Wire Size Calculator

Please fill in the required fields with values greater than 0.
–minimum wire size
–voltage drop (volts)
–voltage drop (%)
–wire ampacity (amps)

Educational estimate for single-phase circuits using the 60 C column of NEC ampacity values. It does not replace local electrical code. Have a licensed electrician confirm.

How to use this calculator

Enter the load in amps, the voltage, the one-way distance and the wire material. Choose the maximum voltage drop you accept, usually 3 percent. Press Calculate to see the smallest wire size that is safe for both current and voltage drop.

The formula

Voltage drop = 2 × length × current × resistance per 1,000 ft ÷ 1,000
Voltage drop % = voltage drop ÷ system voltage × 100
The wire must also be rated for at least the load current.

Example: A 20 A load at 120 V, 100 ft away on copper wire needs 12 AWG for current, but the voltage drop would be 6.6%. To stay under 3% you need 8 AWG, which gives a drop of 3.11 V (2.59%).

Understanding your wire size results

The wire size calculator checks two things at once. First, the wire must be rated to carry the load current safely. Second, the wire must be thick enough that voltage drop stays within your limit over the run. The minimum wire size is the smallest gauge that passes both tests. Remember that a smaller AWG number means a thicker wire.

What changes the wire size

  • Current. More amps need thicker wire, both for heat and for voltage drop.
  • Length. Voltage drop grows with distance. Long runs often need a larger wire than ampacity alone suggests.
  • Voltage. The same drop in volts is a smaller percentage at 240 volts than at 120 volts.
  • Material. Aluminum has more resistance than copper, so it must be larger for the same job.

An example of voltage drop at work

Take a 20 amp, 120 volt circuit running 100 feet one way in copper. 12 AWG handles 20 amps on a short run, but over 100 feet it drops about 6.6%, more than double a 3% target. The calculator steps up to 8 AWG, which drops about 3.11 volts, or 2.59%.

Now run the same 20 amp load at 240 volts. The 12 AWG wire still loses about 7.9 volts, but that is only about 3.3% of 240 volts. Higher voltage is one reason long runs to sheds and well pumps often use 240 volt circuits.

Tips and common mistakes

  • Enter one-way length. The tool already doubles it for the return path.
  • Match the breaker to the wire. A wire must be protected by a breaker no larger than its rating allows.
  • Check terminals. Devices and breakers have their own temperature ratings that can limit ampacity.
  • Do not forget derating. Bundled cables, hot attics and conduit fill can lower a wire’s allowed current.

Planning and safety

Larger wire costs more per foot and can be harder to fit on device terminals, so plan the run before you buy. Bring the calculated gauge and length to the supply house and buy a little extra for bends and connections. This tool is an educational estimate. Follow your local electrical code, pull any required permit and have a licensed electrician design or check the circuit.

Frequently asked questions

What wire size do I need for 20 amps?
12 AWG copper is rated for 20 amps on short runs. Longer runs need larger wire to limit voltage drop.
What is voltage drop?
It is the voltage lost along the wire. Too much voltage drop makes lights dim and motors run hot.
How much voltage drop is acceptable?
A common guide is 3 percent for a branch circuit and 5 percent in total from the panel to the load.
Is aluminum wire as good as copper?
Aluminum is cheaper and lighter but carries less current for the same size, so it must be larger than copper.
Can I use this to wire my house?
It is only a guide. Always follow your local electrical code and have a licensed electrician check the work.
What wire size do I need for a 30 amp circuit?
For short runs, 10 AWG copper is the common choice for a 30 amp circuit, such as a typical electric dryer. Longer runs may need 8 AWG or larger to keep voltage drop in check. Aluminum needs a larger size. Confirm the exact size with your local code and a licensed electrician.
What does AWG mean?
AWG stands for American Wire Gauge, the US standard for wire sizes. The numbers run backward: a lower number means a thicker wire that can carry more current. For example, 10 AWG is thicker than 12 AWG, and 12 AWG is thicker than 14 AWG.
Can I use a bigger wire than needed?
Yes. A larger wire lowers voltage drop and runs cooler, and it is allowed as long as it fits the terminals and boxes. You cannot raise the breaker size just because the wire is bigger, unless the circuit design allows it. The main downsides are higher cost and harder installation.