How this instrument works
Most wire-size tools quietly assume a voltage — 12 V for an accessory circuit, 120 V for a household receptacle, 240 V for an appliance — because most real jobs sit on one of those. This instrument does not assume: System voltage is a field, not a constant, because the same 3% budget means a different number of volts depending on what it's a percentage of. A 208 V panelboard leg, a 277 V commercial lighting circuit, and a 48 V telecom battery plant all use the identical formula and the identical K constant; only the supply voltage entered changes, and the arithmetic follows wherever that number leads.
The formula itself comes straight from Ohm's law rearranged around wire geometry. Resistance runs as K times length over cross-sectional area, R = K·L ÷ CM, with K expressed in ohms per circular-mil-foot — the DC resistance of a conductor one circular mil in section and one foot long, a figure the National Electrical Code itself tabulates for copper in Chapter 9, Table 8. Solve that relation for CM against a target voltage drop, double the length because current leaves along one conductor and returns along another, and the result is the minimum copper cross-section a given current, distance, and volt budget will tolerate — no conversion to square inches or millimetres required, since circular mils are what the AWG tables and the K constant already speak.
One boundary worth knowing: the factor of 2 in this form is specifically a single-phase answer, current out one conductor and back another. A three-phase circuit shares the load across three conductors differently, and its equivalent formula replaces that 2 with √3, about 1.732 — feed a three-phase feeder's numbers through this single-phase form and the tool overstates the copper needed by roughly 15%. The percentage target itself is also softer than it looks: NEC 210.19(A) carries the familiar 3% branch-circuit figure as an Informational Note, and the code says plainly that informational notes are not enforceable requirements, though some inspection authorities adopt them as binding local amendments anyway.
- Enter Circuit current — the steady load in amps the circuit will actually carry, not a breaker's trip rating.
- Enter One-way wire run length, ft — the distance from source to load in one direction; the formula doubles it internally for the return conductor.
- Set System voltage to the circuit's actual supply — 120 V, 208 V, 277 V, 400 V, or any figure the installation uses.
- Set Allowed voltage drop, % to the target for this circuit — 3% for a single branch circuit is the common starting point.
- Read Allowed voltage drop for the volt budget that percentage becomes, and Minimum wire size, circular mils for the copper cross-section needed to stay inside it.
Worked example — a 30 A, 120 V run to an RV pedestal
A homeowner is wiring a 30 A, 120 V shore-power pedestal at the edge of the driveway, 50 ft one-way from the main panel, and wants to hold the loss to the common 3% branch-circuit target. The volt budget comes first: VD = 120 × 3 ÷ 100 = 3.6 V, the most the run may lose before the pedestal reads meaningfully under 120 V. Then the cross-section: CM = 2 × 12.9 × 30 × 50 ÷ 3.6. Working it through, 2 × 12.9 is 25.8, times 30 A is 774, times 50 ft is 38,700, and 38,700 divided by 3.6 V lands on exactly 10,750 circular mils.
Standard 10 AWG copper, ampacity-rated for a 30 A circuit and the obvious pick on a short run, measures only about 10,380 circular mils — just short of the 10,750 this particular run demands, so the correct choice steps up to 8 AWG, about 16,510 circular mils. Shorten the run to 20 ft instead and the same load needs only about 4,300 circular mils, well inside 10 AWG's rating; it is the extra 30 ft of resistive path, not the current, that pushes this specific circuit past what its ampacity rating alone would suggest.
Questions
Why does this calculator ask for system voltage instead of assuming one?
Because the volt budget a drop percentage produces is a fixed fraction of whatever the supply actually is, and that supply varies enormously across real circuits — a 12 V accessory run, a 120 V receptacle, a 208 V three-phase leg, and a 400 V industrial feeder each turn 3% into a completely different number of volts. Fixing the formula to one voltage would make it wrong for every circuit that isn't that voltage, so this instrument leaves system voltage open.
Is the 3% voltage drop figure a hard NEC requirement?
No. NEC 210.19(A) carries the 3% branch-circuit figure as an Informational Note, and Section 90.5 of the code states that informational notes are informational only and not enforceable as requirements. Some inspection authorities adopt the 3%/5% figures as a mandatory local amendment, but the National Electrical Code itself treats them as sound practice for operating efficiency, not a pass or fail line — check the local jurisdiction before treating 3% as the only acceptable answer.
Does the factor of 2 in the formula still apply to a three-phase circuit?
No — that 2 specifically models a single-phase circuit's outbound-and-return path through two conductors. A three-phase circuit replaces it with 1.732, the square root of 3, reflecting how three phase conductors share the current. Running a three-phase feeder's numbers through this single-phase form overstates the required copper by roughly 15%; a 208 V or 480 V three-phase run needs the three-phase version of the formula instead.
What allowed voltage drop percentage should I enter if none is specified?
3% is the standard starting point for a single branch circuit reaching its farthest outlet, and it is what most electricians default to absent other guidance. For a feeder that also has branch circuits hanging off it, a common split is roughly 2% on the feeder and 3% on the branch so the combined total stays near 5%. Sensitive electronics and long agricultural or dock runs often call for a tighter 1-2% instead.
In the worked example, why does the answer need 8 AWG instead of 10 AWG?
Because two different rules were both in play, and the tighter one won. Standard 10 AWG copper is ampacity-rated for a 30 A load and would be the normal pick on a short run, but its actual cross-section, about 10,380 circular mils, falls just short of the 10,750 circular mils this specific 50 ft, 3% run requires. Stepping up to 8 AWG, about 16,510 circular mils, clears the voltage-drop floor as well as the ampacity table.