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Electrical Load Calculations
Every electrical decision on a project traces back to one number: the calculated load. Service size, panel capacity, feeder sizing, all of it depends on knowing exactly how much demand a building actually places on the system.
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Estimate this loosely, and the consequences don’t surface until the building is occupied and equipment can’t run simultaneously without tripping breakers, or a planned expansion turns out to exceed what the service can support.
As your electrical engineer, load calculations are the first step in any electrical scope, not a formality performed after equipment is already selected.
What an Electrical Load Calculation Actually Determines
The calculation totals every demand the building places on the electrical system. Lighting load, receptacle load, HVAC equipment, and any specialized loads specific to the project, kitchen equipment, EV chargers, process equipment, all get accounted for individually rather than estimated as a single lump figure.
The result determines service size, the overall capacity the utility connection needs to deliver, and feeds directly into panel schedules, establishing how much capacity each panel needs to serve its assigned circuits.
This calculation is building-specific. A load calculation for one project can’t be reused for another, even a similar one, since occupancy, equipment and use rarely match closely enough to substitute one calculation for another.
Why Load Calculations Have to Come Before Equipment Selection
It’s tempting to work backward, select equipment first, then size the electrical system to match. This sequence causes problems more often than it saves time.
Calculating load first establishes the actual ceiling the project has to work within. Selecting equipment first risks specifying something the service can’t support, discovered only when the load calculation is finally run, at which point revising equipment choices costs more time than doing the calculation first would have.
How HVAC Equipment Drives a Large Share of the Calculation
Mechanical equipment is frequently the largest single load in a building’s electrical calculation, and that’s especially true for heat pump systems, which draw electrical power for both heating and cooling rather than splitting that demand between electric and gas service.
This is why electrical load calculations are developed in close coordination with mechanical design. A change in HVAC equipment type or capacity after the load calculation is finalized often requires revisiting the calculation, and sometimes the service size, before drawings are issued.
Demand Factors and Diversity
Not every connected load runs at full capacity simultaneously, and code allows this reality to be reflected in the calculation through demand factors, adjustments recognizing that not all circuits draw their maximum rated load at the same time.
Applying these factors correctly requires understanding how a building actually operates, not just what’s connected to it. A restaurant kitchen and an office suite have very different diversity patterns, even if their connected load totals look similar on paper.
How This Connects to Panel Schedules and Service Sizing
The load calculation feeds directly into panel schedules, documenting how calculated demand gets distributed across circuits and panels throughout the building. It also determines whether a service upgrade is needed on a renovation or tenant improvement project, where existing capacity has to be checked against new demand before design can proceed.
We handle this calculation the same way for projects in Irvine, greater Los Angeles, and throughout California, building-specific numbers, not a regional default applied uniformly.
Plan Check Considerations
Reviewing agencies check that panel schedules and service sizing shown on drawings are actually supported by a documented load calculation. A panel rated for more capacity than the calculation justifies, or a service size that doesn’t match the total demand shown, is a common source of comments.
When a correction comes back, plan check response from the team that performed the original calculation resolves it efficiently, since the demand assumptions and diversity factors applied are already documented.
When You Need Electrical Load Calculations
Any project requiring electrical service sizing or panel design needs a documented load calculation as part of the permit submittal. This applies to new construction, renovations and tenant improvements, regardless of project size.
- Common Questions
Frequently asked questions
Calculating load first establishes what the electrical system can actually support. Selecting equipment first risks specifying something that exceeds available capacity, which then requires revisiting the equipment choice after the fact.
Demand factors account for the reality that not every connected load runs at full capacity simultaneously. Applying them correctly requires understanding how the specific building operates, not just totaling connected equipment ratings.
Mechanical equipment, particularly heat pump systems, is often the largest single electrical load in a building. This is why load calculations are coordinated closely with mechanical design rather than calculated independently.
No. Occupancy, equipment and use rarely match closely enough between projects to substitute one calculation for another. Each calculation reflects the specific building it's prepared for.
Usually a panel schedule or service size shown on the drawings that isn't clearly supported by the documented calculation. Keeping these consistent from the start reduces how often this comment appears.
Where we start
We begin with the architectural floor plan and equipment list, then calculate connected load and apply appropriate demand factors before any service or panel sizing is finalized. Send us your plans and project address, and we’ll confirm scope, along with a fee and schedule in writing.
2102 Business Center Drive, Suite 130, Irvine, CA 92612