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    humming, buzzing panel

    I've been wiring a large metal ag building. My helper and I were installing ceiling lights in the shop. There are 12 fluorescent luminaires; each has 3 32-watt 4-ft bulbs and an electronic ballast. They are in three banks of four with each bank having its own switch. They are on their own 20 A, 120 V circuit. As we plugged in each one, they lit-up nicely. When we were done, I noticed buzzing from the panel. The buzzing went away if we turned off the breaker for the lights. The noise also went away if we turned off the light switches. Then if we turned them on again, one at a time, the noise began with the first one and became progressively louder as we turned on more. The noise is not really loud enough to be annoying in the shop.

    The shop subpanel is a 100 A Murray (LC2040B1100), and the breaker is a new Murray MP120. We tried swapping the breaker with another Murray MP120 with the same result. Then it dawned on me that the electronic ballasts are nonlinear, and they would all key off the power company frequency in synchrony. So, I'm assuming harmonics are traveling back down the wires to the panel and are responsible for the buzzing. I've searched the internet and found some reports of this phenomenon (like Fluke's site). If 1500 W of quartz halogen lamps are plugged into an outlet on the same circuit and the fluorescent lights are off, the panel is silent.

    Apparently, with the usual type of thermal/magnetic breaker (I think that the Murray is one), there is nothing to worry about. It's faster responding breakers (peak current type) that have trouble with harmonics.

    Comments? Anything to be concerned about?

    Mike

    #2
    I am closing this thread, in accordance with the forum rules. We are not permitted to provide how-to assistance to a person who is not on electrician.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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