OhmPrepOhmPrep

OhmPrep Calculations Course

Every exam calculation, practiced step by step.

308 worked exercises in seven calculation types, 4 timed practice exams, and a phone app that shows every step after you answer.

Join the launch list See an exercise

Not on sale yet. People on the list get the launch price first.

Exercises308
Topics7
Practice exams4 x 25 questions
Exam timer75 minutes
Code editionsNEC 2020 and 2023
Price$59 once, 30-day refund

What's inside

Seven calculation types. Every one worked out.

Each exercise gives you the values you need from the code tables, names the article or table, and has three wrong answers built from the mistakes people really make.

44 exercises

Voltage drop

Single-phase and three-phase, copper and aluminum: drop in volts and percent, minimum circular mils, smallest size, longest run.

44 exercises

Conduit fill

THHN in EMT with Chapter 9 Tables 1, 4 and 5: how many wires fit, percent fill, smallest trade size, 53% and 31% limits, the 0.8 rule.

44 exercises

Box fill

314.16 counting: yokes, grounds, clamps and pigtails, mixed wire sizes, boxes with plaster or extension rings.

44 exercises

Load calculations

Dwelling standard method: general lighting at 3 VA per square foot, demand factors, appliances, range, dryer, heat or A/C, service amps.

44 exercises

Ohm's law and power

Heater current, power at a different voltage, series and parallel resistance, power factor, three-phase current.

44 exercises

Ampacity and derating

90 degree C starting point, ambient correction, more than three current-carrying conductors, the 75 degree C terminal check.

44 exercises

Motor calculations

125% conductors, branch-circuit breaker and fuse sizing from the 430.52 percentages, next standard size, multi-motor feeders.

4 exams

Timed practice exams

25 mixed questions, 75-minute timer, four choices each. Score by topic at the end, then every question with its steps.

How it works

Answer first. Then see every step.

01

Work it out

Type your number or pick one of four answers. Close rounding is accepted.

02

Watch the steps

The solution appears one line at a time: formula, numbers, result, NEC reference.

03

Learn the miss

Pick a wrong answer and the app tells you which mistake leads to it.

04

Keep the streak

Points, a daily goal, levels for each topic, badges, and a Review list of everything you missed.

Preview

One exercise from the course

This is a real exercise from the app, shown with all four choices, the steps and the explanation for a wrong pick. In the app the steps appear only after you answer.

If you picked 20.6%: You left out the equipment grounding conductor. It must be counted in the fill.

Conduit fill: percent+10 pts

Three 8 AWG THHN conductors and one 10 AWG THHN equipment grounding conductor are in 3/4 in. EMT. What percent of the tubing's total area do they fill?

THHN 8 AWG (Ch. 9, Table 5)0.0366 in²
THHN 10 AWG (Ch. 9, Table 5)0.0211 in²
EMT 3/4 in., total area (Ch. 9, Table 4)0.533 in²
  1. A20.6%
  2. B61.4%
  3. C27.5%
  4. D24.6%Correct

  1. Add the current-carrying conductors

    3 × area3 × 0.0366= 0.1098 in²

  2. Add the equipment grounding conductor

    The ground counts (notes to Ch. 9, Table 1)+ 0.0211Total = 0.1309 in²

  3. Divide by the total area of the tubing

    fill % = wire area / total area × 1000.1309 / 0.533 × 100= 24.6% (within the 40% limit)

24.6%. NEC Chapter 9: Table 1 (percent of cross section: 53% one wire, 31% two wires, 40% over two), Table 4 (EMT areas), Table 5 (THHN areas), Note 7 to Table 1 (the 0.8 rounding rule).

Coming soon

OhmPrep Calculations Course

$59 one-time, planned price

  • 308 worked exercises with animated steps
  • 4 timed practice exams with full review
  • NEC 2020 and 2023 edition tags on every exercise
  • 30-day refund, no questions asked
Join the launch list

Personal license for one person. See the terms and the privacy policy.

Frequently asked questions

Are these real exam questions?

No. Every exercise is original and generated from the formulas and code methods the exam tests. Real exam questions are confidential, and we do not use or imitate them.

Which NEC edition does the course use?

The methods and values are checked for NEC 2020 and NEC 2023. Each exercise shows a tag when the article or table number differs between the two. Your state may test a different edition, so check the candidate bulletin.

Do I still need my code book?

Yes. Each exercise gives you the few table values it needs and names the article or table, so you can practice the math anywhere. On the exam you will look those values up yourself, and practicing with your own book is part of getting faster.

Does it work on my phone and offline?

Yes. It is a web app made for phones. After the first visit it keeps working without a signal, and your progress is saved on your device. You can export a backup and import it on another device.

What if I find a mistake?

Every exercise has a Report an error button. Reports go to us with the exercise ID and your note, and we fix and recheck the exercise.

How does the refund work?

If the course is not useful to you, ask within 30 days of purchase and you get a full refund, no questions asked. See the terms for details.

Can you promise I will pass?

No. Nobody can honestly promise that. The course gives you structured practice on the calculation questions, with every step shown, so you can see where your method breaks and fix it.

Is OhmPrep connected to NFPA or a licensing board?

No. OhmPrep is an independent study resource. It is not affiliated with, endorsed by, or approved by NFPA, the National Electrical Code, or any state licensing board.

Not affiliated. OhmPrep is an independent study resource. Not affiliated with, endorsed by, or approved by NFPA, the National Electrical Code, or any state licensing board. NEC is a registered trademark of the National Fire Protection Association. Exam formats, passing scores and adopted code editions differ by state: always check your state's candidate bulletin.