Background: Traditionally, engineering curricula about electrical circuits use textbook instruction and hands-on lessons, which are effective approaches for teaching terms and definitions, the procedural use of formulas, and how to build circuits. Nonetheless, students often lack conceptual understanding. Purpose (Hypothesis): The aim of this study was to discover how to facilitate the acquisition of conceptual understanding. We hypothesized that adding an instructional approach in the form of inquiry learning in a virtual lab would be more effective than relying on traditional instruction alone. Design/Method: Students in secondary vocational engineering education were randomly assigned to one of two conditions in a quasi-experimental study. In the traditional condition, the curriculum was supplemented with computer-based practice. In the virtual lab condition, the traditional curriculum was supplemented with inquiry learning in a virtual lab. Results: Results showed that students in the virtual lab condition scored significantly higher on conceptual understanding (Cohen's d=0.65) and on procedural skills (d=0.76). In particular, students in this condition scored higher (d=1.19) on solving complex problems. This result occurred for both complex conceptual and procedural problems. Conclusion: Since students in the virtual lab condition acquired better conceptual understanding and also developed better procedural skills than students in the traditional condition, it appears that conceptual understanding and procedural skills develop in an iterative fashion.