Electrician ExamStudy Topic
Electrical Load Calculations for the Electrician Exam
Master electrical load calculations for your journeyman or master electrician exam. Covers NEC Article 220, demand factors, residential and commercial methods.
Topic Overview
Load calculations determine the minimum ampacity required for electrical service, feeders, and branch circuits based on the connected loads. The National Electrical Code provides specific rules in Article 220 for calculating loads on residential, commercial, and industrial systems. Exam questions on this topic require candidates to apply demand factors, read tables, and perform arithmetic accurately.
For residential load calculations, the standard method in NEC 220.82 (optional method) and the standard method in Part III of Article 220 both start with the general lighting and receptacle load, calculated at 3 volt-amperes (VA) per square foot of the dwelling unit floor area. From there, you add loads for small appliance branch circuits (1,500 VA each, minimum two required per NEC 210.11(C)), a laundry branch circuit (1,500 VA), and then all fixed appliances, HVAC (at the largest motor + all other loads), and the electric range (using Table 220.55 for ranges rated over 8 ¾ kW).
Demand factors allow you to reduce the calculated load because not all loads operate at full capacity simultaneously. NEC Table 220.42 applies demand factors to the general lighting load: 100% for the first 3,000 VA, 35% for 3,001 to 120,000 VA, and 25% for the remainder. For fixed appliances (excluding electric dryers, ranges, and HVAC), NEC 220.53 allows a 75% demand factor when there are four or more appliances on the same feeder or service.
For commercial load calculations, the approach depends on occupancy type. NEC Table 220.12 provides unit load values (in VA per square foot or per outlet) for different occupancy types such as offices, banks, and restaurants. Commercial calculations also consider specific loads like motors (Article 430 branch circuit sizing rules often interrelate), sign circuits (NEC 600.5), and HVAC equipment.
A key formula for the exam: Amperes = VA / Volts (single-phase) or Amperes = VA / (Volts x 1.732) (three-phase). For sizing conductors and overcurrent devices, the calculated amperage must not exceed the conductor's ampacity (from NEC Table 310.16 or 310.15) and must comply with the 80% continuous load rule when loads operate for 3 or more hours continuously.
- Forgetting to convert from VA (volt-amperes) to amperes before selecting conductor size; using a VA value directly as an amp value.
- Omitting the small appliance branch circuit loads (1,500 VA each) or using fewer than the two required by NEC 210.11(C).
- Applying demand factors from Table 220.42 incorrectly by using the wrong bracket, especially forgetting that only the first 3,000 VA is at 100%.
- Using the nameplate wattage of an electric range instead of applying NEC Table 220.55 column C (or the appropriate column for the number of ranges).
- Forgetting the 125% multiplier for continuous loads when sizing overcurrent protection and conductors per NEC 210.20(A) and 215.3.
- Calculating three-phase load in VA using single-phase formula (omitting the 1.732 multiplier).
Checkpoint Quiz
Test your understanding of Load Calculations
These questions are for study practice only and are not official exam questions.
1. What is the standard unit used to express electrical power in load calculations?
2. Using Ohm's Law, if a 120 V circuit has a resistance of 12 ohms, what is the current flowing?
3. What does 'demand factor' mean in load calculations?
4. A 240 V, 20 A circuit feeds a water heater. What is the load in volt-amperes (VA)?
5. In a standard residential load calculation, what general lighting load value (in VA per square foot) is commonly used for dwellings?
6. A residential dwelling has 2,000 sq ft of living space. Using 3 VA/sq ft for general lighting and applying 100% demand on the first 3,000 VA and 35% on the remainder, what is the calculated general lighting demand load?
7. What is voltage drop, and why is it important in load calculations?
8. When sizing a service for a residence, which of the following loads is typically added at 100% of its nameplate rating with no demand factor applied?
9. A 120/240 V single-phase service feeds a home. The total calculated load is 28,800 VA. What is the minimum service ampacity required?
10. What is the purpose of adding two small appliance branch circuit loads (each at 1,500 VA) to a residential load calculation?
Frequently asked questions
What NEC article covers load calculations?
NEC Article 220 covers branch circuit, feeder, and service load calculations. It includes the standard method, the optional method for dwellings (220.82), and specific rules for ranges, dryers, and commercial occupancies.
What is the general lighting load for a residence?
The NEC standard method uses 3 VA per square foot of living area for general lighting and general-purpose receptacle loads in dwelling units, per NEC Table 220.12.
What is a demand factor and when do I apply it?
A demand factor is a percentage applied to the calculated connected load to account for the fact that not all loads operate at maximum simultaneously. For example, NEC Table 220.42 allows you to apply a 35% demand factor to residential lighting loads between 3,001 and 120,000 VA.
How do I calculate a three-phase load in amperes?
Divide the total volt-amperes (VA) by the system voltage multiplied by 1.732 (the square root of 3). For example, a 48,000 VA load at 208 volts three-phase equals 48,000 / (208 x 1.732) = approximately 133 amperes.
Does the 80% rule apply to all circuits?
The 80% continuous load rule (125% multiplier on the load) applies to overcurrent device and conductor sizing when a load operates for 3 or more hours continuously. Not all circuits have continuous loads; verify whether the load in question qualifies before applying the multiplier.
