Wire Size Calculator — NEC Ampacity & Voltage Drop
Sizing a conductor means satisfying two separate rules, and the larger answer wins. First, ampacity: NEC Table 310.16 sets how much current a conductor can carry without overheating. Second, voltage drop: over a long run, a conductor that is thermally fine can still starve the load. This calculator runs both, tells you which one governed, and applies the 240.4(D) small-conductor caps and the 110.14(C) termination limit along the way.
Size your conductor
NEC 110.14(C) — use the lowest-rated termination in the circuit, not the wire's insulation rating.
Required conductor
Voltage drop governs — ampacity alone would allow 8 AWG.
Ampacity
65 A
Voltage drop
2.05%
See the breakdown
Planning estimate. Derating for ambient heat or more than three current-carrying conductors is not applied here — run the Ampacity & Derating Calculator for that. A licensed electrician and your AHJ have final say.
The formula, explained in plain English
Conductor sizing is two independent checks run against the same circuit. Here is exactly what this calculator does, in order.
Why two rules?
Ampacity is a heat problem — too much current in too small a conductor melts insulation. Voltage drop is a performance problem — resistance over distance robs the load of voltage. A conductor can pass one and fail the other.
What K actually is
K is resistivity in ohm-circular-mils per foot at roughly 75 °C. Copper's 12.9 versus aluminum's 21.2 is why an aluminum run of the same gauge drops about 64% more voltage.
The 2 and the 1.732
Single-phase current travels out and back, so you count the run length twice. Three-phase uses √3 ≈ 1.732 because the phase currents are 120° apart and partially cancel in the return path.
The 240.4(D) trap
Table 310.16 says 12 AWG copper carries 25 A at 75 °C, but 240.4(D) caps its breaker at 20 A. The table value is real; the usable value is what the small-conductor rule allows.
Worked examples
Three circuits showing ampacity governing, voltage drop governing, and the long-run trap that catches people out.
Kitchen receptacle circuit — 20 A, 40 ft
20 A non-continuous · copper · 40 ft one-way · 120 V single-phase · 75 °C terminations · 3% limit.
ampacity: 12 AWG Cu = 25 A at 75 °C, capped at 20 A by 240.4(D) → 12 AWG
drop = 2 × 12.9 × 20 × 40 ÷ 6,530 = 3.16 V = 2.63%
→ 12 AWG copper — ampacity governs
Result: standard 12 AWG on a 20 A breaker. Both checks pass comfortably, which is why short branch circuits rarely need any thought.
Detached garage feeder — 50 A, 100 ft
50 A non-continuous · copper · 100 ft one-way · 240 V single-phase · 75 °C terminations · 3% limit. These are the calculator's defaults.
ampacity: 8 AWG Cu = 50 A at 75 °C → 8 AWG would do
8 AWG drop = 2 × 12.9 × 50 × 100 ÷ 16,510 = 7.81 V = 3.26% ✗
6 AWG drop = 2 × 12.9 × 50 × 100 ÷ 26,240 = 4.92 V = 2.05% ✓
→ 6 AWG copper — voltage drop governs
Result: 6 AWG, one size up from what the ampacity table alone allows, and 235.1 V arrives at the subpanel instead of 232.2 V.
Long 120 V run to a shed — 20 A, 150 ft
20 A non-continuous · copper · 150 ft one-way · 120 V single-phase · 3% limit.
12 AWG drop = 2 × 12.9 × 20 × 150 ÷ 6,530 = 11.85 V = 9.88% ✗
10 AWG = 7.46 V = 6.21% ✗ · 8 AWG = 4.69 V = 3.91% ✗ · 6 AWG = 2.95 V = 2.46% ✓
→ 6 AWG copper — three sizes up
Result: the same 20 A load needs 6 AWG instead of 12 AWG purely because of distance and low voltage. Doubling the voltage to 240 V would cut the drop percentage in half and bring 10 AWG back into play — which is why long outbuilding runs are almost always fed at 240 V.
Wire size chart by amps
Minimum conductor at the 75 °C column of NEC Table 310.16, with the 240.4(D) small-conductor caps applied. The last column shows what a 100-foot run at 240 V needs to stay inside 3% voltage drop — at 120 V the drop percentage doubles, so those runs need more copper.
| Load | Copper (75 °C) | Aluminum (75 °C) | Copper at 100 ft, 240 V |
|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | 12 AWG upsized |
| 20 A | 12 AWG | 10 AWG | 10 AWG upsized |
| 30 A | 10 AWG | 8 AWG | 8 AWG upsized |
| 40 A | 8 AWG | 8 AWG | 8 AWG |
| 50 A | 8 AWG | 6 AWG | 6 AWG upsized |
| 60 A | 6 AWG | 4 AWG | 6 AWG |
| 70 A | 4 AWG | 3 AWG | 4 AWG |
| 100 A | 3 AWG | 1 AWG | 3 AWG |
| 125 A | 1 AWG | 2/0 AWG | 1 AWG |
| 150 A | 1/0 AWG | 3/0 AWG | 1/0 AWG |
| 200 A | 3/0 AWG | 250 kcmil | 3/0 AWG |
Sources & standards: NEC (NFPA 70) 2023 — Table 310.16 conductor ampacities, 110.14(C) termination temperature limitations, 240.4(D) small-conductor protection, 210.19(A) and 215.2(A) Informational Notes on voltage drop, Chapter 9 Table 8 conductor properties. Local amendments override the model code; confirm the edition your jurisdiction enforces.
Frequently asked questions
Common questions about NEC wire sizing, ampacity, and voltage drop.
What size wire do I need for a 50 amp circuit?
On ampacity alone, 8 AWG copper carries 50 A at the 75 °C column of NEC Table 310.16. But over a 100-foot run at 240 V, 8 AWG drops 3.26% — past the 3% recommendation — so most installers pull 6 AWG. Short runs stay at 8 AWG; long runs go to 6 AWG or larger. The calculator above shows you which of the two rules is governing.
Does voltage drop override the ampacity table?
Neither overrides the other — you satisfy both, so the larger conductor wins. Ampacity is a mandatory NEC requirement that keeps the conductor from overheating. Voltage drop is an engineering recommendation that keeps equipment working properly at the far end of a long run. On short circuits ampacity almost always governs; past roughly 75–100 feet, voltage drop usually takes over.
Is the 3% voltage drop limit actually a code requirement?
No. The 3% branch-circuit and 5% total figures live in Informational Notes to NEC 210.19(A) and 215.2(A), and Informational Notes are explanatory rather than enforceable. They are widely treated as good practice, and many project specifications and local amendments adopt them as mandatory — so the calculator checks against them and tells you when drop is the reason a conductor got upsized. See the Voltage Drop Calculator for the full picture.
Can I use aluminum instead of copper?
Yes, where the equipment terminations are listed for aluminum (look for an AL/CU or AL9CU marking). Aluminum carries roughly two sizes less current than the same copper gauge, and its higher resistivity (K = 21.2 vs 12.9) means more voltage drop over the same run. Aluminum is standard for services and large feeders because of cost; copper dominates branch circuits.
Why is 12 AWG limited to 20 amps when the table says 25?
Because of the small-conductor rule in NEC 240.4(D). Regardless of what Table 310.16 allows, overcurrent protection is capped at 15 A for 14 AWG copper, 20 A for 12 AWG copper, and 30 A for 10 AWG copper (15 A and 25 A for 12 and 10 AWG aluminum). The calculator applies these caps automatically.
Do I use the 60, 75, or 90 °C column?
Use the column matching the lowest-rated termination in the circuit, per NEC 110.14(C) — not the insulation rating printed on the wire. Most breakers, panels, and equipment are listed for 75 °C, which is why 75 °C is the default here. THHN is a 90 °C insulation, but you may only use its 90 °C ampacity for derating math, never as the final usable ampacity. The Ampacity & Derating Calculator walks through that distinction.
Does the neutral count as a current-carrying conductor?
For a normal single-phase two-wire or three-wire circuit, yes for the two-wire case and no for a balanced multiwire branch circuit — NEC 310.15(E) exempts the neutral of a balanced 3-wire circuit from the conductor count. It does count where the load is largely nonlinear or on a 4-wire three-phase system with significant harmonic content. This matters for derating, not for the base ampacity lookup this calculator starts from.
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