3.6 Load Calculation Workflow

Load calculations are organized reading problems

Most journeyman load-calculation questions are not mathematically difficult. They are reading, classification, and order-of-operations problems. The exam gives connected loads, occupancy facts, equipment ratings, voltages, and sometimes table references. Your score depends on turning that information into a clean calculation path.

Use VA as the common currency unless the question or NEC rule directs another method. Watts and VA are the same only when power factor is not part of the problem or when the load is treated as unity power factor. Amperes are useful when sizing conductors and overcurrent devices, but combining many loads is often cleaner in VA.

The seven-step workflow

1. Read the final ask. Is it calculated load, feeder ampacity, service size, branch-circuit rating, conductor ampacity, neutral load, or overcurrent protection?
2. List all given loads with units and voltage.
3. Convert each load to VA where appropriate.
4. Classify loads by NEC category and article: general lighting, receptacles, appliances, cooking, dryers, motors, HVAC, fixed heating, signs, continuous loads, or special equipment.
5. Apply demand factors and special rules only to the correct category.
6. Convert final VA to amperes using the correct single-phase or three-phase formula.
7. Apply conductor, terminal, adjustment, correction, and overcurrent rules if the question asks for installation sizing.

The workflow prevents the classic mistake of adding everything, multiplying by one factor, and hoping the answer matches. Some loads are demandable. Some are not. Some are continuous. Some have nameplate rules. Some require the larger of heating or cooling rather than both. Some motor calculations require table full-load current rather than nameplate current.

Code-navigation mindset

The NEC is arranged by subject. Article 100 definitions support the whole code. Branch circuits, feeders, and services have general rules, but specific equipment articles can modify the general method. Load calculations may send you to dwelling rules, non-dwelling lighting rules, motor rules, appliance rules, or special occupancy articles.

A good open-book strategy is to mark major calculation neighborhoods in the code book using allowed permanent tabs if your testing format permits them. ICC rules for materials depend on delivery method and current bulletin instructions, so confirm what your exam allows. Bound copyrighted references with permanent tabs and ink notes are commonly addressed in ICC contractor/trades guidance, while loose papers and programmable calculators are restricted.

Worked example: branch-circuit load setup

A 120-volt branch circuit supplies four fixed loads: 600 VA noncontinuous, 900 VA noncontinuous, 1,200 VA continuous, and 300 VA continuous. What minimum ampacity is needed before conductor table selection?

Group by duration. Noncontinuous total = 600 + 900 = 1,500 VA. Continuous total = 1,200 + 300 = 1,500 VA. Apply 125 percent to continuous: 1,500 x 1.25 = 1,875 VA. Total adjusted load = 1,500 + 1,875 = 3,375 VA. Convert to amperes: 3,375 / 120 = 28.125 A.

The likely minimum ampacity answer is 28.1 A or 29 A depending on rounding choices offered. If the question asks for overcurrent size, you then move to the standard rating rule and any applicable branch-circuit limitations. If it asks for conductor ampacity, you move to the ampacity table and temperature limitations.

Worked example: three-phase feeder setup

A 208-volt, three-phase feeder supplies 24 kVA of noncontinuous load and 12 kVA of continuous load. No demand factor applies. Find minimum feeder current.

Continuous adjusted load = 12 kVA x 1.25 = 15 kVA. Add noncontinuous load: 15 + 24 = 39 kVA. Convert to VA: 39,000 VA. Three-phase current is 39,000 / (1.732 x 208) = 108.3 A.

If you divide 39,000 by 208, you get 187.5 A, which is a common single-phase error. If you multiply the entire 36 kVA connected load by 1.25, you get 45 kVA and about 125 A, which overstates the load because the noncontinuous portion was adjusted incorrectly.

Motors and special loads

Motor calculations deserve caution. NEC motor branch-circuit, feeder, overload, and short-circuit ground-fault protection rules often use table full-load currents and percentages that are not the same as ordinary continuous-load arithmetic. If the question says motor, horsepower, full-load current, locked-rotor, overload, or largest motor, slow down and navigate to the motor rules.

Heating and cooling questions may ask for the larger load rather than adding both when the loads are noncoincident. Electric space heating may be continuous. Air-conditioning equipment may have nameplate minimum circuit ampacity and maximum overcurrent protection values. The exam may provide nameplate MCA and MOCP to see whether you know not to recalculate from horsepower or compressor watts when the listed equipment marking controls.

Demand factors and dwellings

Dwelling load calculations have their own structure. General lighting and receptacle loads, small-appliance branch circuits, laundry circuits, ranges, dryers, fixed appliances, heating and air conditioning, and optional methods may all appear. Do not import a dwelling demand factor into a store, office, shop, or industrial occupancy unless the NEC rule actually applies.

For non-dwelling calculations, lighting load methods, receptacle demand, show-window loads, signs, and continuous operation can change the setup. The ICC content outline gives branch circuits, wiring methods, services, equipment, and special occupancies meaningful weight, so expect calculation facts to be mixed with code navigation.

Exam traps

The first trap is answering the wrong final ask. If the question asks for calculated load in VA, do not give amperes. If it asks for conductor ampacity, do not stop at connected load. The second trap is mixing NEC editions. R17 uses the 2023 NEC, T17 the 2020 NEC, and G17 the 2017 NEC. Confirm the exam your jurisdiction requires before studying tables.

The third trap is applying a demand factor to a load group that is not eligible. The fourth is forgetting continuous-load adjustment. The fifth is using nameplate amperes where a table value is required, especially for motors. The sixth is rounding down before selecting a conductor or overcurrent device. Carry the exact calculated result until the code rule tells you how to round or choose the next size.