Updated September 28, 2026
A commercial electrical load calculation should reflect the actual equipment, intended occupancy, applicable electrical code, and the part of the distribution system being designed. Adding equipment ratings together is a useful inventory step, but it does not by itself establish the required branch-circuit, feeder, or service capacity.
For an owner planning a tenant improvement, addition, or new commercial building, accurate equipment information is often the most useful contribution to the engineering process. It helps the electrical engineer distinguish existing loads, proposed loads, and capacity reserved for a defined future phase.
Start with the adopted electrical code
The National Electrical Code, NFPA 70, provides the electrical code framework. Confirm which edition and local amendments apply to the project. The newest published NEC is not automatically the edition enforced by every state or local authority.
For a North Carolina project, for example, consult the Office of State Fire Marshal's current and past codes page and confirm the applicable requirements with the permitting authority. For other states, use that state's official adoption information. Record the selected edition in the design criteria so the calculation, drawings, and review responses stay consistent.
Separate connected load from calculated demand
Connected load describes the loads included in the equipment and building inventory. Calculated demand is the result of applying the provisions that govern those loads and that portion of the electrical system. Demand factors are not a blanket discount that can be applied to every commercial project.
Keep quantities and units clear. Watts, volt-amperes, amperes, and horsepower describe different things; they should not be substituted for one another without the relevant equipment and system information. Voltage, phase, equipment ratings, and operating characteristics matter when the design is developed.
Why one universal formula is not enough
- Lighting, receptacles, motors, HVAC equipment, and other fixed equipment can be subject to different provisions.
- A rule that applies to a service or feeder does not automatically apply to each branch circuit.
- Continuous loads require the treatment specified by the applicable code and equipment conditions.
- Motor calculations require attention to the relevant motor rules; simply increasing every motor nameplate value by the same percentage is not a complete method.
- Existing buildings, new work, and changes in occupancy may call for different supporting records and calculation approaches.
For these reasons, a generic “first portion at full load, remainder at half load” shortcut should not be used as an across-the-board lighting calculation. The engineer needs to identify the actual load category, applicable provision, and conditions for any factor used.
Prepare a useful equipment and load schedule
A coordinated schedule reduces the chance that a late equipment selection will force revisions to several drawings. Provide the following information where available:
- Project scope and occupancy: intended use, floor areas, tenant boundaries, and the changes proposed.
- Electrical supply: available utility information, service voltage and phase, existing one-line diagrams, panel schedules, and relevant equipment ratings.
- Equipment details: manufacturer cut sheets and electrical data for HVAC, kitchen equipment, pumps, elevators, process equipment, and other substantial loads.
- Operating expectations: equipment expected to operate together, operating hours, and any documented operating controls.
- Special systems: EV charging, emergency or standby systems, solar or storage equipment, and planned future additions.
- Existing records: previous approved plans and any available utility or measured-demand records that the engineer requests.
Identify unknown values as unresolved items. A preliminary equipment allowance should not silently become a final specification. Maintain a dated equipment list and record substitutions so the electrical and mechanical plans describe the same project.
Coordinate the calculation with the permit documents
The final package should tell a consistent story: the calculation, one-line diagram, panel schedules, equipment selections, and site or floor plans should agree. The design also needs the appropriate checks beyond load totals, such as conductor and protection selection and other requirements relevant to the installation.
Ask the project team who is coordinating with the utility, who will answer plan-review comments, and how revised equipment data will be distributed. A comment response is most useful when it identifies the affected calculation and drawing revisions rather than supplying an isolated number.
Common owner questions
Does unused space in a panel mean there is capacity for new equipment?
Not necessarily. Available breaker positions do not establish available electrical capacity. Provide the existing documentation and proposed equipment information so the engineer can assess the relevant system limits.
Can an old calculation be reused for a new tenant?
It is a starting record, not an automatic approval. New equipment, a change in use, or different operating conditions may change the design requirements.
Should we add a standard percentage for future growth?
Describe the future equipment or expansion you expect. The engineer can then address a defined design allowance and its implications instead of relying on an arbitrary percentage.
Contact Gunderson Engineering with your location, commercial project scope, available plans, and equipment schedule to discuss coordinated electrical engineering.