Ohm's Law, Wire Voltage Drop & Resistor Series/Parallel Calculator

Compute Ohm’s Law (V, I, R, P), NEC wire voltage drop percentage, and series/parallel resistance.

Ohm's Law Power (P = V × I) & Load Resistance (R = V / I)
1800 W (8 Ω)
2-Wire Voltage Drop (100 ft): 4.76 V (3.97%: Exceeds 3% Branch Limit)
Load Resistance (R = V/I) 8 Ω
Line Voltage Drop (2×L×I×R) 4.76 V (3.97%)
Series / Parallel with 10 Ω 18 Ω / 4.44 Ω
Voltage at Load Terminal 115.24 V
Inputs
Volts (V)
Supply voltage (e.g. 12V, 120V, 230V, 240V).
Amps (A)
Load current in amperes.
feet (ft)
One-way distance from panel to load in feet.
Standard NEC Chapter 9 Table 8 uncoated copper resistance.
Ohms (Ω)
Used to compute R_load + R₂ (series) and R_load ∥ R₂ (parallel).
Results & Graph
Ohm's Law Power (P = V × I) & Load Resistance (R = V / I)
1800 W (8 Ω)
2-Wire Voltage Drop (100 ft): 4.76 V (3.97%: Exceeds 3% Branch Limit)
Load Resistance (R = V/I)
8 Ω
Line Voltage Drop (2×L×I×R)
4.76 V (3.97%)
Series / Parallel with 10 Ω
18 Ω / 4.44 Ω
Voltage at Load Terminal
115.24 V

Delivered Terminal Voltage vs. Line Voltage Drop (V)

Comparison against NEC 3% recommended branch-circuit drop threshold
Graph 1 • Annotated Electrical Circuit & Power Readout Live Voltage (V), Current (A), Impedance (Ω), and Active Power (W)
120V I = 15 A 1620 W 8.0 Ω Circuit Operating Point • Real Power: 1620 W (1.62 kW) • Apparent Power: 1.80 kVA • NEC 125% Continuous: 18.8 A • Horsepower Eq.: 2.17 HP
Graph 2 • NEC Table 310.16 Copper Wire Gauge Ladder (75°C) Highlights conductor required for 18.8A (125% continuous load)
14 AWG Copper (15A Max Breaker) 15 A 12 AWG Copper (20A Max Breaker) 20 A 10 AWG Copper (30A Max Breaker) 30 A 8 AWG Copper (50A @ 75°C THHN) 50 A 6 AWG Copper (65A @ 75°C THHN) 65 A 4 AWG Copper (85A @ 75°C THHN) 85 A
Graph 3 • Voltage Drop (%) Across Wire Distances (25 ft to 250 ft) Keep branch-circuit voltage drop below the 3.0% NEC recommendation
0% 5.4% 10.7% NEC 3.0% Max Branch-Circuit Drop Limit 25 ft: 0.99 % 1.0 25 ft 50 ft: 1.98 % 2.0 50 ft 100 ft: 3.97 % 4.0 100 ft 150 ft: 5.96 % 6.0 150 ft 200 ft: 7.94 % 7.9 200 ft 250 ft: 9.93 % 9.9 250 ft
Summary: Power is 1800 W, resistance is 8 Ω, and voltage drop is 4.76 V (3.97%).
Summary: At 120 V and 15 A, Ohm’s Law gives a load resistance of R = 8 Ω and real power P = 1800 W (1.8 kW). Over a 100-foot one-way run (200 ft round-trip at 1.588 Ω/1,000 ft), line voltage drop is 4.76 V (3.97%), delivering 115.24 V at the load.

Formula & Mathematical Methodology: Ohm's Law, Joule's Power Law & 2-Wire Loop Voltage Drop

R = VI,   P = V · I,   Vdrop = 2 × L × I × R1000ft1000,   Rparallel = R1R2R1 + R2
R = V / I; P = V × I; V_drop = 2 × L × I × (R_1000 / 1000); R_parallel = (R₁ × R₂) / (R₁ + R₂)
  • 2 × L: Round-trip conductor length (hot + neutral return path)

Rounding: Rounded to 2–3 decimal places.

Worked Example: 120V, 15A Load Over 100 Feet of 12 AWG Copper Wire

Compute power, resistance, and voltage drop for a 15A branch circuit.

  1. Power P = 120 × 15 = 1,800 W (1.8 kW); Load Resistance R = 120 ÷ 15 = 8.0 Ω.
  2. Voltage drop = 2 × 100 × 15 × (1.588 ÷ 1,000) = 4.76 V (3.97% drop → Delivered voltage = 115.24 V).
Result: 1,800 W power, 8.0 Ω load resistance, and 4.76 V (3.97%) voltage drop.

Frequently Asked Questions

What is the maximum voltage drop recommended by the National Electrical Code (NEC)?

NEC Informational Note recommends keeping branch-circuit voltage drop at or below 3% (and combined feeder + branch drop ≤ 5%) for efficient operation.

Science & Engineering • Electrical Engineering & Circuits Guide

How to Use the Ohm's Law, Wire Voltage Drop & Resistor Series/Parallel Calculator: Practical Guide & Visual Analysis

Solve DC and single-phase AC resistive circuit parameters, check whether branch-circuit wire runs meet the NEC ≤3% voltage drop recommendation, and combine resistors in series or parallel. Whether you are researching ohms law calculator, voltage drop calculator, resistor calculator, wattage calculator, parallel resistor calculator, this interactive tool helps you calculate electrical power (Watts), load resistance (Ohms), two-wire line voltage drop over distance, and series/parallel equivalent resistance.

Interpreting the 3 Custom Visual Graphs & Key Outputs

As you adjust Source Voltage (V), Circuit Current (I), One-Way Wire Run Length, Conductor Gauge (NEC DC Resistance Ω / 1,000 ft), the calculator immediately computes Electrical Power (W & kW), Load Resistance (Ω), Line Voltage Drop (V & %), Series & Parallel Equivalent Resistance (Ω) and updates all three domain-specific SVG charts in real time. Instead of relying on a single static figure, you can inspect the primary domain diagram, component breakdown, and multi-point trajectory curve to see how changing your inputs shifts the final result. Use the Compare (up to 3 scenarios) button to evaluate side-by-side variations before making a decision.

Real-World Benchmark & Planning Tips (Global (SI Units & NEC Reference))

In our benchmark scenario (120V, 15A Load Over 100 Feet of 12 AWG Copper Wire), 1,800 W power, 8.0 Ω load resistance, and 4.76 V (3.97%) voltage drop. Keep in mind that single-phase two-conductor circuit at unity power factor. For related calculations in Science & Engineering, explore EV Charging Cost & Fuel vs. Electricity Calculator, Unit Conversion Calculator, Scientific Calculator.