A cable passing through a joist must be kept at least how many millimetres from the top or bottom face?

• A. 30 mm
• B. 50 mm
• C. 75 mm
• D. 100 mm

Answer: (B)

When routing a cable through a wooden joist, the goal is to minimize the chance of damage from fixings and to preserve the joist’s integrity. You want the cable to be well away from the surfaces that will be finished with plaster, flooring, or other coverings, so that nails, screws, or other fixings won’t strike or compress the cable.

The standard minimum distance from the top or bottom face is 50 mm. This means any hole or bore through the joist should place the cable at least 50 mm away from either face. If you can’t meet that distance, you should protect the cable with a steel plate where fixings could pass, or reroute the cable to keep the clearance.

The other numbers would either be insufficient protection (30 mm) or exceed what’s normally required (75 mm or 100 mm) for this situation, making 50 mm the widely accepted and practical minimum.