What Size Wire Do You Need? Ampacity, Voltage Drop and AWG

Two separate limits, and the larger answer wins

Ampacity is about heat: how much current the conductor can carry without exceeding its insulation's temperature rating. Voltage drop is about performance: how much of the supply voltage is lost in the wire before it reaches the load. The NEC ampacity tables ignore distance — an informational note says they do not take voltage drop into account — so a wire can pass on ampacity and still deliver low voltage at the end of a long run.

The JoistCalc electrical calculator has one tab for each question: Wire & Breaker Size gives the ampacity minimum, and Voltage Drop tells you what a chosen gauge loses over your run. Use both, in that order, and keep whichever gauge is larger.

Why long runs need a bigger wire

The calculator's voltage-drop formula is VD = (K × multiplier × I × D) ÷ CM. K is the conductor's resistance constant (12.9 ohm·cmil/ft for copper, 21.2 for aluminum), the multiplier is 2 for single-phase because current travels out and back, I is the load current, D is the one-way length in feet, and CM is the conductor's cross-section in circular mils. Length sits on top of the fraction, so doubling the run doubles the drop: a 20 A load at 120 V on 12 AWG copper loses 3.29% over 50 ft and 6.58% over 100 ft. Circular mils sit underneath, so the only lever that works against distance is a larger conductor — or less current.

That is also why 240 V circuits tolerate distance better than 120 V ones. The same 1,800 W load draws 15 A at 120 V but 7.5 A at 240 V. Over 100 ft of 12 AWG copper, the 120 V version loses 5.93 V, or 4.94%; the 240 V version loses 2.96 V, or 1.23%. Half the current gives half the volts lost, and dividing by twice the voltage halves the percentage again.

Worked example: 30 A at 240 V over 100 feet, copper

Common circuits at 50, 100 and 150 feet

Every cell below was produced with the calculator's own constants. Inputs: copper, single-phase, load current equal to the breaker rating (a worst case), non-continuous load, one-way run length as shown. The second column is the Wire & Breaker Size result; each distance column gives the smallest gauge that keeps voltage drop at or below 3%, with the ampacity-minimum gauge's drop in parentheses where it fails.

At full rated current, the 120 V circuits pass 3% within 50 ft on their ampacity-minimum wire, while the 240 V rows hold out to roughly 100 ft (the 30 A row just misses there, at 3.11%). Real general-purpose circuits rarely draw their full breaker rating for long, so treat this table as the pessimistic end and run your own load through the calculator. Upsizing the wire does not change the breaker, and a much larger conductor may need terminals and devices rated to accept it.

What gauge wire for a 50 A subpanel?

Enter Amps: 50 in the Wire & Breaker Size tab and the calculator returns 6 AWG copper (55 A in the 60°C column) on a 50 A breaker. Then check the run in the Voltage Drop tab at 50 A, 240 V, single-phase: 6 AWG drops 2.05% (4.92 V) at 100 ft and 3.07% (7.37 V) at 150 ft, where 4 AWG brings it back to 1.93% (4.64 V). At 200 ft, 6 AWG drops 4.1% and 4 AWG 2.58%. Three cautions. A subpanel is fed by a feeder, and its branch circuits add their own drop on top, so the feeder should use less than the whole budget (see the next section). The 6 AWG answer comes from the 60°C column; where the wiring method and every termination are rated higher, the code can permit a different size — the electrician's call. And a feeder also needs a neutral and a grounding conductor, sized by rules this calculator does not cover.

The 3% and 5% figures are guidance, not a code requirement

The figures everyone quotes — no more than 3% drop on a branch circuit, and no more than 5% combined across the feeder and branch circuit to the farthest outlet — come from informational notes in the NEC's branch-circuit and feeder articles (210.19 and 215.4(A)(2) in the 2026 edition), with a cross-reference from the ampacity section. The NEC classes informational notes as explanatory material, not enforceable requirements, and the note numbers have moved between editions. Two qualifications: Article 647, for sensitive electronic equipment, does set mandatory voltage-drop limits, tighter than the notes; and your local authority decides which edition applies, may amend it, or may adopt another code with its own limit, so ask the inspector. "Not mandatory" is not "safe to ignore" — motors and heaters perform worse on low voltage, and 3% remains a sensible design target.

Copper vs. aluminum

Aluminum conducts less well than copper of the same size. In the calculator, its resistance constant is 21.2 against copper's 12.9 — about 1.64 times as much — so every voltage-drop figure is about 1.64 times larger for the same gauge. Rerunning the worked example in aluminum (30 A, 240 V, 100 ft, single-phase): 8 AWG drops 3.21% instead of copper's 1.95%, and 6 AWG aluminum drops 2.02%. The Wire & Breaker Size tab, however, is copper only. Aluminum has its own, lower ampacity column in the NEC tables and needs terminations listed for aluminum, so size its ampacity from the code table or with your electrician, then use the calculator to check its voltage drop.

What the calculator does not cover

Each of these can change the answer, almost always toward a larger wire. The calculator applies none of them, so this page publishes no numbers for them.

• Ambient temperature correction. A conductor in a hot attic carries less current than the table value.
• Bundling and conduit fill. More than three current-carrying conductors together must be derated.
• Termination ratings. The lowest-rated breaker, lug or device in the circuit limits which ampacity column applies.
• Aluminum ampacity. Voltage drop is modeled for aluminum; ampacity is not.
• Motors, air conditioners, welders and EV equipment. These follow their own NEC articles and the equipment nameplate.
• Reactance and conductor temperature. The formula uses resistance only; on large conductors AC reactance adds to the drop, and a hotter conductor resists more.

Where this goes wrong

• Stopping at the ampacity answer. Ampacity tables ignore distance; check voltage drop before buying wire.
• Entering the round-trip length. The calculator wants the one-way distance and doubles it itself.
• Measuring as the crow flies. Use the path the cable will actually follow, up walls and around obstructions.
• Forgetting the feeder. A branch circuit from a distant subpanel inherits the feeder's drop.
• Upsizing the breaker with the wire. A conductor enlarged for voltage drop does not justify a larger breaker.
• Using a copper answer for aluminum. Different resistance, ampacity and terminations.

Estimates only, for planning purposes. Always verify against the current NEC, your local code amendments, and a licensed electrician before any installation.