20.1 Mixed Calculation Lab

20.1 Mixed Calculation Lab

Quick Answer: The Arkansas Journeyman Electrician exam (Prov for the Board of Electrical Examiners / ADLL; NEC 2023 open book; 60 questions; 3 hours; 70% cut-score) does not hand you a “calculations only” block. Expect load, ampacity, motor, box-fill, and grounding electrode conductor (GEC) items interleaved with straight lookup questions. This lab trains you to switch article families and finish clean arithmetic under time.

General Knowledge / Calculations alone is 9 items, but Services/OCP (8), Conductors (7), Motors (7), Raceways (8), and Grounding (6) all generate number problems. Passing at 70% means roughly 42 correct of 60—so every careless VA or ampacity miss is expensive.

Universal calc workflow (use on every stem)

1. Classify in ~10 seconds: dwelling/commercial load, conductor ampacity, motor, box/conduit fill, or grounding conductor?
2. Open the controlling article/table via permanent tabs (220, 310.15, 430, 314.16, 250.66/250.122).
3. Read conditions first: copper vs aluminum, temperature rating, continuous load, number of current-carrying conductors, dwelling vs other, service vs feeder.
4. Apply exceptions and table notes before computing.
5. Compute once, check units (VA vs A vs AWG), then match the closest Code-supported choice.

Do not invent demand percentages. If the stem is silent on optional method, default to the standard Article 220 path unless it clearly points to optional/multifamily rules.

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Lab A — Standard dwelling load slice (Article 220)

Given: One-family dwelling, outside floor area 1,600 ft²; 12 kW range; 5 kW dryer; dishwasher 1,200 VA; disposer 800 VA; water heater 4,500 VA; trash compactor 900 VA; heat 7 kW; A/C 5 kW (noncoincident). Find the Table 220.45 lighting/SA/laundry demand and the 220.53 fixed-appliance demand.

Lighting / SA / laundry

• General lighting (220.41): 1,600 × 3 = 4,800 VA
• Small-appliance + laundry (220.52): 1,500 × 2 + 1,500 = 4,500 VA
• Combined before demand = 9,300 VA
• Table 220.45 dwelling: first 3,000 × 100% = 3,000; remaining 6,300 × 35% = 2,205 → demand = 5,205 VA

Fixed appliances (220.53)

• Four fastened-in-place appliances (exclude dryer/range/HVAC): 1,200 + 800 + 4,500 + 900 = 7,400 VA
• Four or more → 75%: 7,400 × 0.75 = 5,550 VA

Also bank for a follow-up stem

• Dryer (220.54): max(5,000, 5,000) = 5,000 VA
• Range 12 kW → Table 220.55 Column C typically 8 kW (verify in your NEC)
• HVAC (220.60): larger noncoincident = 7 kW heat, not 12 kW sum

Exam trap: selecting 9,300 VA as the “lighting demand” skips Table 220.45.

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Lab B — Ampacity adjustment / correction (310.15)

Given: Six current-carrying 12 AWG THHN copper conductors in one raceway; terminal rating 75°C; ambient 30°C (no ambient correction). What is the allowable ampacity after adjustment for more than three current-carrying conductors?

Workflow

1. Base ampacity from Table 310.16 at the conductor’s temperature column permitted by terminations (often 75°C column when terminals are 75°C).
2. Apply 310.15(C)(1) adjustment for 4–6 current-carrying conductors: 80% of the table value (confirm the exact percentage row in your 2023 NEC).
3. Do not “adjust” ambient when the problem states 30°C and Table 310.16 already assumes that ambient unless a correction table applies.

Illustrative arithmetic (verify table cells in your book): If 12 AWG copper THHN at 75°C is 25 A, then 25 × 0.80 = 20 A after 4–6 CCC adjustment. Many stems then ask whether a 20 A breaker is acceptable for the circuit type—separate OCPD rules from ampacity.

Trap: using the 90°C column for final ampacity when terminations limit you to 60°C or 75°C.

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Lab C — Motor circuit numbers (Article 430)

Given: Single 230 V, 3-phase, 10 hp Design B motor; find approximate Table 430.250 FLC and the minimum conductor ampacity idea under 430.22 (125% of FLC for continuous-duty single motor).

Workflow

1. Ignore nameplate FLC for conductor sizing when the Code directs you to the tables—use Table 430.250 (or 430.248/430.249 as applicable) unless the section says otherwise.
2. Conductors: not less than 125% of table FLC (430.22) for a continuous-duty single motor.
3. Branch-circuit short-circuit/ground-fault protection percentages come from Table 430.52—separate from overload (heater) sizing in Part III.

Illustrative: If Table 430.250 lists 28 A FLC for that motor, minimum conductor ampacity ≈ 28 × 1.25 = 35 A before selecting the next standard conductor size under Article 310. Always re-open the actual table on exam day—do not memorize every hp cell.

Trap: sizing conductors from nameplate FLA when the stem is testing table-based 430.22 sizing.

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Lab D — Box fill (314.16)

Given: Metal box with four 12 AWG conductors entering (all spliced or terminated), two 12 AWG equipment grounding conductors, one internal cable clamp, and two devices on a yoke (or two yokes—read the stem). Find minimum volume using Table 314.16(B) allowances.

Count method (concept)

Illustrative count for 12 AWG (verify 314.16(B) volume per conductor in your NEC): 4 conductor allowances + 1 EGC allowance + 1 clamp allowance + 2 × 2 device allowances = 10 volume units of 12 AWG. Multiply by the 12 AWG free-space value from the table to get required cubic inches, then pick a box ≥ that volume.

Trap: counting each EGC separately, or counting only one volume for a device yoke that requires two.

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Lab E — GEC sizing (250.66) vs EGC (250.122)

Given: Service with 3/0 AWG copper ungrounded service-entrance conductors. What Table 250.66 GEC size applies (copper), and why is 250.122 the wrong table?

Workflow

1. GEC to the grounding electrode system follows Table 250.66 based on the largest ungrounded service-entrance conductor.
2. Equipment grounding conductors for feeders/branch circuits follow Table 250.122 based on the OCPD rating—not service conductor size.
3. Note 250.66 limitations/exceptions for rod, pipe, and plate electrodes (GEC need not be larger than 6 AWG copper to a single rod/pipe/plate in many cases—read the section).

Illustrative: For 3/0 copper service conductors, Table 250.66 commonly points to 4 AWG copper GEC (confirm in your 2023 table). Selecting a 250.122 size keyed to a 200 A breaker is the classic mix-up.

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Mixed set — practice under a 25-minute clock

Run five stems back-to-back without studying between them:

1. Dwelling lighting demand only (Lab A style)
2. Ampacity with CCC adjustment (Lab B)
3. Motor conductor 125% (Lab C)
4. Box volume (Lab D)
5. GEC from 250.66 (Lab E)

Score yourself on article choice first, then arithmetic. A wrong table with perfect math still fails the item.

Pacing inside the real 180 minutes

• Pass A: Grab theory and single-tab lookups; flag multi-table calcs.
• Pass B: Work Labs A–E style items with scratch paper; never write in the codebook (Prov forbids marking during the test).
• Pass C: Recheck unit conversions and “forgot the demand factor” mistakes.

Fees and logistics stay the same whether the stem is calc-heavy or not: CBT $80 or paper $56, schedule at provexam.com/schedule after Board approval with Candidate ID beginning 200. This lab does not change those facts—it changes how many of the 60 you finish correctly before the clock hits zero.