How to Run Wire Across a Ceiling Joist

Running a new circuit across a ceiling in a Toronto or GTA attic or basement almost always means crossing joists, and where you drill matters as much for the structure as for the wiring. This guide covers the actual placement rules — hole location, nail plates, and when notching instead of drilling is (and is not) acceptable.

How to run wire across a ceiling joist

Drill a hole through the centre of the joist, at least 50 mm (2 inches) from the top and bottom edges, sized just large enough for the cable to pass through freely without being pinched. If the hole ends up within 32 mm of either edge, install a listed steel nail plate on both sides to protect the cable from future nails or screws. Never notch the top or bottom edge of a joist for cable — that is a structural cut, not an electrical one, and it weakens the framing member.

Why hole placement matters structurally

A joist resists bending load mostly through its top and bottom edges, where the wood fibres carry the most stress. The centre of the joist, along its neutral axis, carries the least stress — which is exactly why a hole drilled through the middle has minimal structural impact, while a notch cut into the top or bottom edge removes material from the highest-stress zone and can meaningfully weaken the member. This is a building code structural principle, not an electrical one, but it directly shapes where electricians are allowed to drill.

The 50 mm edge rule

• Centre the hole in the joist depth, keeping at least 50 mm of solid wood above and below the hole.
• For a typical 2×8 or 2×10 joist, this leaves generous room to stay well clear of both edges.
• Multiple holes in the same joist along its length should also maintain reasonable spacing from each other, not clustered at one point.
• Size the hole to the cable — oversized holes reduce structural margin for no benefit and can let the cable move around inside, risking chafe.

When a nail plate is required

If the drilled hole (or the cable itself, on a shallow joist) ends up within 32 mm of either the top or bottom edge, the Ontario Electrical Safety Code requires a steel nail plate covering that location on both faces of the joist. The plate protects the cable from a drywall screw, a flooring nail, or any future fastener driven into that same spot from above or below — without it, the cable inside is one careless nail away from a short.

Nail plates are cheap, self-adhesive or nail-on steel plates sold at any electrical supply house. Installing them proactively on every close-to-edge run is standard professional practice, not just a defect fix.

Drilling technique

1. Mark the centre point on each joist along the planned run, keeping the line as straight and consistent as practical.
2. Use a spade bit or auger bit sized appropriately for the cable — typically 12–19 mm for standard NMD90.
3. Drill straight through, perpendicular to the joist face, to avoid an oversized or angled exit hole.
4. Deburr any rough edges that could abrade the cable jacket during pulling.
5. Pull the cable through smoothly, avoiding sharp bends right at the hole.
6. Install nail plates on any hole within 32 mm of an edge before covering with insulation or finish material.

Alternatives to drilling through solid joists

• Running along the top of joists under insulation, secured and protected appropriately, where local layout allows.
• Running below the joists in an unfinished basement, stapled to the joist face rather than drilled through it — simpler but leaves cable more exposed to accidental contact.
• Engineered joists (I-joists, open-web trusses) often have manufacturer-specified knockout locations for utilities — always follow the joist manufacturer’s guide rather than drilling freehand, since these members have different structural behaviour than solid lumber.

What NOT to do

• Never notch the top or bottom edge to lay cable into — removes material from the highest-stress zone of the member.
• Never drill oversized holes “just in case” for future cables — size to the actual run.
• Never drill multiple large holes stacked close together in the same joist — concentrates weakened cross-section at one point.
• Never skip the nail plate on an edge-adjacent hole to save five minutes — it is the single cheapest protection against a future puncture.
• Never drill through engineered joists without checking the manufacturer’s allowed hole zones — these members fail differently than solid lumber and often have specific no-drill zones.

Expert tip from our ESA-licensed electricians

In GTA basement finishes, the most common structural mistake we catch (sometimes from a previous DIY project) is a run of holes drilled too close to the bottom edge of the joists to keep the cable hidden just above a planned drop ceiling. It looks tidy but eats into the joist’s structural margin at every crossing. Centre the hole in the joist depth even if it means the cable sits a little lower in the ceiling cavity — a drop ceiling or bulkhead hides it either way, and the framing keeps its full strength.

Frequently Asked Questions

How big should the hole be for one cable?

Just large enough for the cable to pass freely, typically 12–19 mm for standard NMD90 — avoid oversizing.

Can I notch a joist instead of drilling?

No, not for cable runs. Notching the top or bottom edge removes material from the highest-stress zone of the joist.

Do engineered I-joists follow the same rule?

No. Follow the specific manufacturer’s guide for hole size and location — engineered members behave differently than solid lumber.

What if my hole ended up close to the edge?

Install a nail plate on both faces of the joist at that location before the finish goes up.

Is this a DIY-safe task?

Drilling technique is straightforward, but pairing it correctly with circuit design and code compliance is best confirmed by a licensed contractor.

Related reading from ezsmart.ca

• Can I Staple Romex Wire to Studs?
• How to Run Electrical Wire Through Finished Walls
• How to Fix a Nicked Electrical Wire
• Can You Run Electrical Wire and Ethernet in the Same Hole?
• What Is a Rough-In Electrical Inspection?

Contact us

Running new wiring across ceiling joists in a Toronto or GTA home? Call ezsmart at 416-838-9006 or use our contact page. An ESA-licensed electrician will route it safely and protect the framing.