Circuit Load Calculator | Free Construction Calculators

Calculate circuit load, power consumption, and required breaker size for electrical installations. Our professional circuit load calculator helps electricians and contractors determine total circuit amperage, wattage, and proper overcurrent protection for residential and commercial applications. Enter your load in watts or amps at 120V, 240V, or 480V to get instant NEC-compliant breaker recommendations. Perfect for sizing circuit breakers (15A, 20A, 30A, 40A, 50A) and selecting wire gauge (14 AWG, 12 AWG, 10 AWG, 8 AWG) for safe electrical system design.

NEC Compliant

Automatically applies 80% rule for continuous loads per NEC requirements

Breaker Sizing

Recommends proper circuit breaker size based on actual load calculations

Multi-Voltage

Supports 120V, 240V, 208V, 277V, and 480V electrical systems

How to Use This Calculator

  1. Select your system voltage (120V for standard outlets, 240V for appliances)
  2. Choose input type (Watts if you know power consumption, Amps if you know current draw)
  3. Enter the load value from appliance nameplate or total circuit load
  4. Set power factor (1.0 for heaters/lights, 0.8 for motors/transformers)
  5. Select phase type (single-phase for residential, three-phase for industrial)
  6. Click Calculate to see required breaker size and wire gauge
  7. Add 25% margin for continuous loads (running 3+ hours)

Calculation Formulas

Amps from Watts (Single-Phase): I = W ÷ (V × PF)

Amps from Watts (Three-Phase): I = W ÷ (√3 × V × PF)

Watts from Amps (Single-Phase): W = I × V × PF

Minimum Breaker Size: I × 1.25 (for continuous loads per NEC 210.20)

About Circuit Load Calculations

Proper circuit load calculations are critical for electrical safety and NEC code compliance. Understanding total circuit load helps prevent overloaded circuits, nuisance breaker trips, overheated wires, and potential fire hazards. The National Electrical Code (NEC) Article 210 requires that branch circuits be sized to handle the connected load plus a safety margin, especially for continuous loads that operate for three hours or more.

The NEC 80% rule (210.20) states that continuous loads cannot exceed 80% of the circuit breaker rating. This means a 20-amp breaker can only safely carry 16 amps of continuous load. When sizing circuits, apply the 125% multiplier to continuous loads: if your calculated load is 12 amps continuous, you need a breaker rated for at least 15 amps (12 × 1.25 = 15A). This safety margin prevents thermal stress on breakers and conductors during extended operation.

Common circuit breaker sizes follow standard ratings: 15A (general lighting and outlets with 14 AWG wire), 20A (kitchen and bathroom outlets with 12 AWG wire), 30A (electric dryers and water heaters with 10 AWG wire), 40A (electric ranges and large appliances with 8 AWG wire), and 50A (subpanels and large loads with 6 AWG wire). Wire size must match or exceed breaker ampacity per NEC Table 310.16, and voltage drop should not exceed 3% for branch circuits or 2% for feeders.

Standard Residential Circuit Loads (120V)

  • 15A General Lighting: 1800W maximum (14 AWG wire, bedrooms, living areas)
  • 20A Small Appliance: 2400W maximum (12 AWG wire, kitchen countertop outlets)
  • 20A Bathroom: 2400W maximum (12 AWG wire, GFCI protected outlets)
  • 20A Laundry: 2400W maximum (12 AWG wire, dedicated washing machine circuit)
  • 15A Garage/Outdoor: 1800W maximum (14 AWG wire, GFCI protected)

Standard Residential Circuit Loads (240V)

  • 30A Electric Dryer: 7200W maximum (10 AWG wire, typical load 5000-6000W)
  • 30A Water Heater: 7200W maximum (10 AWG wire, typical 3500-4500W)
  • 40A Electric Range: 9600W maximum (8 AWG wire, typical 6000-8000W)
  • 50A Electric Range: 12000W maximum (6 AWG wire, larger cooktops)
  • 30A AC Condenser: 7200W maximum (10 AWG wire, 3-4 ton units)
  • 40A EV Charger: 9600W maximum (8 AWG wire, delivers 32A continuous)

Frequently Asked Questions

What is the 80% rule for circuit breakers?

NEC 210.20(A) requires that continuous loads (operating 3+ hours) not exceed 80% of circuit breaker rating. A 20A breaker can handle 16A continuous (20 × 0.80 = 16A). When sizing breakers, multiply continuous load by 125% to find minimum breaker size: a 12A continuous load requires a 15A breaker (12 × 1.25 = 15A). Non-continuous loads can use the full breaker rating, though it's good practice to maintain headroom.

How many outlets can I put on a 20-amp circuit?

NEC doesn't specify a maximum number of outlets, but limits total load. A 20A circuit at 120V provides 2400W capacity. Using the 80% rule for general use, limit to 1920W (2400 × 0.80). General guideline: 10-13 outlets maximum on 20A circuits, assuming average 1.5A per outlet. Kitchen small appliance circuits should have fewer outlets due to higher loads. Commercial installations follow more restrictive calculations per NEC Article 220.

Can I use a 20-amp breaker on 14 AWG wire?

No. NEC 240.4(D) prohibits 20A overcurrent protection on 14 AWG wire. Wire size must match breaker rating: 14 AWG maximum 15A breaker, 12 AWG maximum 20A breaker, 10 AWG maximum 30A breaker, 8 AWG maximum 40A breaker. You can use larger wire with smaller breakers (12 AWG on 15A circuit), which reduces voltage drop on long runs. Always match or exceed wire ampacity to breaker rating for safety and code compliance.

What size breaker do I need for a 5000-watt water heater?

For a 5000W water heater at 240V: I = 5000W ÷ 240V = 20.8 amps. Water heaters are continuous loads, requiring 125% sizing: 20.8A × 1.25 = 26 amps minimum. Use a 30A breaker with 10 AWG copper wire. Most 4500-5500W water heaters require 30A/10 AWG circuits per manufacturer specifications and NEC Table 310.16. Always verify nameplate requirements and consult local electrical codes before installation.

How do I calculate total panel load for multiple circuits?

Add all continuous loads at 125% plus all non-continuous loads at 100%. Example: 30A continuous AC + 20A non-continuous outlets + 40A continuous range = (30 × 1.25) + 20 + (40 × 1.25) = 37.5 + 20 + 50 = 107.5A total demand. NEC Article 220 allows demand factors for residential loads - not all circuits operate at maximum simultaneously. A 200A panel can typically serve 300-400A of connected load using diversity factors.

Related Electrical Calculators

→ Amperage Calculator - Calculate current from watts and voltage→ Wire Size Calculator - Determine proper AWG for circuit loads→ Voltage Drop Calculator - Check voltage loss on long wire runs

NEC Circuit Load Requirements

  • NEC 210.20(A): Continuous loads limited to 80% of overcurrent device rating
  • NEC 210.19: Conductors sized for at least 125% of continuous load
  • NEC 210.11(C)(1): Two 20A small appliance circuits required in kitchens
  • NEC 210.11(C)(2): One 20A laundry circuit required
  • NEC 210.11(C)(3): One 20A bathroom circuit required (can serve multiple bathrooms)
  • NEC 220.14: General lighting load calculated at 3VA per square foot
  • Standard Ratings: 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A

Circuit Breaker and Wire Size Reference

BreakerWire SizeMax Watts (120V)Max Watts (240V)Typical Use
15 A14 AWG1800 W3600 WLighting, outlets
20 A12 AWG2400 W4800 WKitchen, bathroom
30 A10 AWG3600 W7200 WWater heater, dryer
40 A8 AWG4800 W9600 WElectric range, EV
50 A6 AWG6000 W12000 WLarge range, welder
60 A6 AWG7200 W14400 WSubpanel, hot tub

Professional Circuit Design Tips

  • Dedicated circuits: Refrigerators, microwaves, and sump pumps should have dedicated circuits
  • GFCI protection: Required for bathrooms, kitchens, garages, outdoors, and within 6 ft of sinks
  • AFCI protection: Required for most 120V circuits in living areas per NEC 210.12
  • Load diversity: Not all circuits draw maximum load simultaneously in residential applications
  • Future expansion: Leave 20% panel capacity available for future circuits
  • Voltage drop: Keep under 3% for branch circuits, 2% for feeders (5% total maximum)
  • Balance loads: Distribute circuits evenly across both legs of 240V service panel
  • Label circuits: Clearly identify each circuit at panel and document loads for future reference

Common Appliance Electrical Loads

Kitchen Appliances (120V)

  • Refrigerator: 600-800W (6-7A) - Dedicated 15A circuit
  • Microwave: 1000-1500W (8-12A) - Dedicated 20A circuit
  • Dishwasher: 1200-1500W (10-12A) - Dedicated 20A circuit
  • Garbage Disposal: 500-900W (4-7A) - Dedicated 15-20A circuit
  • Coffee Maker: 800-1200W (7-10A) - Small appliance circuit
  • Toaster Oven: 1200-1500W (10-12A) - Small appliance circuit

Large Appliances (240V)

  • Electric Range: 6000-12000W (25-50A) - 40-50A circuit
  • Electric Dryer: 5000-7000W (20-30A) - 30A circuit
  • Water Heater: 3500-5500W (15-23A) - 30A circuit
  • Central AC (3-ton): 3500-4500W (15-19A) - 30A circuit
  • Heat Pump: 4000-6000W (17-25A) - 30-40A circuit
  • EV Charger: 7200-9600W (30-40A) - 40-50A circuit