Wire Size Calculator

Two limits decide the wire size

Wire gauge is decided by two limits that pull in opposite directions. The first is ampacity: how much current the metal can carry without cooking its insulation. The second is voltage drop: how much of the supply the run eats before it reaches the load. A short run is a formality on the second count. A long run is where gauge choice turns into money.

Inside about 50 feet, a load fits the gauge its breaker demands: 12 AWG copper on a 20 amp circuit. Past that the wire gets upsized on its own, because resistance adds up over distance. At 100 feet a 20 amp load on 240 volts has already lost 5.12 percent on 14 AWG.

The OSHA wiring design rules in 1910.304 set the safety floor for workplace conductors, and the National Electrical Code from NFPA sets the ampacity tables and small conductor rules most inspectors work from. This calculator takes your load, run length and supply voltage, then reports the smallest copper gauge that carries the current and holds the drop under your target.

The voltage drop formula

The whole calculation is one line for a single phase two wire circuit:

VD = 2 x L x I x R / 1000

Four pieces feed it. L is the one-way length in feet, so a panel 100 feet from the garage counts as 100, not 200. I is the load current in amps. R is the resistance in ohms per 1,000 feet, taken from a table. The 2 covers the trip out and back: current leaves on the hot leg and returns on the neutral, so the wire is walked twice.

Voltage drop as a percentage is the drop divided by the supply voltage times 100. A run that loses 7.72 volts on 240 volts is a 3.22 percent drop, and the same 7.72 volts on 120 volts is 6.43 percent. That is why a 240 volt run of the same length and load is easier on the wire.

Three phase circuits replace the 2 with 1.732, because the return path is shared with the other two phases instead of doubling a single conductor: VD = 1.732 x L x I x R / 1000.

Copper resistance and ampacity per size

The R value comes from the conductor properties table in Chapter 9 of the National Electrical Code, and the numbers below are the solid uncoated copper column at 75 degrees C (167 degrees F). Stranded conductors sit slightly higher: 14 AWG stranded is 3.14 ohms per 1,000 feet against 3.07 for solid. The National Bureau of Standards copper wire tables carry the same data, and the DOE electrical distribution handbook covers the wiring side for equipment rooms.

The ampacity column is the 60 degree C line of NEC Table 310.16 for copper. For 14, 12 and 10 AWG it matches the small conductor caps in NEC 240.4(D): 15 A, 20 A and 30 A, whatever insulation column you quote.

Aluminum runs higher at the same gauge: 12 AWG aluminum is 3.18 ohms per 1,000 feet against 1.93 for copper, which is why an aluminum circuit lands two sizes larger for the same load and run. This calculator works in copper.

Worked example: 20 amps over 100 feet

Default case: 20 amps, 100 feet from the panel, 240 volts, with a 3 percent drop target.

Step 1, check current. 20 amps needs 12 AWG copper or heavier. 14 AWG is rated 15 amps, so it is out before the voltage math starts.

Step 2, voltage drop on 12 AWG. VD = 2 x 100 x 20 x 1.93 / 1000 = 7.72 volts. As a percentage of 240 volts that is 3.22 percent, which misses the 3 percent goal by a hair.

Step 3, step up to 10 AWG. VD = 2 x 100 x 20 x 1.21 / 1000 = 4.84 volts, which is 2.02 percent of 240 volts. The wire carries 30 amps, so the current is comfortable.

The answer is 10 AWG copper. Here is the full ladder:

Read the two failure columns separately. 12 AWG passes the current test and fails the voltage test, and that is why the answer moves up one size. NEC guidance suggests keeping drop under 3 percent on a branch circuit and under 5 percent for the feeder and branch together, and the smallest legal wire lands just over the branch target.

Worked example: 15 amps over 50 feet

Same job, shorter and lower voltage: a 15 amp load, 50 feet away, on 120 volts.

Step 1, current. 15 amps is inside the 15 amp rating of 14 AWG copper, so 14 AWG is allowed.

Step 2, voltage drop on 14 AWG. VD = 2 x 50 x 15 x 3.07 / 1000 = 4.61 volts. On a 120 volt supply that is 3.84 percent, which is over the 3 percent goal.

Step 3, try 12 AWG. VD = 2 x 50 x 15 x 1.93 / 1000 = 2.90 volts, which is 2.41 percent of 120 volts, inside the target. The load draws 15 amps and 12 AWG is rated for 20, so 12 AWG is the answer.

Notice how much harder the 120 volt side works. The same 15 amps over the same 50 feet of 14 AWG costs 4.61 volts either way, but that is 3.84 percent of 120 volts and only 1.92 percent of 240. Move the load to 240 volts and 14 AWG clears the target, with 10 AWG down at 0.76 percent.

Three phase runs use 1.732 instead of 2

Three phase changes the constant, not the logic. Current leaves on one phase and comes back through the other two, so the effective length walked through the conductor is 1.732 times the one-way run instead of 2.

VD = 1.732 x L x I x R / 1000

Worked case: a 30 amp three phase load, 150 feet from the disconnect, on 480 volts.

On 10 AWG: VD = 1.732 x 150 x 30 x 1.21 / 1000 = 9.43 volts, which is 1.96 percent of 480 volts.

On 8 AWG: VD = 1.732 x 150 x 30 x 0.764 / 1000 = 5.95 volts, which is 1.24 percent of 480 volts.

Compare a single phase 30 amp run of the same length on 240 volts: on 10 AWG the drop is 2 x 150 x 30 x 1.21 / 1000 = 10.89 volts, or 4.54 percent, which needs 8 AWG to come back under 3 percent. Three phase gives a shorter effective path and a higher line voltage, and the two together cut the drop by more than half.

Watch the percentage, not just the volts. Nine point four three volts sounds small until you divide by 480 and see the equipment running at 470.57 volts instead of 480.

How far you can run each gauge

Working backwards from your drop limit gives a planning table. For the 2 x L x I x R / 1000 form, the longest run at 3 percent is L = 0.03 x V x 1000 / (2 x I x R). Here it is for two common residential cases:

Each step up in gauge buys about 60 percent more distance. That is why 8 AWG shows up in so many detached garage feeder conversations: on a 240 volt, 20 amp run it stretches to 236 ft, the first size that clears 150 ft under a 3 percent drop.

Two cautions. The 14 AWG row for 20 A is listed for voltage drop only, since 15 A of ampacity rules that pair out at any distance. And a feeder plus its branch circuits share the 5 percent budget, which is tighter than it looks on paper.

What the calculator does not cover

The arithmetic is the easy part. Four checks sit on top of it, and a calculator handles none of them:

• Small conductor caps. NEC 240.4(D) limits overcurrent protection for 14, 12 and 10 AWG copper to 15 A, 20 A and 30 A. A 20 amp breaker on 14 AWG is not allowed, whatever the voltage drop says.
• Continuous load. A load that runs three hours or more is continuous under NEC 210.20, and the conductor and overcurrent device are sized at 125 percent of it. A 20 amp continuous load needs a 25 amp circuit.
• Temperature and bundling. Table 310.16 assumes 30 degrees C ambient and three or fewer current-carrying conductors. An attic in July, or six current-carrying conductors in one conduit, pushes the allowed current down.
• Termination ratings. Breakers and receptacles are marked 60 C or 75 C, and the conductor meets the lower mark. A 90 degree C conductor still gets limited by a 60 C device.

Field work has its own rules too: OSHA 1910.304 covers conductor identification, branch circuit construction and grounding in workplaces. And if the load is a motor, a heat pump or anything with a control board, the equipment instructions set their own minimum wire size, and that beats any table here.