Folks -
After adding another grow light to my spring planting seedling setup, which runs off my shop sub-panel on a 15 amp breaker, I noticed the breaker was making a humming sound. There is nothing on the circuit besides the three lights, which total 860 watts/8 amps according to their labels. If it matters, the lights are Agrobrite T5 high-output fixtures, and are new-ish (2 bought last year, the third just a few days ago).
I tried two other 15 amp circuits in the same subpanel, also with nothing else being powered at that time, and they both immediately started humming. I plugged the lights into a 20 amp circuit, also on that same subpanel,and no hum.
In the interests of experimentation, I grabbed a 1500 watt space heater and plugged it into the same 15 amp circuits, and there was the tiniest, barely audible hum. I'd never have noticed it if I wasn't putting my ear to the subpanel and having absolute quiet around me. The hum when they lights were plugged in was much louder, easily heard even at a distance.
Oh, and there was virtually nothing else running off the subpanel during this time - just a couple overhead fluorescents. THe subpanel is 30 amps,I
Can anyone shed some light on this? Are all of the 15 amp breakers bad, or getting there? Is this just electrical noise? Why do the lights make so much hum even though they are well within the circuit's capacity, and make MORE noise by far than something taking almost 2x as much power? I would love to measure the current directly, but I don't have an ammeter.
And, most important of all, is it ok to run those lights on the 15 amp circuit? The 20 amp circuits run my shop machines, usually, although I can swap where things plug in when I'm working in the shop. Kind of a PITA though.
Thanks for taking the time to read this -
Ken