Tradesman14

Ok i have a 100 amp SD panel inside my 900 sqft home.
I back fed my garage with this also
Now when my ac unit kicks on i believe its a 30 amp.
It dims all the lights in my home. I'm currently attended a JVS for electric and i'm 17 years old worked for a contractor for 2 years now and didn't feel comfortable asking my boss...I was thinking maybe the ac unit is going out and over drawing?? or maybe because the POCO's closet transformer is 3 houses down.
I'm not sure but what could this be.

AllanJ

Most likely the dimming is occurring all through the system up to the power company transformer.

You would need to put a voltmeter to various locations including where the service cables enter your main panel to narrow down where the "bottleneck" is. It helps if you or an electrically inclined friend can do measurements up front since any electrician or contractor will charge you for a service call regardless of what he finds or does not find.

There have been cases when neighbors' lights dimmed when someone's AC started up.

An AC unit might draw more power if the Freon level inside has gone down and/or air has gotten in as the result of a leak. Usually this is not noticed unless the breaker trips or the AC unit tries to start and stops and retries given an overheating sensor inside, or has gone on for so long that the house isn't getting cool.

Appliances may have separate ratings for maximum and normal current draw printed on a foil name plate or in the instructions. If only one current draw rating is shown, it is generally normal for the startup current draw to exceed that for a second or two.

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Did you know?
The most commonly used pattern on a portable lawn sprinkler is a semicircle, which represents half the coverage area advertised on the package.

SD515

To start with I'd shut the power off to the AC and check all it's wiring connections, including the wiring connections at the breaker and the breaker to the panel itself. I'd also use an 'amp-clamp' type tester and check the current draw on the AC, see if that's within specs. Off hand, sounds like a loose 'hot' connection somewhere.

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Kyle

It's an obstacle ...not a barrier

oilseal

Lights dimming as large appliances turn on is not unusual. If you'd connect an Amprobe on the feed to the A/C, it'd likely indicate a surge a good bit more than 30a. I see lights dimming in houses that have 200a services. It's a simple matter of Ohms Law.

Yoyizit

Put a known load on your 240v. Here's how I did mine.

I used an unused dryer outlet to measure the voltage at the panel, so I didn't need to remove the panel cover.

The kitchen wall oven is the load, marked as 5.1 kw at 240v.

21.25A @ 240v for the oven = 11.29 ohms

~247v dropped to ~246.5 when broil is selected and the oven is switched on.

246.5v/11.29 ohms = 21.83A

0.5v/21.83A = 0.0229 ohms

Isc = 247v/.0229 ohms = 10,800A, for resi. service but the pole 'former is right out front.

6 milliohms out of this 23 mΩ could be the 000 SE cable.

So the Thevenin equivalent circuit of my PoCo is a 247v source in series with a 23 milliohm impedance.

If your 'former is down the block your impedance could be higher.

If enough forum members do this test and report results we can make up likely pass/fail limits for this impedance.

MD's PoCo says our resi. voltage should be 240 +/- 5% but some states allow +/- 10%. They also told me they are not obligated to observe any power quality standards.

With #000 wire coming in I probably have 200A service.

CFLs are relatively insensitive to these voltage variations.

AllanJ

>>> ... your impedance could ...
We cannot rule out a possible loose and burned connection (loose connections tend to spark and burn up and increase their impedance and get worse over time) including under the main breaker or the electric meter. As the latter part of the system is sealed by the power company, only they can check it although you may be responsible for paying to fix it.

>>> With 000 wire coming in ...
Your meter may also state an amperage or wattage that could further limit the total main breaker amperage (whether in one or more main panels). Changing or adding main panels may require a separate permit so as to give the POCO a chance to see you are not exceeding the meter's capacity. Actually I would make two measurements at each location, one side hot to neutral which is approximately 120 volts no-load, then the other side hot to neutral. This would detect a loose connection that was only on one side.

As far as putting known loads here and there, all I would bother to check is how far down (in volts) as measured at the air conditioner's breaker screw, at the fins under the breakers best accessed from an empty slot which are the main breaker outputs, at the service cable clamps which are the main breaker input, and if possible somewhere outside the house which would depend on some exposed bare wire or clamp. Each time as the air conditioner comes on.

__________________
Did you know?
The most commonly used pattern on a portable lawn sprinkler is a semicircle, which represents half the coverage area advertised on the package.

J. V.

The AC draws a significant amount of power (current) when starting. This in turn lowers the voltage, which in turn dims your lights. It is perfectly normal and you should not be concerned at all.

petey_c

An analogy I like to use is; Electricity is like water flowing through plumbing pipes in your house. You're taking a shower and someone flushes the toilet, you're briefly taking away pressure while the tank refills. An A/C is a large draw (starting current can be three times running current) and can momentarily dim the lights. I'd tend to think as long as they don't stay dimmed, you don't have a problem. pete

Yoyizit

What gauge wire they use from house to house? It's maybe 5x or 10x the CM of 000.

J. V.

You're killin me Yoyizit. Killin me!

Yoyizit

I'm doin' my best but you ain't dead yet.

The wires that feed other houses seems to be ~1.5x the dia. of the ones that feed my house, so ~2x the CM.
A house 500' from this 'former could have 80 milliohms impedance at the load center so Isc = ~ 3 kA and your voltage drop with the load test would be ~4x higher.

WFO

This is probably the best answer unless your dimming is severe. A brief blink is nothing to worry about. If your fluorescent lights go off, it's severe. If it's brief, make a game of drinking a beer every time it happens and you'll come to love it.

A quick check is to ask your neighbors that are on the same underbuilt. If they all are having problems (usually less so the closer you get to the transformer) then call the POCO.

Viper16

They do sell motor starters...or capacitors that may help prevent it...sort of like a capacitor for a flash on your camera...it stores up a lot of energy for quick dispersion when ever there is a demand for it. This could prevent the lights dimming due to the inrush currents req'd. by your A/C unit.

Yoyizit

http://www.google.com/search?hl=en&s...&aq=f&oq=&aqi=

portclinton

Try updating to a 200 Amp service. I did this on my rental property and the power fluctuations went away. It will also increase the value of your home. Like petey c said above: electricity flow is comparable to plumbing pipes in your home. If you run 3/4 pipes instead of 1/2 inch, the water fluctuation will be a little less. Same with electrical service.