Quick Answer: A greenhouse counts as a damp location under the National Electrical Code, so every outlet in one needs GFCI protection — a weather-resistant GFCI receptacle under an in-use weatherproof cover, not an indoor outlet. If you’re running power underground to reach the structure, standard direct-burial cable needs 24 inches of cover, dropping to 12 inches for a GFCI-protected 120-volt residential circuit (NEC Table 300.5). A single heater or heat mat for one season can run on a heavy-duty outdoor extension cord; anything permanent or multi-device should be a real circuit installed by a licensed electrician.
Most greenhouse buying guides — ours included — treat “add a heater,” “add grow lights,” “add a thermostat” as simple plug-in upgrades. They are, once there’s a safe outlet to plug into. Getting power to the structure in the first place is the part that actually trips people up, and it’s also the part where a mistake is dangerous rather than just inconvenient: you’re running electricity into a small, humid, water-splashed enclosure. Here’s what the code actually requires and where the DIY line sits.
Greenhouse electrical by the numbers:
- 24 inches — standard minimum cover depth for direct-burial cable under NEC Table 300.5, dropping to 12 inches if the entire underground circuit is GFCI-protected upstream of where it goes into the ground.
- 6 inches — minimum cover for rigid or intermediate metal conduit, under the same GFCI-protected-circuit condition — the shallowest option if you’d rather trench less.
- Every 15- and 20-amp, 125-volt receptacle — the scope of NEC 210.8’s GFCI requirement for outdoor and damp locations, which covers a greenhouse outlet regardless of the structure’s size.
Why a greenhouse counts as a “damp location” even without a hose running
Code doesn’t require a puddle on the floor to trigger the damp-location rules — humidity, condensation on polycarbonate panels, and splashback from a misting system or irrigation system are enough. That’s why every outlet inside one needs GFCI (ground-fault circuit interrupter) protection: the device trips power within milliseconds if it detects current leaking somewhere it shouldn’t, which is the difference between a nuisance and a shock when water and electricity share a small enclosed space. Use a receptacle marked “WR” (weather-resistant) — a standard indoor GFCI outlet isn’t rated for the moisture — and cover it with an in-use (“bubble”) weatherproof cover that stays closed around a plugged-in cord, not just a flip-lid cover meant for an empty outlet.
Outdoor Weatherproof GFCI Outlet with In-Use Cover
- WR (weather-resistant) marking required for damp/outdoor locations — a standard indoor GFCI isn't rated for this.
- In-use ("bubble") cover keeps the outlet weatherproof even with a cord plugged in, not just when empty.
- Test the trip button monthly — GFCI protection degrades over years of use and stops working silently.
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Getting power to the structure: extension cord, or a real circuit?
| Situation | Reasonable fix | Notes |
|---|---|---|
| One seedling heat mat, one season | Heavy-duty 12-gauge outdoor extension cord, GFCI-protected at the source | Sunlight-resistant jacket; don't bury or run it under mulch |
| One small heater, occasional use | Same — outdoor extension cord | Never daisy-chain cords; one continuous run only |
| Heater + fan + grow lights running unattended | Dedicated circuit to the structure | Multiple cords across a greenhouse floor multiply trip and moisture-damage risk |
| Any structure with plumbed-in irrigation or misting | Dedicated circuit, GFCI-protected | Water and multiple devices sharing one cord is the failure case GFCI exists to catch |
| Permanent walk-in kit, year-round use | Buried circuit installed by a licensed electrician | Requires an electrical permit in most jurisdictions |
A single seedling heat mat or a small space heater for one growing season is a legitimate reason to run a heavy-duty, sunlight-resistant, 12-gauge outdoor extension cord from a GFCI-protected outlet on the house — plenty of hobby growers do exactly this. What it isn’t is a permanent solution: cords left lying through grass or under mulch degrade faster than their rating suggests, and once you’re running a fan, a grow light setup, and a thermostat at the same time, one cord (or several daisy-chained together) becomes both a trip hazard and a fire-code problem most inspectors will flag. That’s the point where running an actual buried circuit — sized by an electrician for your real load, not guessed at — earns back the up-front cost.
The dig: depth requirements if you’re burying cable
If the run to your greenhouse goes underground, NEC Table 300.5 sets the minimum cover (the distance from the top of the buried cable or conduit to finished grade, not the bottom of the trench):
- 24 inches — standard direct-burial cable (UF-B) in most residential situations.
- 12 inches — a 120-volt residential branch circuit, if and only if the entire underground portion of the circuit is GFCI-protected upstream — at the panel or the first junction box before the wire goes underground, not just at a downstream outlet.
- 6 inches — rigid or intermediate metal conduit (RMC/IMC), under the same GFCI-protected-circuit condition.
These are national minimums; local jurisdictions can and do require more (frost-line depth in colder climates is a common reason to dig deeper than code strictly demands), so confirm the exact number with your building department before you rent a trencher. This is also the same inspection point covered in our guide on whether a greenhouse needs a permit — a buried electrical run almost always needs its own permit and inspection, independent of whatever exemption applies to the structure itself.
Where the DIY line actually sits
Plugging a GFCI-protected extension cord into an existing outdoor outlet, installing a plug-in thermostat or humidity controller, and swapping a cracked outlet cover for a proper WR in-use one are all reasonable homeowner jobs. Opening the main panel, adding a new breaker, or trenching and terminating a buried circuit to a detached structure is electrician territory in most jurisdictions — not just because of code, but because a mistake in a damp, enclosed structure is a shock or fire risk, not just a blown fuse. If you’re already planning the foundation or anchoring for a permanent kit, that’s the right time to have the electrician trench the circuit at the same time, before the structure and any landscaping around it are in the way.
The bottom line
A greenhouse outlet is a damp-location outlet by definition, which means GFCI protection and a weatherproof in-use cover aren’t optional extras — they’re the baseline. A single-season heater or heat mat is fine on a heavy-duty outdoor extension cord; anything running unattended, plumbed to water, or permanent should be a real buried circuit sized and installed by a licensed electrician, at 24 inches of cover (or 12 with upstream GFCI protection) per NEC Table 300.5. Get that part right once, and every other upgrade in our greenhouse automation guide — thermostat, humidity controller, timed irrigation — becomes a simple plug-in decision instead of a safety question.