Every home has a ceiling on how much electricity it can safely draw at one time — and that ceiling is set by the service entrance conductors and main breaker. Before adding a major appliance, an EV charger, or a hot tub, you need to know whether your existing service has the capacity. NEC Article 220 provides the standard methodology for calculating that demand, and this calculator walks you through it.
The NEC Article 220 Load Calculation Method
The National Electrical Code Article 220 defines the standard procedure for calculating electrical service load in residential buildings. The calculation starts with the general lighting load — 3 volt-amperes per square foot of living area — plus mandatory allowances for small appliance circuits (two circuits at 1,500 VA each) and one laundry circuit (1,500 VA). These are combined and then subject to a demand factor: the first 3,000 VA carries no reduction, while everything above 3,000 VA is multiplied by 35%, recognizing that not all lights and receptacles operate simultaneously at full load. Dedicated appliance loads — electric range, dryer, water heater, dishwasher, HVAC — are added at their rated loads or per NEC demand tables. The range demand, for example, is taken from NEC Table 220.55 rather than the nameplate rating; a 10 kW range counts as only 8 kW in the calculation. The final total in VA is divided by 240V to get the calculated ampere demand, which you compare against your main breaker rating.
Continuous Loads and Service Sizing
Continuous loads may require conductors and overcurrent devices to be sized at no less than 125% of the continuous component. That treatment applies to qualifying loads and equipment, not automatically to the entire residential service calculation. EV charging is commonly treated as continuous, so the charger demand should be modeled explicitly and any listed load-management system should be documented. Always verify the adopted code edition, equipment instructions, service rating, and local interpretation with a qualified electrician or the authority having jurisdiction.
Demand Factors and Why They Matter
Demand factors are reductions the NEC permits you to apply to calculated loads because real-world usage is lower than the theoretical peak. The most significant is the general lighting demand factor in NEC 220.42: only the first 3,000 VA of lighting and receptacle load must be carried at 100%; everything beyond that is derated to 35%. This single factor can reduce a large home's calculated lighting load by tens of amperes. NEC 220.55 provides a similar demand table for electric ranges — the higher the nameplate rating, the smaller the percentage you must count. For multiple air conditioning units, NEC 220.60 allows you to omit the smaller of heating or cooling loads since they cannot operate simultaneously. Without demand factors, nearly every 2,500+ square foot home with electric appliances would appear to require 400A service when 200A is actually adequate for the actual demand. Understanding these deductions helps you evaluate whether a service upgrade is truly necessary before calling an electrician.
Signs You Need a Service Upgrade
Several conditions indicate that your electrical service may be undersized for your current or planned loads. Frequent tripping of the main breaker — not individual branch circuit breakers — is the clearest signal that demand regularly exceeds the service rating. Dimming lights when large appliances like a central air conditioner or electric range start up can indicate voltage drop from a heavily loaded service conductor or a severely unbalanced panel. If you are planning to add a Level 2 EV charger, a hot tub, an air source heat pump, or plan to convert gas appliances to electric, run a full NEC 220 load calculation before purchasing any equipment. Panel replacement alone — swapping a full breaker panel for a new one with more spaces — costs $1,500–$3,500 including labor; a full service upgrade requiring the utility to replace the meter base and service entrance conductors typically runs $3,000–$8,000 or more depending on your distance to the transformer and local permit fees. Running the load numbers before starting any project prevents costly mid-construction surprises when the electrician discovers the service is too small.
Load Balancing Across A and B Legs
Single-phase residential service provides two hot conductors (called the A leg and B leg) each at 120V relative to neutral, or 240V between them. Every 120V single-pole breaker circuit draws current from one leg only, while 240V double-pole breakers draw from both legs equally. If all your heaviest 120V loads fall on the same leg, that leg runs hotter and at a slightly lower voltage than the other, which reduces efficiency and can cause nuisance voltage issues for sensitive electronics. Good panel design alternates large 120V loads — refrigerator, microwave, basement circuits — across the A and B legs so the current drawn from each leg stays as close to equal as possible. This is why electricians install breakers in a staggered pattern in the panel rather than filling one side before starting the other. Most modern load calculation software will show you the A-leg and B-leg totals separately so you can spot and correct significant imbalances before they become operational problems.