Service Calculations

A service calculation is not breaker-handle arithmetic. It is a documented model of how the building uses power, built from real loads and the demand rules that apply to that occupancy and edition of the Code.

Define the calculation before reaching for a table

Are you sizing a new service, feeder, panel, transformer, generator, or an addition to an existing building? Is the occupancy a dwelling, office, restaurant, workshop, multifamily building, or mixed-use property? The load categories and permitted calculation methods change with the answer.

Confirm the system voltage and phase, adopted Code edition, local amendments, utility requirements, plans, specifications, and known future loads. Write the scope at the top of the worksheet. A correct calculation for the wrong occupancy or system is still wrong.

Build the connected-load list from evidence

Use equipment nameplates, schedules, approved submittals, mechanical data, appliance ratings, and area information. Separate general lighting, receptacles, appliances, cooking, drying, water heating, space conditioning, motors, elevators, EV charging, process equipment, signs, and other loads as the applicable method requires.

Do not substitute breaker size for load unless the rule or equipment data makes that appropriate. Breakers protect circuits and often exceed normal operating current. Do not count the same equipment twice because it appears on both a mechanical schedule and a panel schedule.

Keep units under control

Load calculations often begin in volt-amperes. Single-phase and three-phase current use different relationships. Convert only when you know whether the value is watts, volt-amperes, or amperes and whether power factor and efficiency belong in the step.

Apply demand factors only where the adopted rules permit

Demand factors recognize that certain groups of loads are unlikely to operate at maximum simultaneously. They are specific allowances, not a blanket percentage to make the service fit. Apply each factor to the correct load category, quantity, occupancy, and method, and show the subtotal before and after.

Some loads are noncoincident, meaning the design can use the larger of two loads that will not operate together. Heating and cooling are a common example, but only when the controls and installation truly prevent simultaneous operation and the applicable method allows that treatment. Supplemental heat and other combinations can change the answer.

Continuous loads and motors get special attention

A continuous load operates at maximum current for a defined extended period. Conductors and overcurrent devices may need additional capacity for that duty. Identify those loads from the operating description instead of assuming every commercial load is continuous or none of them are.

Motor calculations coordinate several different numbers. Conductors, overload protection, short-circuit and ground-fault protection, and the service or feeder load do not necessarily use the same current value or multiplier. Multiple motors may require an addition based on the largest motor plus the others according to the applicable rules. Show which rule each line follows.

Dwelling methods are structured, not casual

Dwelling calculations account for general lighting and receptacle load, required small-appliance and laundry circuits, fastened-in-place appliances, cooking equipment, dryers, heating and air conditioning, motors, and other loads. Standard and optional methods have their own eligibility and steps.

Do not mix the most favorable pieces of different methods. Pick a permitted method and follow it from beginning to end using the tables and notes in the adopted edition. Document appliance ratings and whether loads are included, excluded, or treated as noncoincident.

Convert the final load into a complete service design

After calculating amperes, coordinate the service rating, conductors, terminal temperature limits, overcurrent protection, disconnects, meter equipment, panel ratings, grounding and bonding, available fault current, and interrupting ratings. Check voltage drop and phase balance as design concerns even when the calculation establishes enough ampacity.

For three-phase systems with many single-phase loads, distribute them sensibly and calculate the neutral according to the actual load characteristics and applicable rules. Nonlinear loads can change neutral behavior. A perfectly balanced spreadsheet does not guarantee a balanced building after tenants plug in equipment.

Existing services require measurement and history

For an addition, inspect the actual service and verify ratings, conductor sizes, condition, available spaces, and connected equipment. Permitted load-study methods may use measured maximum demand over a defined period, but the data has to represent the real operating cycle and account for the new load.

A short reading on a mild day does not establish capacity for seasonal heating, cooling, kitchen peaks, or production schedules. Review interval data, records, and operating conditions as required. If the existing labels and schedules are unreliable, fix the information gap before trusting the calculation.

Build a worksheet another electrician can audit

1. State occupancy, system, scope, edition, method, and assumptions.
2. List each connected load with source, units, quantity, and phase.
3. Group loads by the categories used in the calculation method.
4. Show every demand factor, continuous-load adjustment, motor addition, and noncoincident choice.
5. Carry units through each subtotal and conversion.
6. Record the final calculated load and selected standard equipment rating.
7. Check conductors, terminals, protection, fault ratings, voltage drop, and future capacity separately.
8. Have the design reviewed and approved through the required professional, utility, permit, and AHJ process.

The best service calculation is not the smallest number you can defend. It is a transparent, repeatable calculation that matches the building, coordinates with the equipment, and leaves no hidden assumptions for the installer or inspector to discover.