How many grow lights per circuit

Lights per circuit, by breaker and voltage

Each cell is the number of lights a circuit can carry at 80 percent of the breaker. The usable amps are 12 A on a 15 A breaker, 16 A on a 20 A breaker, 24 A on a 30 A breaker, 32 A on a 40 A breaker, 40 A on a 50 A breaker. 120 and 277 V circuits are single pole, 208 and 240 V circuits are two pole.

Free to cite with a link to this page. Amps are the line current on the maker sheet for that voltage. A star marks a voltage the sheet does not list, computed as watts divided by volts, which is close for LED drivers with a power factor near 1. Check the sheet for your exact model, and never count the start up current of a magnetic ballast as running load.

Why only 80 percent of the breaker

The National Electrical Code calls a load that runs for 3 hours or more a continuous load, and grow lights on a 12 or 18 hour cycle are exactly that. Sections 210.19 and 210.20 require the branch circuit conductors and the breaker to be rated at no less than the non continuous load plus 125 percent of the continuous load. Dividing 1 by 1.25 gives 0.8, which is where the 80 percent rule of thumb comes from. A 20 A breaker carries 16 A of lights, a 30 A breaker 24 A.

The exception is a breaker and enclosure listed for 100 percent continuous duty, which the breaker label states. A standard branch breaker is not, so plan on 80 percent unless the label says otherwise.

Monthly electricity use and cost

kWh a month is watts times hours a day times 30, divided by 1000. The table is for a room of 8 lights. Put in your own utility rate above, since rates vary widely by state and by tariff.

Lights only. Air conditioning, dehumidifiers and fans often add as much again, so size the service on the whole room, not the lights.

Moving from 120 V to 240 or 277 V

The three LED fixtures above accept any input from 120 to 277 V. The watts stay the same, so the bill does not change, but the current falls as the voltage rises. A Gavita 1700e draws 5.3 A at 120 V and 2.3 A at 277 V, so a 20 A circuit goes from 3 lights to 6. Fewer circuits means fewer breakers, less copper and a smaller panel. A building with 480Y/277 V service can run lights straight off 277 V, and a 480 to 240 V transformer or a 480 to 120 V transformer feeds anything that cannot. The 220 V versus 240 V guide explains why the same circuit goes by both names.

When the grow outgrows the service

Add up the lights, the cooling and the dehumidifiers in kW. Run the total through the kW to amps calculator and compare it with 80 percent of the main breaker. Forty 630 W LEDs alone are 25.20 kW of continuous load before any air conditioning. A large grow usually ends up on 208 or 480 V three phase with its own transformer. The kVA to amps calculator shows what each transformer size delivers, and the commercial service sizes chart maps every service from 100 to 4000 A to the transformer that feeds it.

Common questions

Sources

• IAEI Magazine, 100 vs 80, choosing the right overcurrent device (NEC 210.20 continuous load rule)
• Fulham, 1000 W HPS ballast specification sheet
• Keystone, HPS-1000A-Q-KIT ballast specification sheet
• EC&M, NEC branch circuit overcurrent protection and continuous loads
• Iluminar, 315 W CMH fixture manual
• Luxx Lighting, 315 W CMH specifications
• Gavita, Pro 1700e LED manual
• Horticulture Lighting Group, 650R manual
• Spider Farmer SF4000 EVO specifications, via Trimleaf

Planning a grow that needs three phase power. Request a transformer quote.