Wire Size & Voltage Drop Calculator

Last updated Oct 2026Code minimum

Enter the load, one-way distance and voltage. Each wire size is checked against NEC ampacity and the voltage-drop limit you pick. The smallest size that passes both is the answer.

Worksheet

Check with your local authority: Codes and regulations vary by state, county and city and are amended over time. Confirm any code-related result with your local building or health department (the authority having jurisdiction) or a licensed professional before you buy, install or permit anything.

How it works

Voltage drop

Single-phase: VD = 2 × K × I × L ÷ CM
Three-phase: VD = 1.732 × K × I × L ÷ CM
VD% = VD ÷ system voltage × 100

K is the resistance of a circular-mil-foot of conductor: about 12.9 Ω for copper and 21.2 Ω for aluminum. I is the load in amps, L the one-way length in feet, and CM the conductor area in circular mils from NEC Chapter 9, Table 8 (for example, 12 AWG = 6,530 CM and 10 AWG = 10,380 CM).

The NEC recommends, in an informational note (not a mandatory rule), a maximum of 3% drop on a branch circuit and 5% total for feeder plus branch circuit.

Ampacity

The wire must also carry the current safely. The calculator uses NEC Table 310.16 (not more than three current-carrying conductors, 30°C ambient) in the column you choose, and caps small conductors at the 240.4(D) overcurrent limits: copper 14/12/10 AWG at 15/20/30 A, aluminum 12/10 AWG at 15/25 A.

Worked example

A 20 A, 120 V single-phase load 100 ft away on copper:

12 AWG: VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V = 6.58% (fails)
10 AWG: 51,600 ÷ 10,380 = 4.97 V = 4.14% (fails)
8 AWG: 51,600 ÷ 16,510 = 3.13 V = 2.60% (passes)

12 AWG passes ampacity (20 A) but drops too much voltage, so 8 AWG copper is the smallest size that meets both ampacity and the 3% target. (The breaker still protects the circuit at 20 A.)

Assumptions and limits

Uses DC resistance at 75°C from Chapter 9 Table 8, which is accurate within a few percent for AC up to about 4/0. Larger conductors and power-factor effects need Table 9.
No ambient-temperature correction, conduit-fill adjustment, or continuous-load (125%) factors are applied. These can require a larger wire.
Termination temperature limits (NEC 110.14(C)) often restrict you to the 60°C or 75°C column even when the insulation is rated 90°C.
Motor, HVAC, EV charger and service conductors have their own NEC rules. Your inspector or a licensed electrician has the final say.

Common questions

What size wire for 20 amps at 100 feet?

At 120 V on copper, 12 AWG meets ampacity but drops about 6.6%. To stay under 3%, you need 8 AWG copper. At 240 V the drop percentage halves, so 10 AWG (about 2.1%) works.

Is the 3% voltage drop rule required by the NEC?

No. It appears as an informational note recommending 3% for branch circuits and 5% total. It is not enforceable on its own, but some local codes and equipment instructions require it.

Why does 12 AWG copper only get 20 amps when the table says 25?

NEC 240.4(D) limits overcurrent protection for small conductors: 14 AWG to 15 A, 12 AWG to 20 A and 10 AWG to 30 A for copper, regardless of the higher table values.

Should I use one-way or round-trip distance?

Enter the one-way distance. The single-phase formula already multiplies by 2 to account for the return conductor.

Sources

NCCER: Electrical Formula Sheet
IAEI Magazine: Voltage Drop Formulas
IAEI Magazine: Voltage Drop Calculations
NEC Mastery: Voltage Drop – Quick Reference
NFPA 70 (NEC 2017) via course reprint: Chapter 9, Table 8 – Conductor Properties
VoltDropCalc: NEC Chapter 9 Table 8
NECTables: NEC Table 310.16 Ampacity
Omni Cable: NEC Ampacity Data (Table 310.16)

Sources checked Oct 2026. Each number on this page matches at least two of them.