Electrical Panel Size Calculator
An electrical panel must be rated above the demand current your loads draw. The demand current is the total connected load in volt-amperes, reduced by a demand factor for diversity, divided by the service voltage. This calculator does that arithmetic, compares the result against a service size you select and the 80 percent continuous-load limit, and reports the next standard service size up. It is a transparent first estimate; a licensed electrician should run the official National Electrical Code load calculation.
Panel load formula
Demand load (VA) = connected load * demand factor / 100
Demand current (A) = demand load / service voltage
Percent of chosen service = demand current / chosen service * 100
Next standard service = smallest of (100, 125, 150, 200, 300, 400) that is at least demand current / 0.8
Dividing the demand load in volt-amperes by the service voltage gives current in amperes (apparent power: VA divided by volts). The 80 percent rule means the recommended service is sized so the demand current is no more than 80 percent of its rating.
Electrical service context
- US residential services are commonly 120/240 volts, single phase.
- Standard service ratings include 100, 125, 150, 200, 300 and 400 amperes.
- The NEC limits continuous loads to 80 percent of the breaker or service rating.
- Demand factors from NEC Article 220 reduce connected load to realistic demand.
- This is an estimate; a licensed electrician must perform the official load calculation.
Electrical panel size: frequently asked questions
How do I find the amps for an electrical panel?
Total amperes equal total connected load in volt-amperes divided by the service voltage. For 24,000 VA on a 240-volt service, that is 24,000 / 240 = 100 amperes. The panel main breaker and service must be rated above this demand load. This calculator does the division and applies a demand factor you set.
What is a demand factor?
Not every load runs at once, so codes apply demand factors that reduce the connected load to a realistic demand. The National Electrical Code defines these factors for dwellings. Enter your demand factor as a percentage; for example, 100 percent uses the full connected load, while a lower figure reflects diversity among loads.
What are standard US service sizes?
Common residential service sizes are 100, 125, 150, 200, 300 and 400 amperes. The calculator compares your computed demand against the next standard size up so you can see which service your load requires. Always confirm with a licensed electrician and your local code.
Why the 80 percent rule?
The NEC limits continuous loads to 80 percent of a breaker's rating. The calculator shows your demand as a percentage of a chosen service size so you can check it stays within the 80 percent limit, leaving headroom for continuous operation and future additions.
Is this a substitute for a code load calculation?
No. A proper service-entrance load calculation follows National Electrical Code Article 220, with specific rules for general lighting, appliances, heating and cooling. This tool gives a transparent first estimate using your own load and voltage figures. Have a licensed electrician perform the official calculation.
Official sources
- U.S. Consumer Product Safety Commission: Electrical Safety.
- National Institute of Standards and Technology: SI Derived Units (volt, ampere).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.