8.1 General & Non-Dwelling Load Calculations

8.1 General & Non-Dwelling Load Calculations

Commercial calculation questions are easiest when treated as an accounting problem. List every load, identify the controlling rule, convert each to volt-amperes, apply only authorized demand factors, and then convert the final VA to current. Skipping the classification step is the usual reason a calculation goes wrong.

Connected load, demand, and conductor sizing

Connected load is the sum before permitted diversity. A demand factor reduces a specified group only where the NEC authorizes it. A continuous load is one expected to run at maximum current for the defined duration; branch-circuit and feeder conductors and overcurrent devices generally account for continuous load at 125%, subject to equipment listing and specific rules.

Do not apply a demand factor merely because loads are unlikely to operate together. Use a noncoincident-load provision only when its conditions are met. Likewise, do not multiply an already demand-adjusted value again unless the code directs that sequence.

Lighting loads

For a specified occupancy, use the general-lighting unit load and floor-area method in the assigned NEC edition. Article 220 was reorganized between editions, so locate the current table rather than assuming a 2017 number. Include the floor area defined by the code and Wisconsin SPS 316.220 where that state definition applies.

Continuous-load treatment for lighting depends on the type and use. A calculation problem may state that lighting is continuous or may place it in an occupancy where the applicable rule establishes that treatment. Keep the base VA and the 125% sizing value in separate columns so you do not confuse calculation demand with conductor sizing.

Show-window lighting uses the specific linear-foot or outlet rule in Article 220. Track lighting uses the load assigned per unit length unless the track is supplied through a listed current-limiting device under the applicable provision. Electric signs and outline lighting have their own minimum branch-circuit and feeder treatment. These loads should be itemized.

Receptacle loads

For non-dwelling general-use receptacles, calculate the load by the VA assigned per single or multiple receptacle on one yoke, or by the rule for a multioutlet assembly. After obtaining the connected receptacle VA, apply a receptacle demand factor only when the assigned table and occupancy permit it.

For example, a bank or office calculation may permit a demand treatment for a sufficiently large receptacle total, but a small group below the threshold remains at 100%. A receptacle intended for a specific appliance may be calculated from that appliance rather than being counted as an ordinary general-use outlet. Read the stem carefully.

Equipment and motor loads

Fixed appliances, commercial cooking equipment, elevators, welders, EV charging, and other equipment each have article-specific rules. Begin with nameplate ratings unless the code supplies another method. Motors require the Article 430 table current for conductor and short-circuit calculations in the situations covered there, and a feeder serving multiple motors adds 25% of the highest motor load to the sum of the others.

Heating and cooling may be noncoincident, but ventilation, pumps, crankcase heaters, or other components may operate with either mode. Drop only the portion that truly cannot operate at the same time.

Single-phase and three-phase conversion

For unity-power-factor apparent power:

$I_{1\phi}=\frac{VA}{V}$

$I_{3\phi}=\frac{VA}{\sqrt{3}\times V}$

For real power, include efficiency and power factor when the problem provides them and the requested quantity requires them. Never divide three-phase VA by voltage alone; that omission overstates current by a factor of √3.

Example

A 208V, three-phase feeder has 24,000 VA of calculated lighting and receptacle load, 18,000 VA of noncontinuous equipment, and 12,000 VA of continuous load after all permitted demand factors. The sizing total is:

$24,000+18,000+(12,000\times1.25)=57,000\text{ VA}$

$I=\frac{57,000}{1.732\times208}=158.3\text{ A}$

The calculation current is not the end of the design. Select conductors, overcurrent protection, and equipment with adequate ratings, apply terminal temperature limits and adjustment factors, and verify any standard-size or next-size permissions.

Exam worksheet

Build the worksheet in this order:

1. List the description and controlling code rule.
2. Enter connected VA and the permitted demand factor.
3. Apply the continuous-load multiplier where required.
4. Sum the final calculated VA only after every row is classified. This method also exposes missing loads: if the problem mentions a sign, track, motor, or show window and no row exists for it, the calculation is incomplete.