# Grow room amps and circuits calculator

> Free grow room electrical calculator: total watts to amps at 120 or 240 V, the 80% continuous-load rule, how many circuits you need, and the GFCI and no-daisy-chain rules.

Interactive: https://genetixbay.com/knowledge/tools/electrical.html

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.

## How it works

### 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.

## Electrical safety

- Use GFCI protection on every grow circuit: water, pumps and power share the room. The code already requires it in basements, garages, outdoors and near sinks.
- Never daisy-chain power strips or plug one into another, and don't use extension cords as permanent wiring.
- Plug heavy loads (lights, AC, heaters) straight into wall receptacles, keep cords off wet floors and hang drivers and power strips above the flood line.
- Adding circuits or a 240 V line is a job for a licensed electrician. Check your local code and your insurance.

## Constants

| Constant | Value | Unit | 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

1. 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
2. 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/
3. 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
4. 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