Electrical Load Calculator
Estimate electrical load for your project with our free calculator. Get accurate material quantities, costs, and specifications.
Formula
Total Demand = General (NEC demand) + Range (8kW for <12kW) + Dryer + Water Heater + max(AC, Heating x 65%)
The NEC standard calculation applies demand factors to general loads (100% of first 3,000W + 35% of remainder), uses 8,000W for ranges up to 12kW, includes dryer and water heater at nameplate, and uses the larger of cooling (100%) or heating (65%) since they do not operate simultaneously. Total amps equal demand watts divided by voltage.
Worked Examples
Example 1: Standard 2,000 sq ft Home
Problem: Calculate the electrical service size for a home with 2,400W general lighting, 3,000W small appliance, 1,500W laundry, 5,000W A/C, 10,000W heating, 8,000W range, 5,000W dryer, and 4,500W water heater at 240V single phase.
Solution: General loads = 2,400 + 3,000 + 1,500 = 6,900W\nDemand: first 3,000 at 100% + remainder at 35% = 3,000 + (3,900 x 0.35) = 4,365W\nRange demand (under 12kW) = 8,000W\nDryer = 5,000W | Water heater = 4,500W\nHVAC: max(5,000, 10,000 x 0.65) = max(5,000, 6,500) = 6,500W\nTotal demand = 4,365 + 8,000 + 5,000 + 4,500 + 6,500 = 28,365W\nAmps = 28,365 / 240 = 118.2A\nWith 125% = 147.7A
Result: Demand: 28,365W | 118.2A | 150A breaker recommended | #1 AWG Copper
Example 2: Small Apartment
Problem: Calculate for an apartment with 1,200W lighting, 3,000W small appliance, 1,500W laundry, 3,000W A/C, 0W electric heating (gas heat), 6,000W range, 4,000W dryer at 240V.
Solution: General loads = 1,200 + 3,000 + 1,500 = 5,700W\nDemand: 3,000 + (2,700 x 0.35) = 3,000 + 945 = 3,945W\nRange demand = 8,000W (standard for under 12kW)\nDryer = 4,000W | Water heater = 0W\nHVAC = max(3,000, 0) = 3,000W\nTotal = 3,945 + 8,000 + 4,000 + 0 + 3,000 = 18,945W\nAmps = 18,945 / 240 = 78.9A\nWith 125% = 98.7A
Result: Demand: 18,945W | 78.9A | 100A breaker sufficient | #4 AWG Copper
Frequently Asked Questions
What is an electrical load calculation and why is it important?
An electrical load calculation determines the total power demand of a building to properly size the electrical service entrance, main breaker panel, and feeder wires. It is required by the National Electrical Code (NEC) Article 220 before any new construction, major renovation, or service upgrade. Without a proper load calculation, you risk undersizing the electrical service, which causes breakers to trip frequently and creates fire hazards from overloaded circuits. Oversizing wastes money on unnecessarily large panels and wiring. The calculation uses demand factors to account for the fact that not all loads operate simultaneously. For example, NEC allows a 35% demand factor for general lighting loads above 3,000 watts because it is unlikely every light and outlet will be used at full capacity at the same time.
What wire size do I need for my electrical service?
Wire sizing depends on the amperage and wire material (copper or aluminum). For 100-amp service, use 4 AWG copper or 2 AWG aluminum. For 150-amp service, use 1 AWG copper or 2/0 AWG aluminum. For 200-amp service, use 2/0 AWG copper or 4/0 AWG aluminum. For 400-amp service, use 400 kcmil copper or 600 kcmil aluminum. These sizes follow NEC Table 310.16 for 75-degree Celsius rated conductors. Aluminum wire is commonly used for service entrance feeders because it costs significantly less than copper and the connections are made with properly rated lugs. The wire must be rated for the full breaker size, not just the calculated load. Additionally, the grounding electrode conductor must be properly sized per NEC Table 250.66, and the neutral conductor must be the same size as the ungrounded conductors for residential services.
What is the difference between connected load and demand load?
Connected load is the sum of all electrical equipment nameplate ratings in a building, representing the theoretical maximum power draw if everything ran simultaneously at full capacity. Demand load is the realistically expected power consumption after applying NEC demand factors that account for usage diversity. For a typical 2,000 square foot home, connected load might be 40,000-60,000 watts, but demand load is usually 15,000-25,000 watts (40-60% of connected). This significant difference exists because appliances cycle on and off, lights are not all on simultaneously, and many outlets are unused. The demand load determines the actual service size needed. Without demand factors, every home would need 300+ amp service, which would be unnecessarily expensive. The diversity factor (demand load divided by connected load) indicates how efficiently the electrical system is utilized.
How do electric vehicle chargers affect residential electrical load?
EV chargers are one of the largest single loads in modern homes and can significantly impact electrical service requirements. Level 1 chargers use a standard 120V outlet and draw only 1,400-1,900 watts (12-16 amps), adding minimal load. Level 2 chargers operate at 240V and draw 3,800-11,500 watts (16-48 amps), with most installations using a 40-amp circuit requiring a 50-amp breaker. A 48-amp charger requires a 60-amp breaker and adds 11,520 watts to the demand calculation. Many homes with 100-amp service cannot accommodate a Level 2 charger without upgrading to 200-amp service. NEC Article 625 governs EV charging calculations, and starting with the 2023 NEC, load management systems can be used to share capacity between the EV charger and other loads, potentially avoiding costly service upgrades.
What are common mistakes in residential electrical load calculations?
The most frequent mistake is using actual room measurements instead of the outside dimensions of the dwelling for floor area calculations, which NEC requires for general lighting load. Another common error is forgetting to include the two required small appliance circuits (3,000 watts total) and one laundry circuit (1,500 watts) that NEC mandates regardless of actual appliances. Failing to apply demand factors correctly, particularly the general lighting step-down from 100% to 35% above 3,000 watts, leads to oversized service calculations. Using running watts instead of starting watts for motor loads (A/C compressors need 125% of nameplate per NEC 430) underestimates peak demand. Not accounting for future loads like EV chargers, pool equipment, or hot tubs can result in needing an expensive panel upgrade shortly after construction. Always calculate for anticipated future needs, not just current loads.
What is the 80% rule in electrical panels?
The 80% rule states that circuit breakers should not be loaded beyond 80% of their rated capacity for continuous loads (those running 3+ hours). A 20-amp breaker should carry no more than 16 amps continuously. This rule is codified in NEC Article 210.20 and exists because breakers are designed to trip at 100% of their rating, but sustained operation near the trip point generates heat that degrades connections and insulation. For the main breaker, this means a 200-amp panel should not sustain more than 160 amps of continuous load. Non-continuous loads can use 100% of the breaker rating. In practice, this means sizing circuits for the expected continuous load times 1.25 (the reciprocal of 80%). Some breakers are listed for 100% duty and can be loaded to full rating continuously, but they are more expensive and must be specifically listed by the manufacturer.