Electrical Load Calculator

Total Circuits is a plain sum of the four circuit-count inputs -- Lighting, Outlet, Appliance, and HVAC Circuits -- and is completely unaffected by any of the four watts-per-circuit inputs, since it counts circuits, not load. Outlet Circuits carries the most weight in that sum at the calculator's defaults (8 of 16 total circuits), so it moves Total Circuits more than Lighting, Appliance, or HVAC Circuits do for an equal percentage change. Total Connected Load, by contrast, is a sum of four category totals (circuits times watts per circuit for each category), and at the defaults the Appliance category is the single largest contributor at 12,000 W of the 29,000 W total -- larger than Lighting (2,400 W), Outlet (9,600 W), or HVAC (5,000 W) -- so Appliance Circuits and Watts per Appliance Circuit each move Total Connected Load more than the corresponding lighting or HVAC inputs do. NEC Demand Load applies the standard demand factor (100% of the first 10,000 W of connected load, 40% of the remainder), which is why it comes out well below Total Connected Load once the total clears 10 kW, and why Panel Size and Main Breaker are sized off that reduced demand figure rather than the raw connected load. Panel Size and Main Breaker only change at specific NEC-standard tier boundaries (100, 125, 150, 200 A, and so on), so a small change to any single circuit input often leaves them unchanged even though it does move the underlying demand amps -- only a large enough swing pushes the recommendation to the next standard size. Panel Size is chosen from a 100-400 A table and Main Breaker from a narrower 100-200 A table; at very large loads the required capacity (demand amps times the NEC's 125% continuous-load factor) can exceed either table.

Exceeds Standard Sizing (1=Yes) fires when required capacity exceeds the Panel Size table, the Main Breaker table, or both -- but because the breaker table's ceiling (200 A) is narrower than the panel table's (400 A), there is a real range of loads (required capacity above 200 A but at or below 400 A) where the flag fires from the breaker table alone while the displayed Panel Size is still a genuinely valid, correctly-sized recommendation. Before discarding both numbers, check which one actually hit its table's ceiling -- e.g. Main Breaker reading exactly 200 A while Panel Size sits comfortably below 400 A means only the breaker needs a custom size; Panel Size itself is fine as shown. Only when Panel Size is also pinned at 400 A (or the underlying required capacity exceeds it) is the panel recommendation itself invalid. This tool applies the general demand-factor method from Article 220 of NFPA 70, the National Electrical Code (NEC) -- the first 10 kW of connected load at 100% and the remainder at 40% -- plus the NEC's 125% continuous-load factor for the panel and breaker sizing step; it does not substitute for a full NEC Article 220 load calculation with your local utility and inspector.