Calculator inputs
Use the responsive three-column, two-column, and one-column grid below.
Formula used
Single-phase branch circuit: VD = 2 × I × L × (R × PF + X × sinφ) ÷ 1000
Three-phase branch circuit: VD = √3 × I × L × (R × PF + X × sinφ) ÷ 1000
Percent drop: % Drop = (VD ÷ Source Voltage) × 100
Delivered voltage: Delivered Voltage = Source Voltage − VD
Resistance correction: Rcorrected = Rbase × [1 + α × (T − 75)]
Here, I is load current, L is one-way length in feet, R is conductor resistance in ohms per 1000 feet, X is conductor reactance, and PF is power factor. The calculator also divides impedance by the number of parallel conductor sets.
How to use this calculator
- Choose single-phase or three-phase service.
- Enter source voltage, load current, and one-way circuit length.
- Select conductor material, conductor size, and any parallel sets.
- Enter power factor and estimated conductor operating temperature.
- Set your preferred maximum percentage drop for comparison.
- Press the calculate button to show the result block above the form.
- Review voltage drop, delivered voltage, efficiency, and the suggested size.
- Use the CSV or PDF buttons to export your result summary.
Example data table
| System | Voltage | Current | Length | Material | Size | PF | Estimated drop | Percent drop |
|---|---|---|---|---|---|---|---|---|
| Single-phase | 120 V | 20 A | 75 ft | Copper | 12 AWG | 0.98 | 5.70 V | 4.75% |
| Single-phase | 120 V | 30 A | 120 ft | Copper | 8 AWG | 0.95 | 5.32 V | 4.43% |
| Three-phase | 208 V | 40 A | 180 ft | Copper | 3 AWG | 0.90 | 2.94 V | 1.41% |
| Single-phase | 240 V | 50 A | 150 ft | Aluminum | 2 AWG | 0.92 | 4.64 V | 1.93% |
FAQs
1. Why does voltage drop matter in a branch circuit?
Excessive drop lowers delivered voltage, can reduce equipment performance, and may increase heating or nuisance issues. Keeping drop controlled helps protect efficiency and system reliability.
2. Should I enter one-way length or round-trip length?
Enter one-way length. The calculator applies the proper circuit multiplier internally for single-phase and three-phase systems, so you should not double the distance yourself.
3. How does conductor material change the answer?
Aluminum has higher resistance than copper at the same size, so it usually shows a larger voltage drop. Larger aluminum sizes can offset that difference.
4. What does power factor do here?
Power factor changes the balance between resistance and reactance in the impedance term. Lower power factor generally increases voltage drop in AC circuits.
5. Why does temperature affect the result?
Hot conductors have higher resistance. As temperature rises, voltage drop and power loss both increase, even if current and length stay unchanged.
6. What do parallel conductor sets change?
Parallel sets share current between conductors. That lowers effective resistance and impedance, which reduces voltage drop and conductor losses.
7. Is the recommended size always code-compliant?
No. The recommendation only checks voltage drop against your selected limit. You still need to verify ampacity, insulation type, termination ratings, and applicable code rules.
8. What drop percentage should I choose?
Many designers aim for about 3% on a branch circuit, but project standards vary. Use the target that matches your design practice or governing requirements.