The AWG you need for the load and the distance — checked against both ampacity and voltage drop, because on a long run it is almost always the voltage drop that decides.
A
Distance from the panel to the load, not there and back — the return leg is added for you.
%
NEC recommends 3% on a branch circuit and 5% overall. Use 2–3% for DC and solar.
Recommended wire size
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Design current—
Smallest by ampacity—
Smallest by voltage drop—
Ampacity of recommended—
Voltage drop—
Voltage at the load—
Circuit resistance—
Power lost in the cable—
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Reference tool only. Always confirm conductor sizing against the current edition of your local wiring code and have the installation checked by a licensed electrician.
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Two tests, and the bigger one wins
Sizing a conductor means passing two independent checks, then taking whichever demands more copper.
Ampacity — can the wire carry the current without overheating? This comes straight from NEC Table 310.16 and depends on the conductor material and the temperature rating of the terminations at each end. It has nothing to do with distance.
Voltage drop — does enough voltage survive the journey? Resistance is proportional to length, so a wire perfectly adequate at 15 feet can be badly undersized at 150. On long runs this is almost always the binding constraint, and it is the one people forget.
The voltage drop formula
Single-phase: VD = 2 × I × R × L ÷ 1000. Three-phase: VD = 1.732 × I × R × L ÷ 1000, where R is ohms per 1,000 feet from NEC Chapter 9 Table 8 and L is the one-way distance. The 2 in the single-phase version is the current travelling out and back.
Why low-voltage systems need fat cable
Voltage drop matters as a percentage. Losing 3 V is nothing on a 480 V feeder — 0.6% — but on a 12 V battery circuit it is 25% of the supply and the load simply will not work properly. This is why 12 V and 24 V solar and RV wiring uses conductors that look absurdly oversized next to their mains equivalent.
The small-conductor rule
NEC 240.4(D) caps overcurrent protection on small conductors regardless of what the ampacity table allows: 15 A for 14 AWG copper, 20 A for 12 AWG, and 30 A for 10 AWG. Those caps are applied here, which is why 12 AWG copper will not be offered for a 25 A load even though the 90 °C column shows 30 A.
Real installations carry adjustments this tool deliberately leaves out, and every one of them makes the wire bigger, never smaller. Ambient temperature above 30 °C derates ampacity — an attic in summer can cost you 20%. Conduit fill derates it again once more than three current-carrying conductors share a raceway: 80% for four to six, 70% for seven to nine. Termination ratings govern the whole circuit, so a 90 °C cable landing on a 75 °C-rated breaker must be sized from the 75 °C column — which is why 75 °C is the sensible default for most equipment.
When to go one size up anyway
Upsizing is cheap at installation and impossible later. It is worth it when the run is long, when the circuit is near its limit, when there is any chance of adding load later, on solar and battery circuits where every lost volt is lost harvest, and anywhere the cable passes through a hot space.
On a short run, 10 AWG copper at the 75 °C column, which is rated 35 A and capped at 30 A by the small-conductor rule. Over a long distance voltage drop takes over — at 120 V and 100 feet, 30 A in 10 AWG drops about 6%, so you would move up to 8 or 6 AWG.
What size wire for 100 amps?
3 AWG copper or 1 AWG aluminium at 75 °C meets ampacity, though 100 A services are commonly run in 2 AWG copper or 1/0 aluminium for margin and voltage drop. On a long feeder, size up further and check the drop figure above.
How far can I run 12 gauge wire?
At 20 A on a 120 V circuit, roughly 60 feet before exceeding a 3% drop. At 240 V the same wire and current reaches about 120 feet, because the drop is the same in volts but half as much in percentage terms.
Is aluminium wire acceptable?
Yes, and it is standard for larger feeders and service entrances. It carries roughly 60% of copper's ampacity for the same size, so you go up about two sizes. It needs terminations rated for aluminium and an antioxidant compound, and it must never be spliced to copper without a listed connector.
Why does my long run need such thick wire?
Because voltage drop scales with distance while ampacity does not. Resistance accumulates along every foot, so beyond roughly 50–100 feet the voltage drop test starts demanding a larger conductor than the ampacity table does — and it only gets worse at low voltages.