How many circuits does my grow need?
Amps = watts ÷ volts. Grow gear runs for hours, so the code treats it as a continuous load: use only 80% of the breaker. Add up everything that can run at once, at full power, and see how many circuits that takes.
Your result
How it works, in plain words
Amps
Amps = watts ÷ volts. We divide by a 0.95 power factor too, because drivers and ballasts draw a little more current than their watts suggest.
The 80% rule
Anything that runs 3 hours or more is a continuous load, so a 15 A breaker carries 12 A (about 1,440 W at 120 V), a 20 A breaker 16 A (1,920 W at 120 V, 3,840 W at 240 V).
240 V
The same watts at 240 V draw half the amps. Many LED drivers accept 120–277 V and run slightly more efficiently at the higher voltage.
Worst case
Size for everything at 100%: lights undimmed, AC, dehumidifier and fans all running together.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
CONTINUOUS | 0.8 × breaker rating | Continuous loads (3 h or more, which every grow light is) may use only 80% of the breaker | Established (code) |
PF | 0.95 power factor | Drivers and ballasts draw slightly more amps than watts ÷ volts; we divide by 0.95 to be safe | Estimate |
Sources
- NFPA 70, National Electrical Code (2023): Art. 100 (continuous load = 3 hours or more), 210.19(A)(1) and 210.20(A) (conductors and breakers at 125% of continuous load, i.e. load ≤ 80%), 210.8 (GFCI locations), 400.12 (flexible cords not a substitute for fixed wiring). https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
- CSA C22.1, Canadian Electrical Code, Part I: Rule 8-104 (continuous loads limited to 80% of the breaker rating) and Rule 26-700 series (receptacles and GFCI). https://www.csagroup.org/store/product/CSA%20C22.1:24/
- GenetixBay Knowledge Base. LED vs HPS: the heat and power-bill hype, and an LED buyer-beware guide (fixture µmol/J tiers, 85% canopy capture, watt-for-watt house advice, 1 W = 3.412 BTU/h). https://genetixbay.com/knowledge/equipment/led-vs-hps-hype-and-buyer-beware.html
- OSHA. 29 CFR 1910.303(b)(2): listed equipment must be used per its listing and labelling (power strips are not to be chained together). https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.303
