Electrical Service & Panel Load Calculator
Uses the NEC 220.82 optional calculation for a single-family dwelling to recommend a service size — for a whole house or panel, not a single circuit.
| General load (before demand factor) | |
|---|---|
| General load (after demand factor) | |
| Fixed appliance load | |
| Total calculated load | |
| Calculated amperage |
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Signs an existing panel is undersized
Before assuming you need this calculator for a brand-new service, a few real-world signs suggest an existing panel may already be maxed out: breakers tripping when multiple large appliances run together, an electrician declining to add a circuit "because there's no room," a panel still using fuses or rated below 100A, or a home that's added significant load since the panel was installed — a garage conversion, a hot tub, an EV charger, a heat pump replacing a gas furnace. Any of these is a reason to actually run the numbers rather than guess.
Why general loads get a demand factor and appliances don't
Not every lighting and receptacle circuit in a house is used at full capacity simultaneously — the NEC accounts for that by taking the first 10,000 VA of general load at 100%, then only 40% of everything above that. Large fixed appliances (heating, cooling, water heater, range, dryer) are counted at their full 100% nameplate rating instead, since those genuinely can run at or near full draw. Only the larger of heating or cooling counts, not both added together — a home doesn't run its furnace and AC at the same time.
How to use this calculator
Enter your home's square footage, small-appliance and laundry circuit setup, and the nameplate wattage of your major fixed appliances. This runs the NEC 220.82 "optional calculation" — the method most residential service sizing is actually done with, since it's simpler than adding up every branch circuit individually and produces a realistic result by applying a demand factor to general loads.
Worked example
A 2,000 sq ft home, 2 small-appliance circuits, laundry circuit, 5,000W heating, 3,500W cooling, 4,500W water heater, 8,000W range, 5,000W dryer: general load is 2,000×3 + 2×1,500 + 1,500 = 10,500 VA, which demand-factors down to 10,200 VA. Fixed load is 5,000 (larger of heat/cool) + 4,500 + 8,000 + 5,000 = 22,500 VA. Total: 32,700 VA ÷ 240V = 136A calculated, which rounds up to a standard 150A service.
Frequently asked questions
What's the difference between this and a wire/circuit calculator?
The wire size and ampacity calculators size an individual circuit's conductor. This tool sizes the main service or panel — the total capacity feeding the whole building, not any one circuit.
Does this work for a panel upgrade, not just a new service?
Yes — the same load calculation determines whether an existing panel has room for added circuits or needs to be upgraded to a larger main breaker/service size.
Is the optional method always allowed?
It applies to a single dwelling unit under NEC 220.82, which covers the large majority of single-family homes. Multi-family buildings, mixed-use, and some larger or unusual dwellings may require the standard method (NEC 220 Part III) instead — a licensed electrician can confirm which applies to your project.
Where does an EV charger fit into this calculation?
Add its nameplate wattage to "other fixed appliances" — a Level 2 home charger commonly draws 7,000-11,500W continuously, which is often the single biggest addition that pushes an older 100A or 125A service into needing an upgrade. It's one of the most common real-world reasons homeowners run this calculation today.
Based on the NEC 220.82 optional calculation method for single-family dwellings. Reference only — verify against the current NEC and local code, and consult a licensed electrician before installing or upgrading a service.