# Calculate it properly: watts, amps, circuits and 240 V for a grow

> Amps = watts ÷ volts (÷ power factor for AC gear). Grow equipment runs for hours, so codes treat it as a continuous load: keep it to 80% of the breaker, about 1,440 W on a 15 A 120 V circuit, 1,920 W on 20 A 120 V, and 3,840 W on 20 A 240 V. A 4×4 ft tent with lights, fans and a dehumidifier fits one 15 A circuit; a 10×10 ft room needs several 120 V circuits or a 240 V circuit plus a dedicated one for the AC. Use GFCI protection, no daisy-chained power strips, and a licensed electrician for new circuits.

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## Questions and answers

**How many watts can I run on a 15 amp circuit for a grow?** On a 15 A, 120 V circuit you can run about 1,440 W continuously. Grow lights, [fans](https://genetixbay.com/knowledge/glossary/fan-leaf.md) and [dehumidifiers](https://genetixbay.com/knowledge/glossary/dehumidifier.md) run for 3 hours or more, so electrical codes treat them as continuous loads and limit them to 80% of the breaker rating: 15 A × 0.8 × 120 V = 1,440 W. Count everything else on the same circuit too, such as room lights or a fridge.

**How do I convert watts to amps for grow lights?** Amps = watts ÷ volts, divided by the power factor for AC equipment. An 800 W [LED](https://genetixbay.com/knowledge/glossary/led.md) with a 0.98 power factor draws about 6.8 A at 120 V and 3.4 A at 240 V. Use measured wall watts at full power, not the dimmed setting, because you might turn it up later. Add every device on the circuit and compare with 80% of the breaker.

**Should I run my grow lights on 240V?** Running grow lights on 240 V halves the current for the same power, so one 20 A 240 V circuit carries about 3,840 W continuously, twice a 20 A 120 V circuit. Most LED drivers accept 100–277 V and run slightly more efficiently at 240 V. You need a 240 V circuit and the right plugs, installed by a licensed electrician to your local code.

**Is it safe to plug power strips into each other for a grow?** No. Plugging power strips into each other (daisy-chaining) can overload the first strip and its cord, which is a fire risk, and listed relocatable power taps are meant to be plugged directly into a wall outlet. Use a properly rated, listed strip directly in the wall for small loads, or add circuits. Keep strips off the floor and away from water.

**Do I need a GFCI for my grow room?** Yes, use GFCI (RCD) protection for every grow-room outlet. A GFCI cuts power within a fraction of a second when current leaks to ground, for example through water or a person, and US code already requires GFCI outlets in basements, garages, outdoors and near sinks. Water, pumps and wet hands make grow rooms exactly the place they're for.

**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](https://genetixbay.com/knowledge/tools/#electrical). For general safety habits read [electrical and water safety](https://genetixbay.com/knowledge/guides/electrical-and-water-safety.md).

> **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](https://genetixbay.com/knowledge/equipment/calc-lighting-sizing.md)); 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.

**Continuous load at 80%.** 1,440 W: 15 A, 120 V; 1,920 W: 20 A, 120 V; 3,840 W: 20 A, 240 V. Source: NEC 210.19/210.20 [1]; [CEC](https://genetixbay.com/knowledge/glossary/cation-exchange-capacity.md) Rule 8-104 [2].

## How many amps for each size?

Lights for about 900 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) with a top-tier LED (see [lighting sizing](https://genetixbay.com/knowledge/equipment/calc-lighting-sizing.md)), 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 |

**Chart: Amps at 120 V by space vs circuit limits.** Lights, fans and a dehumidifier draw about 5 A in a 2 × 4, 8 A in a 4 × 4, 11 A in a 5 × 5 and 13 A in a 4 × 8, against a 12 A continuous limit on a 15 A circuit and 16 A on a 20 A circuit. A 10 × 10 room at about 36 A needs several circuits or 240 V.

Reference line: 15 A circuit: 12 A continuous (12).

Reference line: 20 A circuit: 16 A (16).

| Space | Lights, fans, dehumidifier (A) |
|---|---|
| 2 × 4 tent | 5.1 |
| 4 × 4 tent | 7.7 |
| 5 × 5 tent | 11.2 |
| 4 × 8 tent | 13.4 |
| 10 × 10 room | 36.4 |

Source: Table above; 80% rule [1][2]; power factor [7].

**Will it fit on the circuit?.** Add every watt: At full power, not dimmed → ÷ volts ÷ power factor (amps) → Compare with 80% of the breaker → Too much? Split circuits or go 240 V. Source: [1][2][7]

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](https://genetixbay.com/knowledge/equipment/led-drivers-explained.md)) [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]. 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.

## Sources

1. NFPA. 2023. NFPA 70, National Electrical Code: 210.19(A) and 210.20(A) (continuous loads at 125%, i.e. 80% of the rating), 210.21(B)(2) (cord-and-plug loads on receptacles), 210.8(A) (GFCI in dwelling locations), 400.12 (flexible cord not a substitute for fixed wiring). https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
2. CSA Group. 2024. CSA C22.1:24, Canadian Electrical Code, Part I: Rule 8-104 (maximum circuit loading; continuous loads at 80% of the rating for standard equipment) and GFCI requirements in Section 26. https://www.csagroup.org/store/product/CSA%20C22.1:24/
3. OSHA. 29 CFR 1910.303(b)(2): listed or labelled equipment shall be installed and used per its listing instructions. https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.303
4. UL. UL 1363, Standard for Relocatable Power Taps. https://www.shopulstandards.com/ProductDetail.aspx?productId=UL1363
5. US Consumer Product Safety Commission. GFCI fact sheet (ground-fault circuit interrupters). https://www.cpsc.gov/s3fs-public/099.pdf
6. Electrical Safety Foundation International. Power strip and extension cord safety (don't daisy-chain). https://www.esfi.org/
7. MEAN WELL. HLG-600H series specification (input 90-305 VAC; PF 0.98 at 115 VAC, 0.95 at 230 VAC; efficiency vs input voltage). https://www.meanwell.com/Upload/PDF/HLG-600H/HLG-600H-SPEC.PDF
8. GenetixBay founder. Field experience (first-hand account; GenetixBay house position).