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.
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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

Ohm's Law, Wire Voltage Drop & Resistor Series/Parallel Calculator: inputs, method, and how to read the result

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

Choose inputs for the case you are checking

The defaults are illustrative. Replace them with values for your case and keep the units shown beside each field.

  • Source Voltage (V): Supply voltage (e.g. 12V, 120V, 230V, 240V).
  • Circuit Current (I): Load current in amperes.
  • One-Way Wire Run Length: One-way distance from panel to load in feet.
  • Conductor Gauge (NEC DC Resistance Ω / 1,000 ft): Standard NEC Chapter 9 Table 8 uncoated copper resistance.

What the calculation includes

With the default values shown, Ohm's Law Power (P = V × I) & Load Resistance (R = V / I) is 1800 W (8 Ω). The calculation follows Ohm's Law, Joule's Power Law & 2-Wire Loop Voltage Drop; the formula and variable definitions above show how the inputs combine.

One-input check: With the other defaults held constant, changing Source Voltage (V) from 120 Volts (V) to 132 Volts (V) changes Ohm's Law Power (P = V × I) & Load Resistance (R = V / I) from 1800 W (8 Ω) to 1980 W (8.8 Ω). This is a comparison inside the calculator's stated model, not a prediction.

Where to use caution

  • Single-phase two-conductor circuit at unity power factor.
  • High-inductance AC motor loads require reactance (X) impedance corrections.

Scope: Global (SI Units & NEC Reference) Check the assumptions and limitations against your situation before relying on the estimate.

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