Wire Size Calculator

NEC 2023 Table 310.16 ampacity check plus voltage-drop check using the K-method (Mike Holt / IAEI). Returns the smallest copper or aluminum conductor that satisfies both.

Load & circuit

A
Amps. For watts, divide by voltage (P / V = I).
3 hours or more at full load. NEC 210.19(A)(1)(a).
ft
Source to load — the calculator handles the round-trip factor.
V
Common: 120 / 208 / 240 / 277 / 480 V.
Single-phase uses the 2 × factor; three-phase uses √3.

Conductor

K = 12.9 (Cu) · 21.2 (Al) ohm·CM/ft @ 75 °C.
NEC 110.14(C)(1) requires the column matching the lowest-rated termination.

Voltage drop limit

3 % is NEC 210.19(A)(1) IN4 (branch); 5 % is NEC 215.2(A)(1)(b) IN2 (combined). Both are recommendations, not requirements (NEC 90.5(C)).

Calculation results

Recommended size

— AWG

Actual voltage drop

— V

Adjusted current

— A

Constraint

—

Voltage drop uses the K-method with K = 12.9 (Cu) / 21.2 (Al) at 75 °C. Three-phase uses the line-line factor √3 (assumes power factor = 1).

Informational only. Final wire sizing must be verified by a licensed electrician against the full NEC, including 110.14(C) terminations, 310.15 derating for ambient temperature and conductor count, 240.4 overcurrent protection, and any local amendments. This calculator does not perform short-circuit or arc-flash analysis.

Understanding the formula

Two checks every wire must pass: it must carry the load current without overheating (ampacity, NEC Table 310.16) and it must deliver the load voltage with acceptable drop along the run (voltage drop, NEC informative appendix). The calculator runs both and returns the bigger of the two.

Voltage drop — K-method

Single-phase: VD = (2 × K × I × D) / CM
Three-phase: VD = (√3 × K × I × D) / CM

K is the resistance constant in ohm·circular-mils per foot at 75 °C operating temperature: 12.9 for copper and 21.2 for aluminum / copper-clad aluminum. CM is the conductor circular-mils area. D is the one-way distance — the formula factor (2 for single-phase, √3 for three-phase) handles the geometry. The K-method assumes power factor = 1.

Continuous loads

NEC 210.19(A)(1)(a) and 215.2(A)(1)(a): a continuous load (≥ 3 hours at full load) requires the wire and the breaker to be sized for 1.25 × the load. The calculator multiplies the load by 1.25 when you mark it continuous.

Voltage drop limits

  • Branch — 3 %: NEC 210.19(A)(1) Informational Note 4 — recommendation, not enforceable.
  • Feeder + branch — 5 %: NEC 215.2(A)(1)(b) Informational Note 2 — combined feeder + branch.
  • NEC 647.4(D): 1.5 % branch / 2.5 % combined for sensitive electronics — mandatory.
  • NEC 695.7: stricter limits for fire-pump branches — mandatory.

Worked example

30 A continuous EV charger, 80 ft one-way, 240 V single-phase, copper, 75 °C, 3 % limit:

  • I_adj = 30 × 1.25 = 37.5 A
  • Ampacity at 75 °C: smallest Cu with ≥ 37.5 A is #8 (50 A)
  • VD limit = 240 × 3 % = 7.2 V; CM_min = (2 × 12.9 × 37.5 × 80) / 7.2 = 10,750 CM. Smallest AWG ≥ 10,750 CM is #8 (16,510 CM)
  • Recommend #8 AWG copper; actual VD = (2 × 12.9 × 37.5 × 80) / 16,510 = 4.69 V (1.95 %)

When to use this calculator

Pre-flight check before pulling wire on residential and light-commercial circuits: dedicated appliance branches, EV chargers, well pumps, sub-panels, sub-ground feeders to outbuildings, lighting feeders.

The calculator does not handle:

  • Ambient-temperature derating (NEC 310.15(B)(1)) — assume 30 °C ambient.
  • More than three current-carrying conductors in a raceway (NEC 310.15(C)(1)).
  • Short-circuit current rating, fault duty or arc-flash analysis.
  • Equipment grounding conductor sizing (NEC Table 250.122).
  • Neutral conductor sizing for non-linear loads.
  • Parallel runs (above 500 kcmil at single conductor).

Common mistakes & tips

  • Use one-way distance, not loop length. The formula factor (2 for single-phase, √3 for three-phase) handles the round trip. Entering the loop length doubles the calculated drop.
  • Pick the right temperature column. NEC 110.14(C)(1) requires the column matching the lowest-rated termination. Most residential breakers are 60 °C-rated for circuits ≤ 100 A; modern panels are 75 °C above 100 A.
  • Mind the 240.4(D) small-conductor rule. Even if the 90 °C ampacity says #14 Cu can carry 25 A, the OCPD is capped at 15 A; #12 Cu at 20 A; #10 Cu at 30 A. The calculator flags this in the result note.
  • Aluminum is bigger. Aluminum wire needs ~50 % more circular mils for the same ampacity. The K of 21.2 vs 12.9 means ~64 % more CM for the same voltage drop.
  • Long runs are voltage-drop driven. For runs over 100 ft on small loads, voltage drop usually picks the wire size — not ampacity.
  • Watch sub-panels and feeders. Feeder + branch combined drop is what reaches the load. Use 5 % combined or split between feeder (2 %) and branch (3 %).

Frequently asked questions

Is the 3 % voltage drop limit a code requirement?
No. NEC 210.19(A)(1) Informational Note 4 (branch) and 215.2(A)(1)(b) Informational Note 2 (combined feeder + branch) recommend 3 % and 5 % respectively. NEC 90.5(C) explicitly says Informational Notes are not enforceable. Mandatory voltage-drop rules exist in NEC 647.4(D) (sensitive electronics: 1.5 %/2.5 %) and NEC 695.7 (fire pumps).
Why does the calculator show K = 12.9 for copper instead of an exact resistance?
K = 12.9 ohm·CM/ft is the standard resistance constant for copper at 75 °C operating temperature, derived directly from NEC Chapter 9 Table 8. Mike Holt, IAEI and EC&M all use it. For higher-precision work at non-standard temperatures, use NEC Table 9 values directly.
Can I use this calculator for DC circuits?
Yes — pick single-phase. DC has no phase factor, but the 2× round-trip factor in the single-phase formula gives the same result as a DC voltage drop calculation.
What does NEC 240.4(D) say about small conductors?
It caps the maximum overcurrent protection regardless of ampacity: 15 A breaker for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper. For aluminum: 15 A for 12 AWG, 25 A for 10 AWG. So even though 14 AWG copper is rated 25 A at 90 °C, you can never put it on a breaker bigger than 15 A.
How big should the equipment grounding conductor be?
See NEC Table 250.122. The EGC is sized from the OCPD rating, not the circuit ampacity: 15–20 A breaker → 14 AWG Cu EGC; 30–60 A → 10 AWG Cu; 100 A → 8 AWG Cu. This calculator does not size the EGC.
Is the K-method accurate for low power factor loads?
No. The K-method assumes PF = 1 (resistive). For motor loads, capacitor banks or low-PF lighting, use NEC Table 9 with the impedance method: VD = √3 · I · L · (R · cosθ + X · sinθ). The K-method underestimates voltage drop for low PF.
What temperature column should I use for residential branch circuits?
NEC 110.14(C)(1)(a)(1): for circuits rated 100 A or less and conductors 14–1 AWG, use the 60 °C column UNLESS terminations are rated 75 °C or higher. Most modern breakers and devices are 75 °C-rated; check the markings. The 90 °C column is only used as a starting point for derating, never for direct termination.
Why does aluminum wire need to be larger than copper?
Aluminum has higher resistivity (~64 % more) and lower ampacity (~80 %) than copper. For the same circuit, aluminum typically needs 1–2 sizes larger than copper. The K constant (21.2 vs 12.9) and the lower 310.16 ampacity values reflect this.