I installed 4 recessed lights in an unfinished (now finished) room of my house...the leg was installed to that room by the electricians during original construction. I just replaced the single light with 4 new recessed lights. Now when I turn on those 4 lights the other lights on the circuit give a quick one-time flicker. This is a room off the master bedroom and is on the master bedroom's arc-fault circuit breaker.
Best way to track this down? Loose wire in one of the new lights? Weak breaker?
Sounds like probably was/is a 15A/14ga lighting circuit and you've now added too much wattage w/ the new fixtures and are seeing the temporary voltage drop when the added load comes on.
How much _total_ wattage is now on this circuit when you observe this? And, how much did you add?
As a quick/easy test, take all the bulbs but one out of the new fixtures and make it a small one -- say 60W. Now, does the flicker go away? If so, put the original back in and do the same test sequentially. I'll hazard a guess after one or two, you'll see the symptoms reappear.
I'll add my guess. The new 4 lights are probably 150 watt floods or something like that. They have a very low startup resistance, therefore a high inrush. Now, if the circuit is fairly far from the breaker box, you will get a momentary voltage drop until the filaments get fully hot. The further you are from the breaker, the higher the resistance, the bigger the voltage drop. This is especially seen on circuit with aluminum wiring because of higher per foot resistance. I see this all the time in my 2nd floor bedroom/bath. The
4 halogen lamps in the bathroom, when turned on, will cause a momentary voltage dip for the other bathroom. BTW, this circuit is aluminum and is about 50
I don't see how dirty switch contacts would make the other lights do that bit of flicker.
It sounds to me like voltage drop in the wiring from the surge of current drawn by the freshly-turned-on lights while their filaments are warming up.
Meanwhile, I would suggest to the OP to check total wattage of lights on that circuit and the wire size and breaker size. If the wire is AWG 14, maximum allowed breaker size is 15 amps which means 1800 watts. The code only allows "continuous loads" up to 80% of the breaker rating, and it appears to me that lighting qualifies as a "continuous load".
I also somewhat remember that wire size is supposed to be adequate to have voltage drop less than 5% at full current. For 14 AWG at 15 amps, that is roughly 75 feet from breaker to load. For 12 AWG at 20 amps, that is about 90 feet. 12 AWG with a 15 amp breaker is good for about 120 feet, and I would mark the breaker box so that nobody in the future replaces the 15 amp breaker with a 20 amp one on the basis of wire size.
Another note - I remember reading somewhere recently that a branch circuit must have all permanently installed wiring identical in maximum allowable overcurrent protection. I do not know for sure this is NEC; it might be a local building code for all I know. I guess the reasoning is to prevent someone from putting in a breaker that is too big for all of the wiring on the basis of seeing only some of it.
Meanwhile, since incandescents briefly draw up to 10-15 times their normal current during a cold start, some significant voltage drops in wiring resulting from this can be perfectly normal. Keep in mind that light output of an incandescent is roughly proportional to the cube of applied voltage (normally varies a little more drastically than that even)! A 5% voltage drop causes incandescents to dim by 15-17%.
If 300 watts of incandescents have simultaneous cold starts, that can mean a brief load of 4000 watts or 33 amps, possibly a little more. This is allowed. If this surge flows through just 35 feet (breaker to load) of
14 AWG wire, that can cause a brief voltage drop to already-on lights downstream of this hypothetical 35 foot wire run of 5% and briefly dim them possibly by 15-17%. A fix is not required if a voltage drop of more than 5% occurs only while filaments are warming up. Brief overcurrents are allowable - there are even fuses and breakers specifically designed to not blow/trip from more serious allowable brief overcurrents - moderate and moderately severe overcurrent will not overheat wiring in a split second.
One more solution, should you determine that a fix is needed - put a junction box only a few feet from the breaker box and split the new lighting wire run from the existing one at that point. Keep in mind to have all wiring on that circuit identical in maximum allowable overcurrent protection and to have all lights not exceed 80% of the allowable overcurrent protection - otherwise upgrade all wiring in the circuit or add another circuit.
More questions or if you have doubts that you can competently do any such needed work yourself - that's what professional licensed electricians are for.
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