This calculator suggests an AWG conductor size that keeps voltage drop within the limit you set. It uses the same DC-resistance method and the same National Bureau of Standards data as the voltage drop calculator.
This is not an ampacity calculation. It does not tell you whether a wire can safely carry the current. It does not include ampacity, temperature derating, bundling, terminal temperature, or conduit fill. After you have a suggested size, you must still check that size against the local codebook for current-carrying capacity.
What this tool does
It answers one question: for this voltage, current, one-way length, metal, and allowed voltage drop, what is the smallest commonly stocked AWG size that stays within the drop limit? The sizes offered are 14, 12, 10, 8, 6, 4, 3, 2, 1, and 1/0 through 4/0 AWG. Odd sizes such as 13, 11, 9, 7, and 5 AWG are not offered, because they are rarely available in the field.
What this tool does not do
- It does not select a wire by ampacity.
- It does not apply National Electrical Code (NEC) tables, and it does not reproduce those tables.
- It does not size conductors for motor starting, short-circuit duty, or voltage drop on the utility side of the service.
- It does not replace a licensed electrician or the local codebook.
If the required area is larger than 4/0, the calculator will not invent a size. It will tell you that the result is outside the range of this first version.
What you enter
- Source voltage
- Actual load current, in amperes
- One-way length, in feet (the path the cable actually takes; do not multiply by 2)
- Single-phase or three-phase
- Copper or aluminum
- Allowed voltage drop, default 3%
What you get
- The required conductor area, in circular mils
- The smallest standard AWG size that meets the voltage-drop limit
- The actual voltage drop with that size
- The next size larger, for comparison, when one is available in the list of sizes this tool offers
The formulas
The voltage-drop formulas are rearranged to solve for the required area. Resistivity is first adjusted from 20 °C to 75 °C, as explained on the voltage drop page.
Single-phase:
CM_required = 2 × I × L × ρ75 / VD_allowed
Three-phase:
CM_required = √3 × I × L × ρ75 / VD_allowed
VD_allowed is in volts (your percentage times the source voltage). √3 is taken as approximately 1.732.
The calculator then looks up the smallest size it offers whose nominal area, from NBS Handbook 100, is at least CM_required. Copper resistivity and the temperature coefficient come from Handbook 100. Aluminum resistivity and the temperature coefficient come from NBS Handbook 109 Table 1, EC-H19. Those constants are listed on the voltage drop how-it-works page and on the About page.
What AWG and circular mils mean
AWG is the American Wire Gage, a standard set of conductor sizes used in the United States. In this system, a smaller gage number is a larger wire. 14 AWG is smaller than 10 AWG; 1/0, 2/0, 3/0, and 4/0 are larger still.
A circular mil is a unit of area used for conductors: the area of a circle with a diameter of one mil (0.001 inch). Handbook 100 publishes a nominal circular-mil area for each AWG size. The calculator uses those areas. It does not invent sizes between standard gages.
After you have a result, check the code
Use the suggested size only as a voltage-drop starting point, then:
- Check the conductor’s ampacity in the codebook that applies at the installation, including any required adjustments.
- Confirm the voltage, the load, and the one-way length you entered.
- If the run is long, the conductor is 2/0 or larger, or the load is a motor, remember that this method leaves out reactance and power factor, so the real drop may be higher.
- Follow manufacturer instructions for terminations, especially on aluminum.
If the size that meets your voltage-drop limit is different from the size required for ampacity, the installation has to satisfy both. This tool only helps with the voltage-drop side.
Disclaimer
Results are for learning and estimation only. They are not a substitute for a licensed electrician, the local electrical code, or manufacturer instructions. This calculator does not determine ampacity. The method is written for U.S. practice and is not directly applicable outside the United States. If you use these results, you do so at your own risk.