Free Tool
Free electrical service size calculator
Estimate the service a home needs, in amps, from its square footage and major appliances. A rough NEC standard-method check, not a permit document. Free, no signup.
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A gut check on the panel
Before you quote a panel upgrade, it helps to know roughly what the house actually needs. This runs a simplified version of the NEC standard method to get you a ballpark service size.
It starts with general lighting at 3 volt-amps per square foot, adds the small-appliance and laundry circuits every home has, then applies the code demand factor: the first 10,000 volt-amps count in full and the rest at 40 percent. On top of that it adds the big fixed loads you check, like the range, dryer, and water heater, plus the air conditioning.
Divide the total by 240 volts and you have the calculated load in amps, which the calculator rounds up to the next standard service size. A house that lands near 140 amps calculated usually gets a 150 or 200 amp service for headroom.
This is a sanity check, not a permit. The real load calculation depends on the exact appliances, the code cycle your area is on, and local amendments, and a licensed electrician has to run it and pull the permit. Use this to sniff out whether a panel is undersized before you write the quote.
Electrical Service Size Calculator questions
What size electrical service do I need?
It depends on the square footage and the major appliances. This estimates the calculated load and suggests a standard service size, as a starting point.
What is the NEC standard method?
A load calculation that totals general and appliance loads, applies a demand factor, and divides by the service voltage. This tool uses a simplified version.
Can I use this for a permit?
No. It is a rough estimate. A licensed electrician must run the actual load calculation and pull the permit for the work.
Is this really free?
Yes. No email, no signup, and nothing you type leaves your browser.
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Disclaimer: These free tools and templates are provided for general information and estimating purposes only. All results — including quantities, material and cost estimates, load, sizing, and span figures, scores, and any guidance or warnings — are rough approximations that may not fit your specific project, site conditions, local building codes, or current prices. They are not engineering, architectural, structural, legal, financial, or other professional advice, and are not a substitute for a licensed engineer, architect, contractor, or other qualified professional. Always confirm quantities, sizing, and structural, electrical, mechanical, and code requirements with a licensed professional and your local authority having jurisdiction before purchasing materials or performing any work. Werx makes no warranty as to accuracy and accepts no liability for any decision made using these tools. Use at your own risk. See our Terms of Service.