Construction Calculators

Voltage Drop & Wire Size Calculator

Enter voltage, amps, one-way distance, metal and AWG to see the voltage drop in volts and percent, and the smallest wire gauge that stays under your limit. Works for 120/240 V AC, three phase and 12 V DC.

Inputs

Results

Fill the form and press Calculate.

Voltage drop only, not an ampacity or safety check. Check ampacity in your adopted NEC edition and with a licensed electrician before wiring anything. This tool does not reproduce NFPA 70 Table 310.16 and does not check breaker size, insulation, conduit fill or derating.

How it works

Wire size: the diameter of AWG number n is d = 0.005 × 92^(36−n)/39 inch (the ASTM B258 standard nominal diameters; 1/0 to 4/0 use n = 0, −1, −2, −3). Area in circular mils is (d in thousandths of an inch)², so 10 AWG is about 10,383 cmil and 4/0 is 211,600 cmil.

Resistance: R per foot = resistivity ÷ area. At 20 °C annealed copper (100% IACS, 1.7241 µΩ·cm, ASTM B3 / B193) is 10.371 Ω·cmil/ft. Aluminum is taken as 17.0 Ω·cmil/ft (about 61% IACS, as in ASTM B230). Resistance is then scaled to your conductor temperature with R×(1 + α×(T − 20)), α = 0.00393 per °C for copper and 0.00403 for aluminum (ASTM B193 / B230 values, NIST-published copper constant). Hotter wire has more resistance, which is why the default is 75 °C, a typical rated operating temperature; set 20 for a cold-wire textbook figure.

Drop: single phase and DC: V_drop = 2 × I × R × L (current goes out and back, L is the one-way length). Three phase: V_drop = √3 × I × R × L. Percent = V_drop ÷ source voltage. The smallest-AWG result tries sizes from 18 AWG to 4/0 in order and returns the first whose percent drop is at or under your target. It ignores AC reactance and power factor, which are small for branch-circuit sizes but grow with large conductors; for those, use a reactance-aware method.

The 3% and 5% flags: the informational notes to NEC 210.19(A) and 215.2(A) recommend no more than 3% drop on a branch circuit and 5% for feeder plus branch combined. They are recommendations, not requirements (reference by section only; no NFPA text or table is copied here). Your target field lets you use another limit.

Ampacity: the minimum AWG shown here is a voltage-drop minimum. Current-carrying capacity can require a larger wire. Look it up in your adopted NEC edition (310.16 and 240.4(D)) and confirm with a licensed electrician.

Sources: ASTM B258 (standard nominal diameters of AWG wire); ASTM B3 and B193 (annealed copper, resistivity and temperature coefficient); ASTM B230 (aluminum 1350 conductivity). Formulas only; no standard text copied. Runs entirely in your browser; nothing is sent or stored.

Frequently asked questions

How do I calculate voltage drop?

For DC or single-phase AC, drop in volts = 2 x current x resistance per foot x one-way length. Three-phase uses 1.732 instead of 2. Divide by the source voltage for percent. This page does exactly that, using the resistance of the AWG you pick.

What wire gauge do I need for 30 amps?

Two separate checks apply. Ampacity (how much current the wire may carry) comes from the NEC; see 310.16 and 240.4(D) in your adopted edition, and ask a licensed electrician. Voltage drop depends on distance: enter 30 A, your voltage and your one-way length, and this tool shows the smallest AWG that stays under your percent limit. Use whichever size is larger.

Is 3% voltage drop a rule?

No. The informational notes to NEC 210.19(A) and 215.2(A) recommend sizing so branch-circuit drop is no more than 3% and feeder plus branch together no more than 5%. They are recommendations, though your local code, the equipment maker or the inspector may require more.

How does 12V DC (RV, solar, boat) differ?

The math is the same but the voltage is small, so the same volts lost is a much bigger percent. 3 V lost on 12 V is 25%. Marine and solar guidance often aims for 3% or less, so wire sizes grow quickly with distance. Select single phase / DC and enter 12 as the voltage.

Why does my result differ from another calculator?

Differences usually come from the resistance table used, the conductor temperature (hot wire has higher resistance; this tool lets you set it), whether round-trip length is used, and whether AC reactance is included. This tool is resistance-only and uses 10.371 ohm-cmil/ft for copper at 20 C.

Does this size wire for safety?

No. It checks voltage drop only. It does not check ampacity, breaker or fuse size, insulation, conduit fill, derating or terminals. Have a licensed electrician confirm any real installation.