Short answer, in plain words. Electricity is like water in a pipe: watts are how much work you're doing, amps are how much is flowing, and the breaker is the size of the pipe. Amps = watts ÷ volts. Grow gear runs for hours, so the rules say: only fill a circuit to 80%. That's about 1,440 W on an ordinary 15 A, 120 V circuit. A 4 × 4 tent usually fits on one; a 10 × 10 room needs several circuits or 240 V. Plug into GFCI-protected outlets, never chain power strips together, and have a licensed electrician add circuits. Established (electrical codes and Ohm's law).
Use the calculator: electrical calculator. For general safety habits read electrical and water safety.
Safety. This guide is for planning. New circuits, 240 V outlets and panel work must be done by a licensed electrician, to your local code and with permits where required. Water and electricity kill: protect every outlet with a GFCI (RCD).
What are the formulas?
- Amps (A) = watts (W) ÷ volts (V) ÷ power factor (PF). For resistive loads PF = 1. Good LED drivers run at about 0.95–0.98 at full load [7]; use the label's input current if it's given.
- Continuous-load limit: loads that run 3 hours or more count as continuous, and codes require the circuit to be rated at 125% of them, which is the same as loading it to no more than 80% of the breaker [1][2].
- Circuit capacity (W) = breaker amps × volts × 0.8.
- Use full power, not the dimmed setting. You'll turn it up one day. GenetixBay's house position is to oversize lights and dim them (founder's field experience) [8] (see lighting sizing); size the wiring for 100%.
How much can each circuit carry?
| Circuit | Maximum | Continuous (80%) |
|---|---|---|
| 15 A, 120 V | 1,800 W | 1,440 W (12 A) |
| 20 A, 120 V | 2,400 W | 1,920 W (16 A) |
| 20 A, 240 V | 4,800 W | 3,840 W (16 A) |
| 30 A, 240 V | 7,200 W | 5,760 W (24 A) |
| 40 A, 240 V | 9,600 W | 7,680 W (32 A) |
Everything already on the circuit counts too: room lights, a fridge, a space heater in the next room. Find out what else your breaker feeds before you plug in. Most North American homes have 120 V outlets and 120/240 V service at the panel.
- 1,440 W15 A, 120 V
- 1,920 W20 A, 120 V
- 3,840 W20 A, 240 V
How many amps for each size?
Lights for about 900 µmol/m²/s with a top-tier LED (see lighting sizing), fans scaled to size, and a dehumidifier's running watts (assumed; check its label). PF 0.98.
| Space | Lights | Fans | Dehumidifier | Total | Amps at 120 V | Amps at 240 V | Fits |
|---|---|---|---|---|---|---|---|
| 2 × 4 ft tent | 260 W | 40 W | 300 W | 600 W | 5.1 A | 2.6 A | One 15 A circuit |
| 4 × 4 ft tent | 525 W | 80 W | 300 W | 905 W | 7.7 A | 3.8 A | One 15 A circuit |
| 4 × 4 at 50 W/ft² (house rec.) | 800 W | 80 W | 300 W | 1,180 W | 10.0 A | 5.0 A | One 15 A circuit, near the limit |
| 5 × 5 ft tent | 820 W | 100 W | 400 W | 1,320 W | 11.2 A | 5.6 A | 15 A at the limit; 20 A better |
| 5 × 5 at 50 W/ft² | 1,250 W | 100 W | 400 W | 1,750 W | 14.9 A | 7.4 A | 20 A, 120 V or 240 V |
| 4 × 8 ft tent | 1,050 W | 120 W | 400 W | 1,570 W | 13.4 A | 6.7 A | 20 A, 120 V |
| 4 × 8 at 50 W/ft² | 1,600 W | 120 W | 400 W | 2,120 W | 18.0 A | 9.0 A | Two 120 V circuits, or one 20 A 240 V |
| 10 × 10 ft room | 3,280 W | 300 W | 700 W | 4,280 W | 36.4 A | 18.2 A | Three 20 A 120 V, or a 30 A 240 V |
| 10 × 10 at 50 W/ft² | 5,000 W | 300 W | 700 W | 6,000 W | 51.0 A | 25.5 A | A 40 A 240 V circuit, or split across two |
- Add every wattAt full power, not dimmed
- ÷ volts ÷ power factoramps
- Compare with 80% of the breaker
- Too much? Split circuits or go 240 V
Air conditioners and mini-splits usually need their own dedicated circuit, sized by the installer from the unit's nameplate. Don't add them to the lighting circuit.
Worked example, 4 × 8 tent at 50 W/ft²: 2,120 W ÷ (120 V × 0.98) = 18.0 A, over the 16 A continuous limit of a 20 A circuit. Options: split lights across two circuits, or run the lights on a 20 A 240 V circuit (9 A, well inside 16 A).
Should I run my lights on 240 V?
Often, yes, for bigger grows. Same watts, half the amps. A 20 A 240 V circuit carries twice the continuous power of a 20 A 120 V one. Most quality LED drivers accept about 90–305 V AC and run a little more efficiently at 230–240 V than at 115–120 V (see LED drivers explained) [7].
What it takes: a 240 V circuit from the panel, the right receptacles and plugs (for example NEMA 6-15 or 6-20 in North America), fixtures and cords rated for it, and a licensed electrician. Established.
Power strips, extension cords and daisy chains
- Don't plug power strips into each other or into extension cords. Listed relocatable power taps are meant to plug straight into a wall outlet, and chaining them can overload the first strip and cord [3][4][6].
- Don't use extension cords as permanent wiring [1].
- A cord-and-plug load on one receptacle should stay at or under 80% of that receptacle's rating, for example 12 A on a 15 A outlet [1].
- Use heavy-duty, listed strips and timers rated above the load (a "15 A" timer running 12 A continuously is at its limit). Warm plugs, discoloured sockets or a burning smell mean stop now.
- Keep everything off the floor and above the highest water line, with drip loops on cords.
Why GFCI?
A GFCI (RCD outside North America) compares the current going out with the current coming back and cuts power within a fraction of a second if some is leaking to ground, for example through water or you [5]. US code already requires GFCI protection on outlets in basements, garages, outdoors, laundry areas and near sinks [1]; Canadian code has similar requirements [2]being reviewed. A grow room has water, pumps and wet hands; protect every outlet.
Common mistakes
- Sizing circuits for the dimmed setting, then turning the lights up.
- Counting only the lights. Fans, dehumidifiers, pumps, heaters and whatever else is on that breaker all count.
- Running a 15 A circuit at 15 A for 12 hours. The continuous limit is 12 A [1][2].
- Daisy-chained power strips and extension cords [3][4][6].
- No GFCI anywhere near water [5].
- DIY 240 V wiring. Hire a licensed electrician.
