Pick a calculation mode, set the circuit parameters, and click Calculate. A wire must clear both the NEC ampacity check and the voltage-drop limit to pass.
Percent voltage drop for your circuit across neighboring conductor sizes. Your selected (or recommended) size is highlighted in gold; the dashed line marks the NEC 3% branch-circuit recommendation.
Results are engineering estimates based on published NEC Table 310.16 general ampacity (30°C ambient, ≤3 current-carrying conductors) and conductor resistance and reactance at the selected operating temperature and conduit type. Actual ampacity and voltage drop vary with installation conditions. Verify with a licensed electrician before any electrical work.
NEC Table 310.16 copper ampacity (60/75/90°C) and AWG↔mm² equivalent cross-sectional area, for quick lookup without re-entering numbers.
| AWG / kcmil | mm² Equiv. | 60°C (Cu) | 75°C (Cu) | 90°C (Cu) |
|---|
Copper, general ampacity per NEC Table 310.16, 30°C ambient, not more than 3 current-carrying conductors. Verify with a licensed electrician for your actual installation.
Cord & Signal Wire Reference (16 AWG–40 AWG) — these gauges fall outside NEC Table 310.16 (fixed branch-circuit wiring), which stops at 14 AWG. Flexible cord ampacity is governed by NEC Table 400.5 instead, and varies by cord construction (S, SJ, SJO, SO, etc.) — the calculator above does not use this table and does not accept these sizes as inputs. Geometry is standard AWG; check the cord's jacket marking or manufacturer spec for its actual ampacity rating.
| AWG | Diameter | Area | Typical Use |
|---|
Reference only — not part of this calculator's NEC 310.16-based ampacity or voltage-drop engine. Not a substitute for the cord's actual NEC Table 400.5 rating or manufacturer spec.
Three real-world scenarios, worked step by step. Click "Load This Example" to run it in the calculator above.
Two independent checks, both must pass
Choosing a wire size means clearing two unrelated limits: ampacity (can the conductor carry the current without overheating, per NEC Table 310.16) and voltage drop (does the run's length cause too much voltage loss for equipment to work properly, per NEC 210.19/215.2). A wire can pass one check and fail the other — long, lightly loaded runs are usually voltage-drop-limited, while short, heavily loaded runs are usually ampacity-limited. This calculator checks both and recommends the smaller conductor that satisfies whichever constraint is tighter.
Continuous vs. standard loads
NEC 210.19(A)(1) defines a continuous load as one expected to run at maximum current for three hours or more — think EV chargers, water heaters, or commercial lighting left on for a full shift. Continuous loads must be sized at 125% of their current for ampacity purposes, which effectively pushes many circuits up one wire size compared to an intermittent load of the same nameplate current.
Combining parallel conductors
Running multiple smaller conductors in parallel instead of one large conductor is permitted by NEC 310.10(H) for 1/0 AWG and larger, provided every parallel set matches in length, material, size, insulation, and terminations. Combined ampacity is the sum of each conductor's individual rating, and the combined cross-sectional area corresponds to an equivalent single conductor size — useful for comparing a paralleled feeder against a single large conductor on cost, weight, or conduit-fill grounds.
AWG and mm² are the same idea, different units
Both systems describe a conductor's cross-sectional area — AWG numerically (lower number, thicker wire) and mm² directly. Switching the Standard toggle here converts your current selection to its closest cross-sectional-area match in the other system, the same conversion published in wire manufacturer datasheets.