Breaker Box Load Calculator — Free Online Calculator
Check your breaker box total load and remaining capacity. Find out if you can add more circuits.
How to Use This Calculator
Select panel rating, enter current total load and the new load you want to add.
The Formula Explained
NEC 80% rule: Continuous loads should not exceed 80% of the panel rating. Total load with new addition must stay under this limit.
Panel Load Calculation: The Foundation of Service Sizing
Before you can choose a breaker panel size or plan a service upgrade, you need to know the actual electrical demand of the building. Simply adding up every appliance nameplate overstates the load because appliances rarely run simultaneously at full power. The NEC recognizes this with "demand factors" that discount certain load categories based on typical usage patterns. Two methods exist for residential: the standard method (NEC 220 Part III) and the optional method (NEC 220.82). Both are approved; the optional method usually gives lower numbers and is preferred for new construction.
The calculation answers critical questions: is my current service adequate for a new EV charger or heat pump? Does my proposed 3,000 sq ft addition require a 400A service upgrade or can I stay at 200A? How much capacity do I have available for future expansions? Getting the calculation right avoids expensive over-installations and protects against dangerous overloaded services. Many old 100A panels on modern homes with electric dryers, electric ranges, and AC are marginal or outright overloaded by current standards.
Worked Example: 2,500 Sq Ft Home, Standard Method
Home characteristics: 2,500 sq ft habitable area, 40-gallon electric water heater (4,500W), electric dryer (5,500W), electric range (12,000W), 3-ton central AC (5,000W at 240V), no electric heat (gas furnace). Calculation per NEC 220 Part III:
General lighting: 2,500 × 3 = 7,500 VA. Small appliance circuits: 2 × 1,500 = 3,000 VA. Laundry circuit: 1 × 1,500 = 1,500 VA. Subtotal general: 12,000 VA.
Apply demand factor: first 3,000 VA at 100% = 3,000 VA. Next 117,000 VA at 35% (applies to 12,000 - 3,000 = 9,000 VA) = 3,150 VA. General load demand: 3,000 + 3,150 = 6,150 VA.
Specific appliances: Water heater 4,500 VA. Dryer (NEC 220.54): 5,000 VA (minimum). Range (NEC 220.55 Column C): 8,000 VA (demand for single range up to 12 kW). Total specific: 17,500 VA.
Larger of heat or cool: cooling 5,000 VA (no electric heat).
Total: 6,150 + 17,500 + 5,000 = 28,650 VA. At 240V: 28,650 / 240 = 119.4 amps. Service size: 125A (minimum standard size above 119.4) or more practically 150A or 200A for future margin. Most new homes of this size are built with 200A service standard.
Worked Example: All-Electric Home with EV, Optional Method
Home: 3,200 sq ft, heat pump (5 tons / 18,000 VA cooling, 15,000 VA heating with backup heat strips), electric water heater (4,500W), electric dryer (5,500W), induction range (9,600W), Tesla Wall Connector (11,520 VA continuous). Calculation per NEC 220.82 optional method:
Step 1 — General lighting and receptacle loads: 3,200 × 3 = 9,600 VA. Small appliance: 2 × 1,500 = 3,000 VA. Laundry: 1,500 VA. Fixed appliances (non-motor): water heater 4,500, dryer 5,500, range 9,600, EV charger 11,520. Total non-heating load: 45,220 VA.
Step 2 — Apply 40% rule: first 10,000 VA at 100% = 10,000. Remainder 35,220 at 40% = 14,088. Non-heating demand: 24,088 VA.
Step 3 — Heating or cooling, whichever larger: 18,000 VA cooling (vs 15,000 heating). Use 18,000 at 100% per 220.82.
Total demand: 24,088 + 18,000 = 42,088 VA. At 240V: 175 amps. Service size: 200A minimum, though 200A is marginal (87.5% utilization). Consider 300A or 400A service for long-term capacity, especially if adding a second EV or ground-source heat pump later.
Note how the optional method gave 42,088 VA total vs summing all nameplates (about 72,000 VA without demand factors). The diversity factor is the difference between what you could theoretically use and what you actually use.
Five Panel Load Mistakes
1. Adding all nameplates without demand factors. This overstates load by 40-60% and leads to unnecessary service upgrades. Use NEC 220 methods correctly.
2. Forgetting the 125% continuous load rule for EV chargers. A Tesla Wall Connector at 48A continuous requires a 60A breaker and counts as 60A × 240V = 14,400 VA in the load calculation, not 48 × 240.
3. Missing specific appliance demand tables. NEC 220.54 (dryer) and 220.55 (range) have specific tables that usually reduce the nameplate value. A 12 kW range counts as only 8 kVA in load calcs.
4. Not planning for future loads. A 100A service that is 70% utilized today has little room for a future EV, heat pump, or solar battery. Design for 10-20 year needs, not current loads only.
5. Confusing panel rating with calculated load. A 200A panel main breaker does not mean 200A of actual load — it means the panel can handle up to 200A safely. Calculated load should be 60-80% of panel rating for reasonable headroom.
Standard Service Sizes and Typical Homes
60A service: Historic/old homes, cabins. Inadequate for modern appliances. Cannot support electric dryer + electric range + AC simultaneously.
100A service: Small homes (under 2,000 sq ft) with gas heat and hot water. Adequate for basic appliances. Marginal with EV charger; cannot support heat pump upgrade.
150A service: Medium homes with gas heat. Comfortable with one EV charger. Still marginal for all-electric retrofit.
200A service: Standard for new construction. Most common residential size. Handles typical 2,500-4,000 sq ft home with EV and heat pump.
300A service: Not a standard size, uncommon.
320A combination service: Common upgrade when 200A not enough — two 200A panels fed from 320A meter.
400A service: Large homes (5,000+ sq ft), all-electric with multiple EVs, or homes with heat pumps plus electric water heating plus significant general load.
For commercial services, sizes go to 800A, 1,200A, 1,600A, and 2,000A for typical small-to-medium businesses. Above that, medium-voltage service becomes economical.
NEC Code References
NEC 220.12 — general lighting load (VA per square foot by occupancy type). NEC 220.14 — receptacle loads. NEC 220.52 — small appliance and laundry loads. NEC 220.54 — dryer demand. NEC 220.55 — range demand with Column C table. NEC 220.60 — non-coincident loads (heating OR cooling, whichever is larger).
NEC 220.82 — optional method for dwelling units (the 40% rule). NEC 220.83 — optional method for existing dwelling unit calculations when adding loads. NEC 220.84 — multifamily dwelling optional method. NEC 230.79 — service disconnect rating minimum (60A for single-family dwelling). Use the standard method when in doubt; use the optional method when you want the lower number for new construction.
Panel load calculation: NEC 220 standard method for residential service sizing
The panel load calculation determines the minimum service size for a residence. The NEC 220 standard method sums general lighting load, small appliance circuits, laundry, and specific named appliances, then applies demand factors that account for the unlikelihood of all loads running simultaneously. The result is the minimum service rating in amps.
The formula and what it does
General lighting and small-appliance loads (3 VA/sq-ft for lighting, 1500 VA each for SA circuits and laundry) get demand factors per Table 220.42: first 3000 VA at 100 percent, next 117,000 VA at 35 percent. Major appliances (range, dryer, water heater, HVAC, EV charger) get their own demand factors per their respective NEC sections.
Worked example
Scenario: 2400 sq-ft home, 12 kW range, 5 kW dryer, 4.5 kW water heater, 4-ton heat pump (28 A LRA), 7.2 kW EV charger. Find service size.
General lighting: 2400 sq-ft x 3 VA = 7200 VA. Small appliance (2 circuits): 3000 VA. Laundry: 1500 VA. Subtotal general: 11,700 VA. Demand factor: first 3000 at 100% + remainder (8700) at 35% = 3000 + 3045 = 6045 VA.
Range (NEC 220.55): 12 kW at column C demand = 8 kW = 8000 VA. Dryer (220.54): 5000 VA at 100% = 5000 VA. Water heater: 4500 VA. Heat pump: 28 A x 240 V x 1.25 = 8400 VA. EV charger (625.41): 7200 VA at 100%.
Total: 6045 + 8000 + 5000 + 4500 + 8400 + 7200 = 39,145 VA. Service amps: 39,145 / 240 = 163 A. Minimum service: 200 A (next standard above 163). 200 A is the typical residential service for this loadout.
Code references and standards
NEC 220.42 general lighting demand factors: first 3000 VA at 100%, 3001-120,000 at 35%, above 120,000 at 25%.
NEC 220.52 small-appliance and laundry branch circuits: 1500 VA each.
NEC 220.53 appliance load: 75% demand factor if four or more appliances.
NEC 220.54 dryer load: 5 kW or nameplate, whichever larger.
NEC 220.55 Table range loads with demand factors by number of ranges and total kW.
NEC 220.82 optional calculation: simpler method allowed for dwelling units, often gives slightly higher load.
Common mistakes to avoid
Confusing standard and optional methods. NEC offers two: 220 Part III (standard) and 220.82 (optional). Both are code-compliant. Optional is simpler; standard usually gives a slightly lower required service.
Forgetting future EV/electrification load. Many older 100 A and 150 A services are at capacity; adding EV charging or heat pumps requires a service upgrade. Calculator helps verify before installing.
Ignoring HVAC LRA vs running amps. NEC 440.32 motor compressor circuit ampacity is 125% of largest motor running current.
Frequently asked questions
When do I need a service upgrade?
When the calculated load exceeds your current service size. Common triggers: adding EV charging, electric heat pump replacing gas furnace, electric water heater conversion, adding a hot tub or pool heater.
What size service does an average home need?
Modern code-compliant calculations typically yield 150-200 A for an all-electric 2000-3000 sq-ft home. Older homes with gas heat and gas water heating often run on 100 A successfully.
Why are old houses on 60 A or 100 A service?
NEC minimum service was 60 A from 1959-1971, 100 A from 1971-1983. Pre-EV, pre-heat pump, pre-electric-everything, those services were adequate. Modern electrification pushes most homes to 200 A.
Can I add EV charging without upgrading my panel?
Often yes, with load management. NEC 625.42 allows Energy Management Systems that throttle EV charging when other loads are active. Several products (Wallbox, Span Panel, ChargePoint) do this. Lets a 100 A service support a Level 2 charger without an upgrade.
What is the difference between standard and optional calculation?
NEC 220 Part III (standard method) sums calculated demand-factored loads. NEC 220.82 (optional) applies single demand factors to total load. Optional usually shows a slightly higher load requirement but is much faster to calculate.
How does this differ from individual circuit sizing?
Panel load is the total service requirement. Individual circuit sizing (separate calc) is the breaker for each branch. Both required; serve different purposes.