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Wire Size Calculator
Use this free wire size calculator to find the smallest wire that keeps voltage drop within your limit. Enter the supply voltage, the current, the one-way length of the run, and the maximum voltage drop you will accept, then pick copper or aluminum — the calculator works out the wire area you need and recommends the smallest standard AWG size that meets it.
Standard wire sizes
| Wire size | Circular mils | Typical use |
|---|---|---|
| 14 AWG | 4,110 | Lighting circuits, 15 A |
| 12 AWG | 6,530 | Outlets and small appliances, 20 A |
| 10 AWG | 10,380 | Water heaters, air conditioners, 30 A |
| 8 AWG | 16,510 | Ranges and sub-panels, 40 A |
| 6 AWG | 26,240 | Large feeders and EV chargers, 55 A |
| 4 AWG | 41,740 | Feeders, 70 A |
| 2 AWG | 66,360 | Feeders, 95 A |
| 1 AWG | 83,690 | Feeders, 110 A |
| 1/0 AWG | 105,600 | Service entrances, 125 A |
| 2/0 AWG | 133,100 | Service entrances, 145 A |
| 3/0 AWG | 167,800 | Service entrances, 165 A |
| 4/0 AWG | 211,600 | Service entrances, 195 A |
How it works
Every wire resists current a little, and that resistance turns some of your voltage into heat before it reaches the load. The longer the run, the heavier the current, and the thinner the wire, the more voltage you lose. A wire size calculator works backwards from the drop you are willing to accept: it finds the smallest wire area that keeps the loss inside your limit, then picks the smallest standard wire size with at least that much area.
The formula is CM = 2 × K × I × L ÷ Vd. CM is the wire's cross-sectional area in circular mils, K is a constant for the wire material — 12.9 for copper and 21.2 for aluminum — I is the current in amps, L is the one-way length in feet, and Vd is the largest voltage drop you will accept, in volts. The factor of 2 is there because current travels out along one conductor and back along the other, so the electricity covers twice the one-way length.
Your drop limit comes from your percentage choice. On a 120-volt circuit with a 3 percent limit, the allowed drop is 3.6 volts (120 × 0.03). On a 240-volt circuit with the same 3 percent limit, the allowed drop is 7.2 volts, which is one reason higher-voltage circuits can run longer distances on the same wire. The usual guideline is 3 percent or less on branch circuits and 5 percent or less for the feeder and branch circuit combined.
Wire size is given in AWG, or American Wire Gauge, where a smaller number means a thicker wire. Each step down in gauge roughly increases the area by about a quarter: 12 AWG has 6,530 circular mils, 10 AWG has 10,380, and 8 AWG has 16,510. The calculator compares the area you need against this standard list and recommends the smallest size that meets it — never a size below it. If you need 14,333 circular mils, 10 AWG at 10,380 is too small, so the answer is 8 AWG at 16,510.
Material matters a lot. Aluminum has a higher resistivity than copper, so its K value is 21.2 instead of 12.9, and the same run needs about 64 percent more wire area in aluminum than in copper. That is why aluminum runs usually need a wire one or two sizes larger. Aluminum is lighter and cheaper, which is why it is common for service entrances and large feeders, but the connections need special care and the right fittings.
Voltage drop is only half the sizing job. The other half is ampacity — the maximum current a wire can safely carry without overheating. A wire can pass the voltage-drop check and still be too small for the current, so always check both. For example, 14 AWG copper is generally limited to 15 amps and 12 AWG to 20 amps for ampacity, no matter how short the run is. This calculator sizes for voltage drop only, and its results are estimates for planning. Confirm the final size with a licensed electrician and your local electrical code before buying wire.
Worked example
A homeowner is running a 120-volt, 20-amp circuit 100 feet to a workshop, using copper wire, and wants no more than 3% voltage drop. What size wire is needed?
Step 1: Find the allowed drop — 120 V × 3% = 3.6 V.
Step 2: Apply the formula — CM = 2 × 12.9 × 20 × 100 ÷ 3.6.
Step 3: Work it out — 2 × 12.9 × 20 × 100 = 51,600, and 51,600 ÷ 3.6 = 14,333 circular mils needed.
Step 4: Pick the smallest standard size that meets it — 10 AWG has only 10,380 CM, so it is too small; 8 AWG has 16,510 CM, so 8 AWG is the answer.
Step 5: Check the real drop — with 8 AWG, Vd = 51,600 ÷ 16,510 = 3.13 V, which is 3.13 ÷ 120 × 100 = 2.60%, inside the 3% limit.
If the same run used aluminum instead, the K value would be 21.2, the area needed would rise to 23,556 circular mils, and the recommendation would jump to 6 AWG at 26,240 circular mils.
FAQ
What is the smallest wire for a 20-amp circuit?
For ampacity, 12 AWG copper is the usual minimum for a 20-amp circuit. But on a long run, voltage drop can demand a larger wire — a 100-foot run at 120 volts needs 8 AWG copper to stay within 3% drop. Always check both ampacity and voltage drop, and use the larger of the two sizes.
Why does the calculator recommend a bigger wire than the ampacity table?
The ampacity table only covers overheating. Voltage drop is a separate limit: a wire can carry the current safely yet lose too much voltage over a long distance. Long runs, heavy loads, and low supply voltages are when drop takes over and pushes the size up.
Is aluminum wire as good as copper?
Aluminum carries current well and costs less, but it has higher resistance, so it needs a larger size for the same voltage drop — about 64% more area. It also needs fittings rated for aluminum and careful installation. For small branch circuits, copper is the usual choice.
What if no standard wire size is big enough?
If the area needed is larger than 4/0 AWG at 211,600 circular mils, a single wire cannot do the job. The usual fixes are a higher supply voltage, a shorter run, a lower current, or parallel runs of wire — all of which are jobs for a licensed electrician to design.