Electrician Calculator
7 professional electrical tools in one calculator. Ohm's Law, power triangle, voltage drop, wire sizing, load calculations, motor FLA, and conduit fill. All calculations follow NEC 2023 standards.
⚡ Ohm's Law Calculator
Enter any 2 values and the others are calculated. V = I × R, P = V × I.
Calculated Values
Ohm's Law Triangle
The Ohm's Law Triangle is a visual mnemonic: V at top, I and R at bottom. Cover the unknown variable and the remaining two show the formula.
Power variations: P = V × I = I² × R = V² / R. These derived formulas let you calculate power without measuring current or voltage directly.
Practical tip: For troubleshooting, measure resistance of loads (with power off). Then use V and R to predict current draw.
Wire Size vs Distance at 240V / 20A
| Wire | VD at 75ft | VD at 100ft | VD at 150ft | VD at 200ft |
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Electrical Fundamentals: Complete Reference Guide
Ohm's Law: The Foundation of Electrical Engineering
Ohm's Law states that current (I) through a conductor is proportional to voltage (V) across it and inversely proportional to resistance (R): V = I × R.
Power variations: P = V × I = I² × R = V² / R. These derived formulas let you calculate power without measuring current or voltage directly.
Practical tip: For troubleshooting, measure resistance of loads (with power off). Then use V and R to predict current draw. Most household loads are between 10-20 ohms at 120V.
Power Triangle: Real, Apparent, and Reactive Power
Real Power (P): Watts (W) - actual work done. Apparent Power (S): Volt-Amps (VA) - total power supplied. Reactive Power (Q): VAR - power in magnetic fields.
Power Factor = P / S. Higher PF means less wasted energy. Industrial facilities install capacitor banks to correct PF above 0.95, reducing demand charges by 5-15%.
Voltage Drop: Why Wire Size Matters Over Distance
Every wire has resistance. When current flows, some voltage is lost as heat. This is voltage drop. Formula: VD = 2 × L × I × R / 1000.
NEC Recommendation: Max 3% for branch circuits, 5% total. Exceeding these limits causes motor burnout, flickering lights, and nuisance breaker trips.
Wire Sizing: NEC Table 310.15(B)(16)
Ampacity: Maximum current a wire can carry continuously. Based on conductor material, insulation type, and ambient temperature.
125% Rule (NEC 210.19): Continuous loads (3+ hours) require wire rated for 125% of load current. A 20A continuous load needs wire rated for 25A minimum.
Breaker Protection (NEC 240.4): Wire ampacity must equal or exceed the breaker rating. Never put a 30A breaker on #14 wire (rated 15A).
Motor Circuits: NEC Chapter 430 Requirements
Wire (430.22): 125% of motor FLA. Breaker (430.52): 250% of FLA (inverse-time). Overload (430.32): 115-125% of FLA. Motor circuits follow unique rules separate from general branch circuits due to high inrush current (5-7x FLA at startup).
Frequently Asked Questions
What size wire do I need for a 20-amp circuit?
For a 20A circuit, use #12 AWG copper wire minimum (rated 20A). For runs longer than 75ft at 120V or 150ft at 240V, upsize to #10 AWG to stay within 3% voltage drop.
How do I calculate voltage drop?
VD = 2 x L x I x R / 1000. L is one-way distance in feet, I is current in amps, R is resistance in ohms per 1000 feet. The 2 accounts for the round-trip (hot + neutral).
Can I use aluminum wire instead of copper?
Yes, for service entrance and feeders. Aluminum is lighter and cheaper but requires larger gauge (#4 AL = #6 CU ampacity). Use anti-oxidant paste at connections. Never use aluminum for branch circuits.
What breaker do I need for a 5 HP motor?
A 5 HP single-phase 230V motor has FLA of ~28A (NEC Table 430.248). Use a 70A breaker (250% rule) and #8 AWG wire (125% of FLA = 35A minimum).
What is the NEC maximum voltage drop?
NEC recommends (not mandates) 3% for branch circuits and 5% total. Exceeding limits voids equipment warranties and can cause motor burnout and flickering lights.
How do I fill a conduit properly?
Use NEC Chapter 9 tables. Max fill: 1 wire = 53%, 2 wires = 31%, 3+ wires = 40%. For example, 4 #12 THHN wires need at least 1/2 inch EMT.
References & Sources
- NFPA 70 - National Electrical Code (NEC) 2023 - The authoritative standard for safe electrical installation. Published by the National Fire Protection Association.
- NEC Table 310.15(B)(16) - Allowable ampacities of insulated conductors. The primary reference for wire sizing.
- NEC Chapter 430 - Motors, Motor Circuits, and Controllers. Comprehensive requirements for motor branch circuits.
- NEC Chapter 9, Table 1 - Percent Area of Conduit or Tubing for Conductors. Standard for conduit fill calculations.
- NEC Table 430.248 - Full-Load Current, Single-Phase AC Motors. Reference for motor FLA calculations.
- IEEE Standard 141-1993 - IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book).
- U.S. Department of Energy - Improving Power Factor to Reduce Electricity Demand Charges.