Existing Service Capacity Calculator
Check an existing one-family dwelling service against proposed EVSE, HVAC, and other electrical loads.
Service and load details
Uses 2 VA per square foot in the existing-load calculation.
Add existing ranges, dryers, fixed appliances, EVSE, and other connected equipment. Do not include HVAC entered below.
New equipment loads
Include new appliances and equipment that are not EVSE or HVAC.
Calculated at 240 V line-to-line.
About this calculator
Use this worksheet to screen an existing one-family dwelling service when adding EVSE, HVAC equipment, or other electrical loads. It calculates the existing dwelling load, applies the listed demand factors to proposed loads, and compares the result with the service rating you enter.
Enter connected loads before applying the 2026 existing-dwelling demand factors. Use equipment nameplate data and the actual service configuration.
Method and assumptions
The existing-load subtotal uses 2 VA per square foot, 1,500 VA for each small-appliance circuit and laundry circuit, entered existing appliance nameplates, and the larger combined air-conditioning or space-heating load.
The calculator applies the 2026 NEC existing-dwelling method: the first 8 kVA of combined existing and new load is counted at 100%; the remaining existing load is counted at 40%; new EVSE and new central electric resistance space heating are counted at 80%; and other new loads are counted at 50%.
To prevent double counting, the first-tier allocation counts other new loads first, followed by EVSE and central electric resistance heat, then existing load. Confirm this calculation against the adopted edition, installation details, and the authority having jurisdiction.
Interpret the result
Calculated load current is the calculated VA divided by the selected line-to-line voltage. Remaining capacity compares calculated load VA with the entered service ampacity at that voltage. A positive value does not verify service equipment or conductor suitability.
FAQs
What number should I enter for the existing appliance load?
Enter the connected nameplate loads for existing ranges, dryers, fixed appliances, existing EVSE, and other equipment. Leave out the general dwelling load and HVAC because the calculator has separate inputs for those.
How are existing heating and air conditioning handled?
The calculator compares the combined existing and proposed cooling load with the combined heating load. It includes the larger total and excludes the other, based on the entered operating conditions.
Why are new loads entered in separate categories?
The 2026 NEC existing-dwelling method applies different demand factors to new EVSE, central electric resistance space heating, and other new loads. Enter each load once in its matching field.
Does a result below the service rating mean an upgrade is approved?
No. This is a screening estimate. It does not evaluate every Article 120 rule, service equipment rating, conductor ampacity, load-management system, utility requirement, or local amendment.
Sources
- NFPA 70, National Electrical Code (NEC), 2026 edition, §120.83 and Table 120.83.
- Confirm the edition adopted by the authority having jurisdiction, current errata, equipment instructions, and local amendments.