Example Data Table
These examples show how length, load, and material can affect conductor choice.
| Load |
Voltage |
Length |
Material |
Max Drop |
Typical Result |
| 20 A |
120 V |
50 ft |
Copper |
3% |
12 AWG or larger |
| 40 A |
240 V |
100 ft |
Copper |
3% |
6 AWG or larger |
| 60 A |
240 V |
150 ft |
Aluminum |
3% |
1/0 AWG or larger |
The calculator first adjusts the load for continuous operation. The design current is:
non-continuous load + continuous load × 1.25.
It then applies ampacity correction:
adjusted ampacity = base ampacity × ambient factor × conductor count factor.
Voltage drop is estimated with circular mil area. For single phase and DC systems:
VD = 2 × K × I × D ÷ CM.
For three phase systems:
VD = √3 × K × I × D ÷ CM.
Copper uses K = 12.9. Aluminum uses K = 21.2. The final conductor is the larger size required
by ampacity and voltage drop.
How to Use This Calculator
- Choose copper or aluminum conductor material.
- Select the temperature column that matches the conductor and termination rules.
- Enter load amps, voltage, one-way length, and system type.
- Enter the continuous load percentage if the load runs for long periods.
- Set the allowed voltage drop percentage.
- Add ambient temperature and current-carrying conductor count.
- Press the calculate button and review the recommended conductor size.
- Download the CSV or PDF report for record keeping.
Why Wire Size Matters
Correct wire size protects equipment and people. A conductor must carry the design current without overheating.
It must also deliver useful voltage to the load. Long runs can lose voltage even when ampacity looks acceptable.
That is why this calculator checks both limits.
Load and Continuous Operation
Many electrical loads do not run all day. Some loads run for long periods. Continuous operation needs extra
margin. This tool lets you enter the percent of load that is continuous. It then applies a stronger design
current to that portion. This gives a more careful result for lighting, heaters, pumps, chargers, and similar loads.
Temperature and Conduit Conditions
Wire ampacity changes when conductors are installed in warmer areas. It also changes when several
current-carrying conductors share a raceway or cable. The calculator includes both correction types. These
values help show why a wire that works in open conditions may fail in a crowded or hot installation.
Voltage Drop Review
Voltage drop is not only a comfort issue. Motors may start poorly. Lights may dim. Electronic devices may
behave badly. A common design target is three percent for branch circuits, but projects may need different
targets. The calculator compares the computed voltage drop with your selected limit.
Professional Verification
This page gives a practical engineering estimate. It does not replace official code review. Real projects
may involve special insulation, raceways, terminals, equipment labels, local amendments, rooftop temperature
rules, motor rules, and overcurrent protection rules. Always confirm the final conductor size before installation.
Frequently Asked Questions
1. What does this wire size calculator estimate?
It estimates conductor size from load current, length, voltage, material, ampacity derating, and voltage drop. It gives a practical starting point for planning.
2. Is voltage drop included?
Yes. The calculator checks voltage drop using conductor circular mil area, material constant, circuit length, load current, and system type.
3. Why is the final size larger than expected?
The result may increase because of continuous load, long distance, high ambient temperature, or many current-carrying conductors in one raceway.
4. Can I use aluminum conductors?
Yes. Choose aluminum in the material field. Aluminum has higher resistance than copper, so larger sizes are often needed for the same load.
5. What is one-way length?
One-way length is the distance from the power source to the load. The calculator handles return path factors inside the voltage drop formula.
6. What temperature column should I select?
Select the column allowed by conductor insulation, equipment terminals, and applicable rules. When unsure, use the more conservative lower temperature column.
7. Does this replace official electrical design?
No. It is an estimating tool. Final installations should be reviewed against local rules, equipment labels, terminal ratings, and professional guidance.
8. Why add CSV and PDF downloads?
Downloads help save the calculation inputs and results. They are useful for estimates, job notes, client reviews, and later checking.