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Discussion Starter · #1 ·
I'm finishing my basement and have a 90A subpanel I wired myself (1-1-1-3 AL SER cable). I have one circuit (15A) for all lights except the bathroom (all lights are LED), one single outlet on a separate circuit (15A) for tools, and a circuit (20A) for the bathroom with lights and a duplex outlet. I do have more circuits wired but they are not powered yet and no outlets hooked up before inspection. All of the lights on the dedicated light circuit total about 700W based on the fixture labeling (all are LED recessed lights).



When running my miter saw on the circuit with just the outlet, I notice that the lights on both the bathroom circuit and the dedicated light circuit get brighter for a fraction of a second, and then return to normal.


I suspected a loose neutral, so I set my meter to max/min. I'm unable to easily test the dedicated light circuit as I don't have an outlet there, so I tested the bathroom circuit. Normal voltage on both outlets across the two circuits is about 122. When I run the saw on the 15A circuit, bathroom voltage goes up to 124.5 watts max. When I run the saw on the bathroom circuit, the lights in the bathroom still get brighter (same circuit), but the voltage shows a drop to 113, with max being 122.


Is there something in the design of LED lights that would cause this quick increase in brightness? Some sort of compensating adjustment based on the large startup power draw of the saw? Does a 2W increase on the separate circuit indicate a loose neutral somewhere?



Thanks for any help you can provide.
 

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Discussion Starter · #3 ·
No, everything across all circuits is standard wiring, either 14-2 (lights and outlet circuit), or 12-2 (bathroom). I do have (4), three-way wired sets of lights on the circuit. Bathroom lights are not three-way.
 

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Discussion Starter · #4 ·
I did some additional testing by hooking up a plug-in lamp to the outlet in the bathroom. The lamp has an old incandescent lightbulb in it. When I run the saw, the LED lights get momentarily brighter, while the lamp bulb gets dimmer, as I would expect. So it must be something with the LED light cans?
 

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No.....torque every connection in the distribution box and outlets to proper specs.

You have a loose connection.....somewhere.

A properly torqued connection is much different than you think.
 

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Discussion Starter · #6 ·
I've double checked the connections at the breakers and at the main panel - everything is tight. I don't have a torque screw driver, but I'm pretty strong, and I can't get it any tighter.


I was able to check the voltage at the last light on the light circuit. It's roughly 121. When I run the saw momentarily on the 15A circuit, it dips down to 118 (min), but the lights still get brighter momentarily, before the voltage comes back to normal while the saw is running.

When I run the saw on the bathroom outlet, the min voltage is 121 (standard), but the max jumps up to 122.4.

So on one circuit, the voltage on the light circuit drops slightly, and on the other it goes up slightly.

Would the proximity of the lighting circuit breaker to the other two breakers in the panel make a difference? I've attached a picture of the panel. The lighting circuit is the AFCI one and is right next to the 15A outlet circuit (voltage drops when saw is run) but above the bathroom circuit breaker (voltage increases slightly when saw is run).
 

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It doesn't surprise me that the lights flicker. The motor on the saw can cause a substantial voltage transient, and that's part of what you are seeing with your measurements.



LEDs all have a power supply that reduce the AC line voltage to something that the LEDs can handle. These power supplies often use the fewest components necessary to get the job done and don't deal with fluctuating voltages very well. Hence the change in brightness when you start the saw.



If you've checked all the connections, then I'd say there is little to worry about. You'll just need to get used to the flickering, or switch back to incandescent bulbs.
 

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Discussion Starter · #8 ·
Thanks. I can live with it, I just wanted to make sure I hadn't made a mistake that had the potential to start a fire. Seems like things are wired correctly.


I also wired up an outlet to the last light on the circuit and used a plug-in tester, and it shows everything is wired correct, for whatever that is worth.



Thanks again for all the help. If I missed something I should test, let me know.
 

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Discussion Starter · #9 ·
So, I did a little more testing and I thought I'd share the results.


When I run the miter saw, the circuit on the same bus bar drops down 2-3 volts, from about 122 to 119.



However, the circuit on the opposite bus bar INCREASES in voltage about 1.5V, goes from 122 to 123~124 for an instant. The voltage then settles down a little bit to about ~122.5, before returning to 122 once the saw stops running.


I've switched the circuits between bus bars, and the results are the same.


Is it expected that the opposite bus bar will increase voltage by 1-2 volts to compensate for the large power draw of the saw before settling down a little bit? Or is there a loose neutral somewhere? I have access to all the electrical connections, and after double checking I didn't find a loose neutral.
 

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When the motor first starts, it looks like something close to a short circuit until the motor gets up to speed. That causes it to draw very high current on one of your hot feeds, and since it is a 120V motor, the current returns on the neutral.

The hots and the neutral wires have pretty low resistance, but the high current can cause a small, momentary voltage drop on the one side the motor is powered from.

The other hot leg is 180 degrees out of phase from the one the saw is on. So that current flowing through the neutral that you see as a voltage drop on the side the saw is on, looks like a voltage increase on the opposite leg.
 

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The drop in voltage read on one 120V circuit also produces an increase in voltage above Ground on the Neutral.
While it a bit hard to explain in simple terms, this change in voltage is anti-phase to the other 120V supply and INCREASES the voltage of that supply, which you are measuring.

The greater the drop on one side, the higher will be the rise on the other.
 

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The rise on one is half the drop on the other because half the drop is on the Line wire and the other half is on the Neutral
 
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