Journeyman/Master Electrician License Exam Electrical Load Calculations Flashcards

24 sample cards from the Electrical Load Calculations deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What is a "continuous load" per the NEC, and how does it affect conductor and OCPD sizing?

   A continuous load is one expected to operate for 3 hours or more. The branch-circuit conductor and overcurrent device must be sized for 100% of noncontinuous plus 125% of the continuous load (NEC 210.19 / 215.2 / 215.3).

2. Under NEC 220.84, what is the Optional Calculation for a multifamily dwelling, and what conditions allow its use?

   The Optional method for multifamily dwellings (3 or more units) applies Table 220.84 demand factors to the total connected load, permitted when no dwelling unit is supplied by more than one feeder and each unit has electric cooking and either electric heat, A/C, or both.

3. What general loads are summed at 100% before applying Table 220.84 demand factors in a multifamily Optional calculation?

   $3 \text{ VA/ft}^{2}$ lighting, $1500 \text{ VA}$ per small-appliance and laundry circuit, nameplate of all appliances and motors (including ranges, dryers, water heaters), and the larger of heating or A/C — totaled for all units.

4. What is the Table 220.84 demand factor for a multifamily building with 10 dwelling units (Optional Method)?

   For 10 units the demand factor is 43%. (The factor decreases as unit count increases — e.g., 45% for 7-9 units, 41% for 11 units.)

5. Under the Standard Method for a multifamily dwelling, how are house loads and feeder loads typically handled differently from the Optional method?

   In the Standard Method each unit's load is calculated per Part III (general lighting demand 220.42, ranges via Table 220.55, etc.), then combined; ranges for multiple units use Column C demand, and house/common loads are added at their applicable demand. There is no single building-wide demand factor as in 220.84.

6. For 20 household ranges (each 12 kW) in an apartment building, what is the Column C demand from Table 220.55?

   For 20 ranges, Column C gives a maximum demand of $35 \text{ kW}$.

7. What is the basic formula relating service/feeder VA, voltage, and current for a single-phase system?

   $I = \dfrac{\text{VA}}{E}$, where for single-phase $\text{VA} = E \times I$.

8. What is the formula for line current in a balanced three-phase system given total VA and line-to-line voltage?

   $I = \dfrac{\text{VA}}{\sqrt{3} \times E_{LL}} = \dfrac{\text{VA}}{1.732 \times E_{LL}}$.

9. Per NEC 230.42(A), how is the minimum service-entrance conductor ampacity determined relative to the calculated load?

   The service conductors must have an ampacity not less than the calculated load, sized at 100% of noncontinuous plus 125% of continuous load (or per the 83% rule in 310.12 for certain dwellings).

10. What is the minimum ampere rating for the service disconnect and service-entrance conductors of a one-family dwelling with 6 or more 2-wire branch circuits, per NEC 230.79?

    A minimum of $100 \text{ A}$, 3-wire, for a one-family dwelling.

11. Under NEC 310.12, what percentage of the service rating may dwelling service/feeder conductors be sized to (the '83% rule')?

    For a single-phase, 120/240 V dwelling service that is the main power feeder, conductors may be sized at 83% of the service rating. For example, a 200 A service: $200 \times 0.83 = 166 \text{ A}$ minimum conductor ampacity.

12. When selecting a conductor for a calculated load, which NEC table gives allowable ampacities for insulated conductors rated up to 2000 V?

    Table 310.16 (the 60°C/75°C/90°C allowable ampacity table for conductors in raceway, cable, or earth, not more than three current-carrying conductors).

13. What temperature column of Table 310.16 is normally used for terminations on equipment rated 100 A or less (or marked for #14–#1 conductors)?

    The 60°C column, unless the equipment is listed and marked for higher-temperature terminations, per NEC 110.14(C)(1).

14. What termination temperature rating applies to circuits rated over 100 A or sized larger than #1 AWG under NEC 110.14(C)?

    The 75°C column may be used, unless the equipment is marked otherwise. Conductors with 90°C insulation may be used for ampacity-adjustment calculations but the final ampacity cannot exceed the termination rating.

15. What is the role of the 90°C column in Table 310.16 if terminations are only rated 75°C?

    The 90°C ampacity is used only as the starting point for derating/adjustment (temperature correction and bundling factors). After adjustment, the conductor's final ampacity must not exceed the 75°C termination rating.

16. State the voltage drop formula for a single-phase circuit using conductor resistance.

    $V_{D} = \dfrac{2 \times K \times I \times L}{A}$, where $K$ is the resistivity constant (≈$12.9$ for copper, $21.2$ for aluminum at 75°C), $I$ is current in amperes, $L$ is one-way length in feet, and $A$ is the conductor area in circular mils.

17. State the voltage drop formula for a three-phase circuit using the K-constant method.

    $V_{D} = \dfrac{\sqrt{3} \times K \times I \times L}{A} = \dfrac{1.732 \times K \times I \times L}{A}$, where $L$ is the one-way length.

18. What are the NEC's recommended (informational, non-mandatory) maximum voltage drops for branch circuits and for the combined feeder-plus-branch?

    A maximum of 3% voltage drop on a branch circuit, and 5% total for the combined feeder and branch circuit (NEC 210.19 and 215.2 Informational Notes).

19. Calculate the voltage drop for a single-phase load: copper, $I = 20 \text{ A}$, one-way length $L = 100 \text{ ft}$, conductor #12 AWG ($A = 6530 \text{ cmil}$), $K = 12.9$.

    $V_{D} = \dfrac{2 \times 12.9 \times 20 \times 100}{6530} = \dfrac{51{,}600}{6530} \approx 7.9 \text{ V}$.

20. Rearrange the single-phase voltage drop formula to solve for the minimum conductor area (circular mils) needed for an allowable $V_{D}$.

    $A = \dfrac{2 \times K \times I \times L}{V_{D}}$ (single-phase). For three-phase, replace the 2 with $\sqrt{3}$.

21. What is the ampacity adjustment factor when 7 to 9 current-carrying conductors are bundled together in a raceway (NEC Table 310.15(C)(1))?

    70% — the allowable ampacity of each conductor is multiplied by 0.70 when 7–9 current-carrying conductors are in the raceway or cable.

22. State the temperature correction factor concept: how does an ambient above 30°C affect a 90°C conductor's ampacity, and where is it found?

    Higher ambient temperature reduces ampacity. Multiply the table ampacity by the correction factor from Table 310.15(B)(1) (or use $\sqrt{\frac{T_{c}-T_{a}}{T_{c}-30}}$). For example, a 90°C conductor in 40°C ambient is corrected by 0.91.

23. When both temperature correction and conductor-bundling adjustment apply, how are they combined to get the final adjusted ampacity?

    Multiply them together: $\text{Adjusted ampacity} = (\text{90°C ampacity}) \times (\text{temp correction factor}) \times (\text{adjustment factor})$. The result must still not exceed the termination temperature rating (60°C/75°C).

24. How many current-carrying conductors require ampacity adjustment, and when is the neutral counted as current-carrying per NEC 310.15(E)?

    Adjustment is required when more than 3 current-carrying conductors are in a raceway/cable. The neutral counts as current-carrying when it carries harmonic currents (e.g., from electric-discharge lighting or nonlinear loads) or in a 3-wire circuit from a 4-wire wye where it carries unbalanced plus harmonic current; it is NOT counted when it carries only the unbalanced current of other conductors.

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