Load Calculator

Apparent power is the total power drawn from the source — voltage multiplied by current. Real power is what actually gets converted into useful work, after accounting for the power factor. For resistive loads like electric heaters or incandescent lights, the two numbers are nearly identical. For inductive loads like motors, HVAC compressors, and pumps, the gap widens — meaning the circuit is drawing more current than the wattage alone suggests. Always size conductors and panels on apparent power (VA), not just watts, to avoid undersizing.

Use 0.8 as a safe default for mixed residential or light commercial loads. Purely resistive loads (baseboard heat, incandescent lighting) sit close to 1.0. Motor-driven equipment — HVAC units, well pumps, compressors — typically falls between 0.7 and 0.9. When in doubt, go lower. A conservative power factor builds in headroom and keeps your calculations on the safe side of NEC minimums.

Run a load calculation using the total connected load across all circuits. If the calculated demand exceeds the service rating — typically 100A for older homes and 200A for newer construction — the panel is undersized for safe operation. Common warning signs include frequently tripped breakers, flickering lights under load, or a panel already maxed out on breaker slots. A load calculator gives you the numbers to back up your recommendation before the customer even sees the panel.

Start with a load calculation. If total demand exceeds service capacity, a service upgrade is likely unavoidable. If you’re under capacity but still tripping breakers on specific circuits, the issue is usually load imbalance across phases — redistributing circuits or adding a subpanel can resolve it without touching the meter. The load calculator helps you make that call quickly, so you’re not guessing on-site or over-scoping the job.

The calculator works well for straightforward commercial loads — retail spaces, small offices, or light industrial setups with standard voltage and current inputs. For large commercial or industrial projects with complex demand factors, motor loads, or three-phase systems, NEC Article 220 Part IV applies and the calculations get more involved. Use this tool to build initial estimates and ballpark service sizing; have a licensed engineer verify the final numbers on permitted commercial work.

Skipping a proper load calculation is one of the fastest ways to blow a bid. Undersizing a panel means a costly change order mid-job. Oversizing means you priced yourself out of the work. Running the numbers before you quote lets you specify the right service size, conduit fill, wire gauge, and breaker count from the start — so your materials list is accurate and your labor estimate isn’t built on guesswork.