Engineering interview questions, solved the way interviewers want to see it.

Engineering interviews often hand you a quick calculation and then a real-world problem to reason through. Here’s one of each, fully worked.

Free question 1 (electrical calculation): Current and voltage drop on a branch circuit

A 10 kW resistive heater runs on a 240 V single-phase supply. It sits 30 m from the panel and is fed with 6 AWG copper. What current does it draw, and what’s the voltage drop? Is it acceptable?

Model answer

Current. It’s a resistive load, so the power factor is 1.

I = P / V = 10,000 W / 240 V = 41.7 A

Conductor resistance. Current flows out and back, so the wire length is 2 × 30 m = 60 m. Copper resistivity is about 1.72 × 10⁻⁸ Ω·m at 20°C, and 6 AWG is about 13.3 mm².

R = ρL / A = (1.72 × 10⁻⁸ × 60) / (13.3 × 10⁻⁶) = 0.078 Ω

Voltage drop.

V drop = I × R = 41.7 A × 0.078 Ω = 3.2 V, which is 3.2 / 240 = 1.35%

Then say what a careful engineer adds:

• Resistance rises with temperature. At a 75°C conductor temperature it’s about 0.094 Ω, so the drop is about 3.9 V, or 1.6%. Still low.
• The Canadian Electrical Code (Rule 8-102) limits voltage drop, commonly 3% in a branch circuit and 5% from the service to the load. Confirm the limits in the edition your province has adopted.
• Voltage drop is only one check. You’d also confirm the conductor’s ampacity and the breaker size from the code tables, with any correction factors for temperature or bundling.

What interviewers listen for: you state your assumptions (resistive load, round trip, temperature), your units stay clean, and you know which code governs the answer.

Free question 2 (mechanical case): A bracket keeps cracking in service

A steel mounting bracket on a production machine has cracked three times in six months, always in the same spot. Walk me through how you’d find the root cause and fix it.

Model answer

1. Make it safe first. Inspect the other machines with the same bracket, and decide whether to add temporary supports or shorten the inspection interval while you investigate.

2. Read the fracture. Look at the broken surface. Beach marks spreading out from one starting point, with a small rough area where it finally broke, point to fatigue, not one overload. Note where the crack started, often at a sharp corner, hole, weld toe or tool mark.

3. Compare the real loads to the design. Measure vibration or strain on the machine if you can. Check whether the machine runs faster, heavier or with more vibration than the bracket was designed for. Fatigue depends on the stress range and number of cycles, so a small increase in vibration can shorten life a lot.

4. Check the design details. A sharp inside corner or a hole near a bend concentrates stress. Calculate the local stress with a stress concentration factor, or run a simple FEA, and compare it to the material’s fatigue strength.

5. Check material and manufacturing. Confirm the steel grade and heat treatment against the drawing, using the mill certificate or a hardness test. Look for a rough surface finish, weld defects or a changed supplier.

6. Use a structured method. A fishbone diagram or 5 Whys keeps you from stopping at the first answer. Write down each cause you rule out and how.

7. Fix it and prove it. Typical fixes: a larger fillet radius, moving a hole away from the bend, a thicker section, a better weld profile, shot peening, or isolating the vibration. Then confirm with testing or monitoring, and update the drawing and the design record so it doesn’t come back.

What interviewers listen for: safety first, evidence before conclusions, and a fix that’s verified and documented, not just a thicker bracket.