This page is researched general information and orientation, not professional electrical advice. Electrical installation is governed by local code, and a supply run to an outbuilding involves conductor sizing, burial depth, conduit, a disconnect, bonding and grounding decisions that depend on the specific installation. That work is a job for a licensed electrician. What follows is what you need to know to have a useful conversation with one, and to use the equipment safely once it is installed.
The reason it matters so much in a greenhouse is that a greenhouse is a wet location containing metal, standing water, damp soil and a person, all at the same time. It is one of the least forgiving electrical environments on a domestic property.
What a circuit can actually carry
The arithmetic is simple and it constrains everything else about a greenhouse.
A 120 volt 15 amp circuit supplies 1,800 watts at full load. A continuous load, defined as one running for three hours or more, should not exceed 80 percent of the circuit rating, which is 12 amps or about 1,440 watts. A greenhouse heater is a continuous load by definition. A 1500 watt heater draws 12.5 amps, which is at or fractionally over that limit before you plug in anything else at all.
| Equipment | Typical draw | Amps at 120 V | Notes |
|---|---|---|---|
| Electric heater, 1500 W | 1,500 W | 12.5 A | Continuous. Fills a 15 A circuit on its own |
| Electric heater, 2800 W at 240 V | 2,800 W | 11.7 A at 240 V | Needs a dedicated 240 V circuit |
| Seedling heat mat, 10 x 20 in | 17 to 25 W | Under 0.25 A | Negligible, which is the whole argument for bottom heat |
| LED grow light strip, 4 ft | 20 to 40 W | Under 0.4 A | Several strips still total well under an amp |
| Shutter exhaust fan, 12 in | 40 to 90 W | Under 1 A | Intermittent, driven by a thermostat |
| Circulation clip fan | 3 to 10 W | Negligible | Runs continuously and costs almost nothing |
| Evaporative cooler | 150 to 250 W | 1.5 to 2 A | Plus a water supply |
| Small dehumidifier | 30 to 60 W | Under 0.5 A | Removes water without removing heat |
The practical consequence is worth stating plainly. If the heater BTU calculation returns much above 5,000 BTU per hour, one 15 amp circuit is not enough, and the honest options are a lower target temperature, better insulation, a smaller house, or a 240 volt supply. Run the numbers in the heater BTU calculator before deciding, because insulation measures frequently bring a house back inside the limit and save the entire cost of a new circuit.
GFCI protection, which is not optional
A ground fault circuit interrupter compares the current flowing out on the hot conductor with the current returning on the neutral. If they differ by more than about 5 milliamps, some current is finding another path to ground, and the most concerning possible path is through a person. The device cuts the circuit in a fraction of a second.
In a wet location containing metal benching, damp soil, standing water and a person with wet hands, that is the difference between an unpleasant shock and a fatal one. Every greenhouse circuit should be GFCI protected, whether at the breaker, at the receptacle or by an in-line cord device.
Where the supply itself is not GFCI protected, an in-line cord such as a DEWENWILS 3 ft Auto-Reset Outdoor GFCI Extension Cord ($26.99) or a heavier 10 ft Heavy-Duty Outdoor GFCI Extension Cord ($44.60) adds protection at the point of use. These are a supplement to a properly installed circuit rather than a substitute for one, and the auto-reset feature on some models matters in a greenhouse specifically because a nuisance trip that stays tripped means the heater is off all night in a frost.
Layout inside the house
- Get receptacles off the floor. Mount outlets at bench height or higher, on the frame or on a post, well above anywhere water pools. Water finds floor-level outlets.
- Use weatherproof in-use covers. An outdoor receptacle cover that closes over a plugged-in cord is different from one that only closes when empty, and in a greenhouse where things stay plugged in for months, the in-use type is the relevant one.
- Drip loop every cord. Let each cord hang below its outlet before rising to it, so water running down the cable drips off the low point rather than running into the socket.
- Keep heaters clear. Manufacturers specify minimum clearances to combustible material for a reason. Trays, bags of mix and fleece stacked against a running heater are a fire risk, and a heater buried in foliage cannot circulate air either.
- Route cords where spades do not go. The most common damage to greenhouse wiring is mechanical, from digging, mowing or dragging a heavy pot over a cable.
- Label the circuit. Knowing which breaker kills the greenhouse matters at the moment you most need to know it.
Timers, controllers and failure modes
Almost everything electrical in a greenhouse benefits from being on a controller rather than running continuously, and a controller introduces its own failure modes worth thinking about.
A temperature controller such as an Inkbird ITC-308 Digital Temperature Controller ($36.00) switches a heater on a setpoint measured at plant height, which is both more accurate and more efficient than a built-in heater thermostat. A BN-LINK 7-Day Programmable Digital Timer Outlet ($13.99) runs a lighting photoperiod without anyone remembering. A BN-LINK 24-Hour Mechanical Timer Outlet ($8.99) does the same job with no firmware and no clock to lose, which some people prefer for exactly that reason.
The failure mode that matters is what the system does when it fails. A controller that fails with the heater on cooks the house. A controller that fails with the heater off freezes it. Consider a simple mechanical backstop: many greenhouse heaters have their own internal thermostat, so setting the heater dial to a low frost-protection setting and letting the external controller do the fine control means a controller failure leaves the heater doing something sensible rather than nothing.
Monitoring closes the loop. A logging sensor such as a Govee H5179 WiFi Thermometer and Hygrometer ($33.99) that reports to a phone tells you the house went cold at 3 a.m., which is information you cannot get any other way and which turns a lost crop into a fixed thermostat.
What you can reasonably do yourself, and what you cannot
Rules vary by jurisdiction and this is not legal or professional advice, but the general shape is consistent.
Plugging appliances into an existing, correctly installed, GFCI-protected outdoor receptacle is normal use. Fitting an in-line GFCI cord is normal use. Mounting a heater at the clearances the manufacturer specifies is normal use. Choosing where the bench and the cords go is normal use.
Running a new circuit, trenching a supply to an outbuilding, installing a subpanel, adding a disconnect, sizing conductors, and anything involving a breaker panel is licensed electrician work in most places, and it is inspected for good reason. A 240 volt supply to a greenhouse is a project with a permit, not a weekend job.
Electrical safety and control equipment
The electrical supply available at the site is the single hardest limit on what a greenhouse can ever do, which is why it belongs in the siting decision rather than in the equipment decision. That is covered in siting and orientation.