{{ summaryHeading }} {{ summaryPrimary }} {{ summaryLine }} {{ statusLabel }} {{ methodLabel }} {{ formatNumber(values.load_percent, 0) }}% of rating
Optional modes use one full-value block plus a remainder factor; Standard screen keeps general-load tiers separate.
Accepted planning range is 100–25,000 square feet.
sq ft
The modeled demand is compared with this rating and the planning target below it.
A
Use 80% for the starter planning screen; change it only to match the decision you are comparing.
%
circuits
kW
W
W
W
W
A
Example: Dishwasher,1200,75,fixed. Keep permit-critical nameplates separate and explicit.
Worksheet floor-area allowance.
VA/sq ft
Allowance applied to each entered small-appliance circuit.
VA
Fixed dwelling laundry allowance.
VA
Full-value portion of the optional-style base.
kVA
Demand percentage above the first block.
%
Multiplier applied to entered charger current and service voltage.
%
Presentation rounding only.
MeasureValueMeaningCopy
{{ row.measure }}{{ row.value }}{{ row.meaning }}

Planning checks before installation

{{ note.title }}

{{ note.body }}

Wire Gauge Calculator Size an AWG conductor with voltage-drop and ampacity checks using current, run length, material and a temperature reference column.
Extension Cord Voltage Drop Calculator Estimate extension-cord voltage drop and load voltage from copper AWG, length, and current with checks for heat loss and safe run length.
Generator Watts Calculator Size generator running and starting wattage from a load inventory, with startup overlap and site-derating assumptions for equipment comparison.
Power Factor Calculator Calculate power factor from kW and kVA, size leading or lagging reactive correction, and estimate per-phase capacitance when voltage is known.

Adding the nameplate ratings of every appliance usually overstates a home's expected service demand because many loads do not run together and electrical codes apply specific demand factors. Simply reading the main breaker's ampere rating understates the problem because it says nothing about the calculated load already assigned to the service.

A dwelling load calculation converts floor-area allowances, required circuits, cooking and laundry equipment, water heating, heating or cooling, electric-vehicle charging, and other fixed loads into one demand value. That demand is then divided by the service voltage to estimate amperes and compared with the existing service rating.

TermMeaningWhy it matters
Connected loadThe total equipment or allowance value before demand factorsIt is an input to the code method, not necessarily the service demand.
Demand loadThe load remaining after the selected method's factors and combinationsThis is the value converted to service amperes.
Continuous loadA load expected to run at maximum current for the code-defined durationEquipment such as EV charging may require a 125% circuit-sizing treatment.
Headroom targetA planning limit below the full service ratingIt is a chosen comparison, not a universal code pass/fail rule.

The calculation method and jurisdiction cannot be separated. Optional and standard dwelling methods group loads differently, and adopted code editions or local worksheets can change what belongs in each group. Heating and cooling are often noncoincident, so only the larger applicable load is counted, but auxiliary heat, motors, energy-management systems, and unusual appliances may need special treatment.

Future electrification makes small assumptions consequential. A 40 A EV charger at 240 V becomes 12,000 VA when a 125% multiplier is used. Heat-pump backup strips can exceed the compressor load. An appliance nameplate, locked charger setting, or listed load-management control may change the correct value more than rounding ever will.

A favorable estimate does not authorize a panel change or prove conductor, breaker, bus, meter, utility, fault-current, or physical-space adequacy. Permit-critical work should use the locally accepted worksheet and nameplates, with the final design reviewed by the authority having jurisdiction and a licensed electrician.

How to Use This Tool:

Use the closest local calculation path, then replace every starter allowance with verified dwelling and equipment data.

  1. Select Calculation method and enter the dwelling floor area, existing service rating, service voltage, and planning target. Confirm that the chosen method matches the worksheet accepted for the project.
  2. Enter general and appliance loads from nameplates or the governing allowance. Keep a zero only for equipment that is genuinely absent.
  3. Choose the HVAC basis. For electric heat or heat-pump backup, enter both cooling and heat values so the larger modeled load can be selected.
  4. Add EV and other loads. Each additional-load row uses name, watts, demand percent, and class; correct a flagged row before trusting the result.
  5. Review Demand amps, Target spare, and Recommended service. Treat the recommendation as a planning size, then have the complete load calculation and installation constraints checked professionally.

Interpreting Results:

  • Spare capacity means modeled demand is below both the selected target and the entered rating.
  • Limited spare means demand is at or below the target but no more than 5% of the service rating remains before that target.
  • Above target means the planning target is exceeded while the full entered rating is not.
  • Over entered rating means modeled demand is greater than the service rating.

Exactly meeting the target is classified as limited spare. The recommended service is the first common size whose selected target can hold the modeled demand; it is not a permit approval or a claim that the existing equipment can be uprated.

Technical Details:

Loads are accumulated in volt-amperes (VA), then converted to amperes using the selected service voltage. The model is a dwelling-service planning screen built from public jurisdiction worksheets plus explicit planning extensions. It does not reproduce every rule in NFPA 70 or any local amendment.

Formula Core:

After the selected demand method produces the base load, HVAC, EV, and nonfixed additions are added before the current conversion.

Stotal=Sbase+SHVAC+SEV+SotherIdemand=StotalVservice
Itarget=Irating×P100,Ispare=Itarget−Idemand
QuantitySourceUnit
SbaseGeneral load, range, dryer, water heater, and fixed-load demand under the selected methodVA
SHVACCooling, electric heat, or the larger of cooling and entered heatVA
SEVEV current × service voltage × selected continuous-load percentageVA
PSelected planning target%

Rule Core:

StageModeled treatment
General connected loadFloor area × allowance + small-appliance circuits × allowance + laundry allowance.
Optional baseThe first entered demand block is counted at 100%; the remainder uses the entered percentage.
Standard general loadFirst 3,000 VA at 100%, next 117,000 VA at 35%, and amounts above 120,000 VA at 25%.
Standard range and dryerA range from more than 0 through 12 kW uses 8,000 VA; an entered dryer uses at least 5,000 VA.
Standard fixed appliancesFour or more positive fixed loads use 75% of their connected total; fewer use 100%.
HVACCooling-only uses cooling. Electric-heat and heat-pump-backup paths use the greater of cooling and entered heat.
EVEntered current × voltage × the selected multiplier, 125% by default.

For the default 2,200 ft² example, the optional base is 20,120 VA. Cooling adds 4,800 VA, a 40 A charger at 240 V and 125% adds 12,000 VA, and a nonfixed freezer adds 600 VA. Total demand is 37,520 VA or about 156.3 A. Against a 200 A service and 80% target, only about 3.7 A remains before the 160 A target, so the result is limited spare.

Limitations:

Do not use this estimate as the sole basis for electrical work.

  • Adopted code edition, local amendments, service configuration, and accepted calculation forms vary by jurisdiction.
  • Nameplate loads, motor rules, neutral calculations, optional-method eligibility, energy-management controls, and existing measured demand may require different treatment.
  • Panel spaces, bus rating, conductor ampacity, meter and utility limits, available fault current, grounding, and breaker compatibility are outside the calculation.
  • Installation and service changes can cause fire, shock, and equipment hazards; use a licensed electrician and required permits.

Worked Examples:

Electric heat presses a 150 A service

A 2,000 ft² dwelling under the standard screen has a 24,025 VA base. With 10,000 W electric heat, total demand reaches 34,025 VA or about 141.8 A at 240 V. That is below the 150 A rating but 21.8 A above an 80% target, so the result is above target and the planning recommendation moves to 200 A.

References: