Can You Bury Indoor Electrical Wire Underground?

Homeowners planning a garden light run or a garage sub-feed often ask the same question at the hardware store — can I just bury the indoor cable I already have? It looks similar to underground-rated cable, it is cheaper, and it saves a trip. The answer under the Ontario Electrical Safety Code is a firm no, and the reasons are more than paperwork. This guide walks GTA homeowners through whether you can bury indoor electrical wire underground, what happens if you do, and the alternatives that are code-compliant and safe.

Short answer: no

Direct answer: burying NMD-90 (indoor Romex) directly in the ground violates the Ontario Electrical Safety Code and fails every ESA inspection. The jacket is not rated for wet or underground exposure, and moisture wicks along the conductor insulation, degrading it over years. Use NMWU (Non-Metallic Wet Underground) or install NMD-90 in listed underground conduit — not direct-buried.

Why NMD-90 fails underground

• Jacket permeability. NMD-90 outer jacket allows moisture through over time. Water reaches the conductor insulation.
• Insulation degradation. Conductor insulation on NMD-90 is not rated for continuous wet exposure. It cracks and delaminates.
• Corrosion at splices. Any moisture-contacted marrette corrodes and eventually fails.
• Frost heave damage. Ontario winters expand and contract the soil. NMD-90 jacket is not tough enough for buried freeze-thaw cycles.
• Rodent and insect damage. Underground NMD-90 is chewed within a year or two.

The Electrical Safety Authority lists moisture-degraded underground cable among the residential incident causes tracked in the Ontario Electrical Safety Report. Slow-developing failures in buried cable are difficult to trace when they eventually cause a trip or an outdoor fixture failure.

Code-compliant options for underground runs

• NMWU (Non-Metallic Wet Underground): Canadian standard for direct-buried cable. Buried at 900 mm (36 inches).
• UF-B (US Underground Feeder): equivalent to NMWU in most CSA installations.
• NMD-90 inside PVC conduit: the conduit provides moisture protection. Conduit buried at 450 mm (18 inches).
• THHN conductor in PVC conduit: for feeder runs. Buried at 450 mm minimum.
• Direct-buried MC or TECK cable: more expensive but often the choice for commercial or heavy residential feeders.

When PVC conduit makes sense

Direct answer: PVC conduit around NMD-90 is often cheaper than direct-buried NMWU on shorter runs (under 15 metres) because NMD-90 is significantly cheaper per metre. The conduit adds material cost but the labour is comparable. For longer runs, direct-buried NMWU wins on total cost.

Conduit also gives you future flexibility — upgrading from 60 A to 100 A requires only pulling new cable, not re-trenching.

Burial depth requirements

• Direct-buried NMWU or UF-B: 900 mm (36 inches).
• PVC conduit: 450 mm (18 inches) minimum; 600 mm (24 inches) under roadways.
• Rigid metal conduit (RMC): 300 mm (12 inches) is sometimes acceptable per OESC.
• Warning tape: 300 mm above the buried cable or conduit, marking the electrical line.

Our post on running power to a detached garage covers the full underground-run process.

Special cases: shallow burial

Direct answer: low-voltage landscape lighting cable (12 V or 15 V) can be buried at 150-300 mm because it is low-voltage and the failure consequence is dim lights, not a shock. Standard 120 V branch circuits do not have a shallow-burial exception.

Our post on landscape lighting covers the low-voltage side.

What happens if you buy NMD-90 anyway

• ESA inspection failure. Guaranteed writeup on the first inspector visit.
• Insurance denial risk. If a fire ever originates on the buried run, the claim is denied once the ESA record shows non-compliant cable.
• Home inspection flag. Every purchase inspection includes buried cable review.
• Utility interconnection refusal. Utility will not connect a circuit fed by non-code-compliant buried cable.
• Retrenching cost. Removing and replacing buried cable is roughly double the cost of doing it right the first time.

Expert tip from our ESA-licensed electricians

In our experience diagnosing failed outdoor circuits across GTA back yards, the most common root cause we find is an indoor cable buried by a previous owner or handyman. It worked for 5 or 10 years, then gradually became unreliable. When we dig it up, the NMD-90 jacket is soft and cracked, the conductor insulation is powdery, and marrettes at any splice points are corroded green. The fix is always the same — replace with NMWU or install proper conduit. Customers often ask if we can splice a section — no, once NMD-90 has been buried for years, the whole run is compromised. Do it right the first time. Extra $50 in materials up front saves $500-1000 in retrenching later. Our post on wire types covers the material selection.

Related reading from ezsmart.ca

• Romex vs THHN vs UF wire
• Power to detached garage
• Landscape lighting transformers
• DIY wiring up to code?
• Outdoor lights burning out?

Contact us

Buried NMD-90 in your GTA yard and worried about compliance? Book an ESA-certified electrician to inspect. Call us at 416-838-9006 or visit our contact page — we will get back to you the same day.