Can't figure this one out - Dimming lights under load - Page 2 - Electrician Talk - Professional Electrical Contractors Forum
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Old 09-21-2018, 03:14 PM   #21
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With a panel that ancient, I'd start to worry about surface corrosion on the bus bars.

I've seen amazing things with much younger panels.

The extra resistance doesn't show up until the circuit is loaded.

That thing looks fifty-years old... even older.
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Old 09-21-2018, 06:49 PM   #22
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Originally Posted by NDC View Post

Before they showed up I measured 110 and 124. They apparently corrected it and is now 120 and 123.

Both are wired for 120, the vac has a plug end on it. I plan to wire the pump for 240 if I swap the panel.
Ok you had a leg problem that the poco has now fixed (if it was a neutral problem the numbers would still add to 243v so if leg A went to 110v leg B would have been around 134)

Electricity is like water think of a hose pipe with a spray on the end. If the sprayer is turned on water shoots out under pressure. Good flow (amps) good pressure (volts). Now kink the hose and up to the restriction you still have good pressure but after the restriction you have reduced flow which means you also have reduced pressure.
Funny thing is if you turn sprayer at the end of the hose off even if the hose is kinked the water will slowly fill up the hose and build up pressure. This is what you read with your meter. Its 120v under no load yet will drop once a load has been applied (sprayer is opened / something big has been turned on).

Now all you have to do is find the restriction and to do that add a load like turning on the stove. Measure before the main breaker. If your 240 drops by more than 12v (5%) then it sounds like the wire may be to small. If it drops 24v then it is to small for the load or theirs still a bad connection. poco has a load (heater) they plug in the meter socket which is probably how they traced the first problem they fixed.

Now test the voltage after the breaker is it the same.(same means good). If not measure incoming A leg to outgoing A leg it should read close to zero, Repeat for b leg. (zero means good).


If all of these tests pass then move on to the 120 items. measure between breaker in or out and after the fuses if it reads 240 switch to the other leg at the breaker. Zero is good. Have some one turn on the vac or pump and watch the reading. (vac and pump can pull 7 times name plate fla for a fraction of a second so they add a real high load).
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Old 09-21-2018, 10:44 PM   #23
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The voltage of the incoming mains is not constant at 120v.
It varies from minute to minute depending on many variables.
Because in the past lighting was primarily incandesant,
incandesant lights have a high thermal inertia
So they don't easily show variations in voltage
So we don't see it, doesn't mean it's not there thou !
Now along comes LED lighting almost no thermal inertia,
shows every sign of varing voltage
NOW WE HAVE A PROBLEM !
even thou it was always there.
Better design of LED bulbs will sort it out.
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Old 09-23-2018, 03:35 PM   #24
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A lot of people keep bring up the neutral. Why?

For the sake of voltage drop, there is no difference if the loose connection is in the neutral or the hot wiring.

The neutral seems to have some type of urban legend & proverbial folklore associated with it. Blaming loose neutrals for everything, shocks from neutrals hurting more, neutrals touching you in bad places. Jeeze!


Not to mention at least one of those two loads are most likely a 220v load. Neutral would not even be in play.


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Old 09-23-2018, 03:39 PM   #25
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Originally Posted by NDC View Post
I wish I had checked to see if the A/C dims them as well as it's wired for 240 and no neutral. It's pretty evident that there was a short in the main compartment at some point in time (top left)
I also read different voltages from the main breaker mounting screws and the load side. A small difference but still a difference.
Also if this is a line side issue, shouldn't the neighboring homes using the same xformer have a similar problem?


At first sight Iím like change that panel now. But besides being old it is probably working quite well I havenít seen a panel like that in the 20 years Iíve been in the trade. With a breakered main and then fuses on the bottom. Thatís just crazy and belongs in a museum. But honestly any panel over 50 years old needs to be changed for good measure.


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Old 09-23-2018, 03:56 PM   #26
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Originally Posted by HackWork View Post
NDC, try it out on the bench. Wire up an outlet but have one of the wires so that the tip is just barely touching the screw. Measure the voltage and then turn something with a large draw on and watch where the voltage goes.

This is why many of us use a heat gun when troubleshooting.


This is also a very good tool to have. Replaces the heat gun for testing and also tests wire connection quality. As well as voltage drop tests.

Great tool when finding loose connections.


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Old 09-23-2018, 03:59 PM   #27
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Originally Posted by tmessner View Post
We see this all of the time in rural areas even with a 200 amp service and 4/0 or 250 kcmil al urd. It is just a fact of motor starting current pullinjg the voltage down momentarily. Unless it is something that just started to happen or is getting worse I would not worry about it.


Iím those instances there is also a quick thing to try that fixes it a lot of the time. The reason it is dimming the lights many times is because the buss bar only has so much surface area. If you put the high current load closest to the main incoming power / main breaker. Then when it turns on, it takes all the power before anything else can. (Path of least resistance) now move the highest load items to the furthest point in the buss bar. And watch the lights not dim.


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