Enter circuit details
Formula used
Required ampacity = (Continuous amps × 1.25 + Non-continuous amps) × (1 + Safety margin ÷ 100).
Adjusted ampacity = Base ampacity × Ambient correction factor × Conductor count adjustment factor.
Usable ampacity = Lower value of adjusted ampacity and terminal temperature ampacity.
Single phase or DC voltage drop = 2 × Current × Resistance × One-way length ÷ 1000.
Three phase voltage drop = √3 × Current × Resistance × One-way length ÷ 1000.
Voltage drop percent = Voltage drop ÷ Circuit voltage × 100.
This calculator uses common ampacity planning values. Local code, equipment labels, conductor type, and installation conditions control real work.
How to use this calculator
- Enter the continuous and non-continuous amperage values.
- Select circuit voltage, phase type, and power factor.
- Choose copper or aluminum conductor material.
- Select insulation and terminal temperature ratings.
- Enter ambient temperature and conductor count for derating.
- Add the one-way run length and allowed voltage drop.
- Press calculate to see the first passing wire size.
- Review the detailed table before choosing a final conductor.
Example data table
| Example | Load | Voltage | Material | Length | Goal |
|---|---|---|---|---|---|
| EV branch circuit | 32 A continuous | 240 V | Copper | 80 ft | 3% drop |
| Garden pump | 18 A non-continuous | 120 V | Copper | 140 ft | 5% drop |
| Shop feeder | 48 A continuous | 240 V | Aluminum | 100 ft | 3% drop |
| Three phase motor | 40 A non-continuous | 480 V | Copper | 160 ft | 3% drop |
Amp to Wire Size Guide
Why wire size matters
Wire size is a safety decision. A conductor must carry the load current without overheating. It also needs enough area to limit voltage drop over the run. A small wire can waste energy. It can also create heat at terminals, splices, and devices. This calculator helps compare these limits in one view.
Ampacity and load type
Ampacity means the safe current a conductor can carry. Continuous loads are handled more carefully. Many codes treat a continuous load as one that runs for three hours or more. The calculator adds twenty five percent to that part of the load. It then adds the non continuous current. The result is the minimum design ampacity before optional safety margin.
Material choices
Copper and aluminum behave differently. Copper carries more current in a smaller size. Aluminum is lighter and often costs less. It usually needs a larger size for the same amperage. Terminals must also be listed for the conductor material. Always use the correct antioxidant compound, lugs, and torque values when required.
Temperature ratings
Insulation temperature affects the base ampacity. Terminals can limit the final value. A wire may have high temperature insulation, but the device lug may allow a lower temperature column. This is why the calculator checks both conductor rating and termination rating. Ambient heat and bundled conductors may reduce the usable ampacity.
Voltage drop
Voltage drop is the lost voltage caused by wire resistance. Long runs need more attention. A circuit can be safe by ampacity and still perform poorly. Motors may start slowly. Lights may dim. Electronics may reset. The calculator estimates drop for direct current, single phase, and three phase circuits. The result helps select a practical size.
Derating factors
Derating adjusts ampacity for heat and conductor count. More current carrying conductors trap heat together. Higher ambient temperature also reduces cooling. The calculator applies common correction and adjustment factors. These estimates are useful for planning. Final installations should be checked against local rules and equipment labels.
Using the result
The recommended size is the first size that passes ampacity and voltage drop. The table also shows nearby wire sizes. This helps you compare cost, space, and performance. Larger wire may reduce energy loss. Smaller wire may fail one or more checks. Use the notes to understand why a size passed or failed.
Practical safety notes
Wire sizing is not only a math task. Breaker size, insulation type, raceway fill, equipment terminals, and environment all matter. Motors, welders, chargers, and heaters may have special rules. Underground and wet locations may also require specific cable types. Treat the result as a planning aid. Confirm important work with a licensed electrician.
Planning examples
A kitchen circuit may be short. A garden pump may be far away. The same amperage can need different wire sizes. Distance and voltage both change the final choice.
FAQs
What does an amp to wire size calculator do?
It estimates the smallest conductor size that can carry a load current. It also checks voltage drop and common derating factors. The result helps with planning before final code review.
Why is continuous load multiplied by 125%?
Continuous loads can heat conductors for long periods. A larger design ampacity gives extra thermal margin. This calculator applies that multiplier to the continuous portion only.
Does voltage affect wire size?
Voltage affects the voltage drop percentage. The same wire drop is a larger percentage on low voltage circuits. Long 12 volt or 120 volt runs often need larger conductors.
Is copper better than aluminum?
Copper usually carries more current in a smaller size. Aluminum is lighter and often cheaper. Aluminum must use compatible terminals and correct installation practices.
What is voltage drop?
Voltage drop is voltage lost as current moves through wire resistance. Excessive drop can reduce equipment performance. Larger conductors lower resistance and reduce that loss.
What conductor count should I enter?
Enter the number of current-carrying conductors grouped together. Equipment grounding conductors are usually not counted. Some neutrals may or may not count, depending on the circuit.
What temperature rating should I choose?
Choose the insulation temperature rating for the conductor. Also match the terminal rating to the connected equipment. The lower limit can control the final ampacity.
Can this calculator size motor circuits?
It can estimate a starting point. Motors may need special rules for overloads, starting current, conductor sizing, and short circuit protection. Verify motor circuits separately.
Why did a larger wire pass when ampacity was already enough?
The smaller wire may have failed the voltage drop limit. Long runs often need larger conductors even when ampacity is acceptable.
Can I use the result for final installation?
Use it for planning only. Final work must follow local codes, equipment labels, raceway fill, breaker rules, and inspection requirements. Consult a qualified electrician for critical work.
Why is breaker size included?
The breaker field helps flag a possible mismatch. A breaker should generally protect the conductor and connected equipment. Special loads can have different rules.