Enter Circuit Details
Example Data Table
| Example | Load | Voltage | Length | Material | Temperature Rating | Expected Use |
|---|---|---|---|---|---|---|
| Small branch circuit | 20 A | 120 V | 60 ft | Copper | 60°C | Lighting or receptacles |
| Workshop feeder | 60 A | 240 V | 125 ft | Copper | 75°C | Panel feeder estimate |
| Three phase equipment | 125 A | 480 V | 180 ft | Aluminum | 75°C | Motor or machinery feed |
Formula Used
Design current:
Design Amps = Load Amps × Continuous Load Multiplier
Adjusted ampacity:
Adjusted Ampacity = Table Ampacity × Ambient Factor × Conductor Factor × Parallel Runs
Required table ampacity:
Required Base Ampacity = Design Amps ÷ (Ambient Factor × Conductor Factor × Parallel Runs)
Voltage drop:
Single Phase or DC Drop = 2 × Amps × Length × Resistance × Power Factor ÷ 1000
Three Phase Drop = √3 × Amps × Length × Resistance × Power Factor ÷ 1000
Voltage drop percentage:
Drop % = Voltage Drop ÷ System Voltage × 100
Resistance values are approximate ohms per 1000 feet. Local code tables, terminal ratings, cable type, and installation conditions may change final selection.
How to Use This Calculator
- Enter the expected load current in amperes.
- Select the circuit voltage and circuit system.
- Enter one way conductor length in feet.
- Choose copper or aluminum conductor material.
- Select the insulation temperature rating column.
- Add ambient temperature and conductor count for derating.
- Set the acceptable voltage drop percentage.
- Press the calculate button and review the result above the form.
- Use the table, chart, CSV, and PDF options for documentation.
Reliable Wire Sizing Matters
Ampacity is the current a conductor can carry without overheating. It depends on metal, insulation, temperature, and installation. A wire that looks large enough can still be unsafe. Heat builds inside raceways. Long runs also lose voltage. This calculator checks both concerns together.
How the Calculator Supports Planning
The tool starts with the load current. Continuous loads are raised by twenty five percent. This follows common design practice for steady equipment. Then ambient temperature and current carrying conductors reduce the usable rating. These reductions are called derating. The calculator compares the adjusted need with standard conductor ampacity values.
Voltage drop is checked next. Drop rises with current, length, and conductor resistance. Copper has lower resistance than aluminum. Larger conductors reduce drop. Three phase circuits use a different multiplier than single phase circuits. The result shows volts lost and percentage lost. This helps you protect motors, lighting, controls, and sensitive devices.
Important Input Choices
Material changes both ampacity and resistance. Insulation temperature affects the ampacity column and correction factors. The number of current carrying conductors affects raceway heat. One way length should be entered, not the total loop length. Parallel runs divide current across identical conductors. Use them only when permitted and installed correctly.
Using Results Wisely
The recommended size is the first conductor that passes ampacity and voltage drop checks. The table also shows nearby sizes. This makes comparison easier. You can adjust the voltage drop target for feeders or branch circuits. Many designers use tighter limits for sensitive loads.
Always verify the final selection with your local electrical code. Termination ratings can limit usable ampacity. Equipment instructions may require special conductor sizes. Motors may need separate overload and short circuit checks. Bundled cables, rooftops, wet areas, and special insulation can change the answer.
This calculator is a planning aid. It gives transparent steps, formulas, and export options. It does not replace a licensed professional. Use it to estimate, compare, and document early design choices before final review. Save reports for clients, inspections, or project notes. Recheck every change in load, distance, conduit fill, temperature, or supply voltage before ordering cable. This prevents costly field changes.
FAQs
1. What is wire ampacity?
Ampacity is the maximum current a conductor can carry under stated conditions without exceeding its temperature rating. It changes with insulation, material, ambient temperature, raceway fill, and installation method.
2. Why does continuous load increase the design current?
Continuous loads operate for long periods. Many design methods size conductors at 125 percent for these loads. This provides extra thermal margin and helps reduce overheating risk.
3. Why does voltage drop matter?
Voltage drop reduces the voltage available at equipment. Excessive drop can cause dim lights, motor heating, poor startup, and control problems. Longer runs usually need larger conductors.
4. Should I use copper or aluminum?
Copper carries more current for the same size and has lower resistance. Aluminum is lighter and often cheaper. Use only terminals, lugs, and methods rated for the selected conductor material.
5. What is conductor derating?
Derating reduces usable ampacity when heat cannot escape easily. High ambient temperature and many current carrying conductors in one raceway are common reasons for derating.
6. What length should I enter?
Enter the one way distance from source to load. The calculator applies the correct multiplier for the voltage drop path based on the selected circuit system.
7. Can this calculator replace code tables?
No. It is an estimating and planning tool. Always confirm the final wire size with local rules, equipment labels, terminal temperature ratings, and a qualified electrical professional.
8. What if no wire size passes?
Try a larger conductor range, more parallel runs, shorter length, lower voltage drop target changes, or revised installation conditions. A professional design review may be needed.