 | 3.60 V | 3.0% |
| Pump run C | 240 V | 20 A | 60 m | 3.0 | 7.20 V | 3.0% |
Lighting B and pump C both reach 3 percent, a figure often used as a design target for branch circuits in informational notes of the National Electrical Code. Treat that as guidance and check the edition and rules that apply to your project.
What does this model ignore?
- Three-phase circuits use a different factor, about 1.732 times current for line-to-line drop.
- AC cables also have reactance, which matters for large conductors and long runs.
- Resistance rises with temperature, so a hot conduit drops more than the data sheet value.
- Motor starting current causes a larger, short drop than running current.
The voltage drop calculator works through each circuit as a row. To size the circuit in the first place, see the panel load schedule and how NEC 220.44 applies to receptacle demand, and use the energy cost calculator to price what the load consumes.