Calculate wire voltage drop, load voltage, percentage loss, heat loss, and a planning gauge—or solve maximum current from a target drop for copper, CCA, or aluminum.

Resistance-only estimate at 20 °C. AC assumes unity power factor and negligible reactance. This is not an ampacity check or an installation design.

Copper: 1. Aluminum: approximate 1.61. For CCA, enter the manufacturer’s resistance divided by the same-size copper resistance; no universal CCA factor applies.

Planning target

3% is an editable planning target, not a compliance determination.

Amp Drop Formula

The calculator estimates voltage drop from conductor resistance at 20 °C. For AC, the resistance-only estimate assumes unity power factor and negligible reactance.

r = R₁₀₀₀ L / 1000 \ Δ V2 - wire = 2 I r \ Δ V₃₋ₚₕₐₛₑ = √(3) I r
  • Where I is the current (amps)
  • r is one conductor’s resistance over the one-way run (ohms)
  • R1000 is the copper reference resistance per 1,000 feet, multiplied by the selected material factor
  • L is one-way length in feet; meters are converted before calculation
  • ΔV is the two-wire voltage drop or balanced three-phase line-to-line drop (volts)

To solve current at a target drop, use I = (source voltage × target percentage / 100) / (path factor × r), with path factor 2 for two wires or √3 for balanced three-phase. Total conductor heat is 2I²r or 3I²r, respectively. Gauge planning checks voltage drop only, not ampacity or installation compliance. CCA resistance requires manufacturer data; the aluminum factor 1.61 is approximate.

Definition

What is an amperage drop?

In most electrical contexts, “amp drop” is not a standard engineering term. In a simple series circuit, the current is (approximately) the same everywhere; what “drops” along a wire is voltage due to the wire’s resistance. People sometimes use “amp drop” informally to mean a reduction in current caused by lower available voltage at the load or increased resistance in the circuit. This calculator estimates conductor voltage drop, current at a chosen drop target, and conductor heat loss using a resistance-only model. The voltage delivery ratio is not overall equipment efficiency.

Example Problem

How to estimate wire voltage drop?

Select voltage drop, two-wire circuit, 12 AWG copper, and the copper resistance multiplier 1.

Assume a 100-foot one-way run, 120 V source, and 10 A load. The reference resistance is 1.588 Ω per 1,000 ft.

Finally, calculate resistance, voltage drop, and wire heat using the formulas above:

r = 1.588 × 100 / 1,000 = 0.1588 Ω per conductor

ΔV = 2 × 10 × 0.1588 = 3.176 V (about 2.647%)

Estimated load voltage is 116.824 V; total wire heat is 31.76 W. Check the actual conductor and installation separately.