Hi,
I think this was discussed on the old forum style but I’ll be damned if I can find a thing on this new style pile of crap! But, I still need your help.
We are building a new house and have a 50 amp breaker for the heating system(geothermal) and whenever the unit kicks on the lights dim for a split second. It is annoying. We have a 200 amp service panel and right now the only thing on in the house is a 500 watt work light (still hanging the drywall) so I know that I can not be overloading the box. What gives?
Thanks,
Jason
Replies
Have your electrician tighten all the connections inside the panel box and call the power company to check their connections at the meter base and the service entrance.
However, until someone proves to me otherwise, some dimming is normal. If you have a perfect situation where all connections are tight and you throw a 50 amp load across the lines, lights will dim. Some welders are not 50 amps.
I look down my nose at people who dare to look down their nose at people.
I'm with Pete. Sounds normal. No reason not to check the connections and have the utility confirm their stuff is right, but what's the puzzle? An induction motor #### a lot more than name-plate amperage upon start-up. As much as 4 to 6 times as much. I'm assuming 50 amps is partly resistive heating for use during cold temperatures. But if the nameplate on the motor itself is 25 amps, it could pull 150 amps on start-up. That would make light flicker a bit. You could hang a clamp-on ammeter and an analog voltmeter in the breaker panel and see how low the voltage goes and how high the amperage goes during start-up. Then call the manufacturer and confirm that those readings are normal for the equipment.
Edit: #### = "s u c k s". Guess I should have said "draws"
Ask the utility about (or check it yourself) the length and gauge of your service feeder and run those numbers through a voltage drop calculator (there are several on line). 5% is the upper limit of acceptable (i.e. at 200 amps) and limiting line losses to 3% is better. If someone skimped on the size of your service feed wires, then that would 1) further explain the flickering, 2) not bode well for getting your full 200-amps at full voltage, and 3) cause all your lights and heating elements to run at reduced outputs (a 10% voltage drop results in a 19% power drop). Good Luck
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Edited 4/11/2002 12:43:49 PM ET by David Thomas
Whenever a large reative load (like a motor) turns on, it momentarily appears like something closer to a short circuit to the power line until current starts to flow. When the motor reaches its normal operating speed, that is when it consumes its "rated" power and draws its rated amount of current from the line. The effect you are seeing is called "inrush" and it is very common. In my neighborhood, my lights flicker a bit when my neighbor's 3.5 ton air conditioner kicks in. Its just physics you are experiencing-nothing more.
Edited 4/11/2002 3:23:27 PM ET by Ray
If your lights flicker when your neighbors a/c comes on the power company needs to add a capacitor to thier circuit. They have special crews that are called "power quality" techs. or something like it. Thier job is to record and correct sittuations like you describe. It is called power factor correction. Lots of things can cause the problem as mentioned. I have even seen temporary storm repair splices in distrbution lines cause the problem. If your local utility company is like the one I work for, it can be months before a correct repair is made to a line damaged durring a storm. If a customer calls and says they are having such and such a problem, they get moved up the list, simply because we didn't know that the repair could be causing a problem.
Dave
Right, you are talking about power factor correction, right? Since most real-world loads tend to be more more resistive or inductive, you guys add capacitors across the line in an attempt to swing the current vector closer to the voltage in phase thereby lowering the overall current by making the system load look more resistive. I wonder though, don't you guys aim for correcting the average power factor? I wouldn't think you worried about transients that much. If you did, you might tend to overcorrect and put too much capacitance on the circuit.
Edited 4/11/2002 5:45:05 PM ET by Ray
Follow Pete's advice. Call the power company and/or an electrician and have the terminals, breaker, etc in your panel and meter checked. An undersized or damaged transformer can sometimes cause the problem as can poor drop connections. The power company will also check the output voltage as this too can cause increased current draw at start and shorten motor life. Low voltage is usually more hazardous than high, within reason , voltage.
If you know a friendly AC contractor have him/her check the motor and drive on your system. While a motor start can cause an inrush current of 3 times normal a bad motor, bearings or weak motor start capacitor can increase this inrush at start. If everything checks out ask about adding a hard start kit. This could, if applicable, increase the energy storage available to the motor and decrease the current inrush during start. A good AC tech can advise you after checking the system.
One more item that gaurantees that a large transient load on a commercial distribution system (even the neighbors on the same distribution transformer) will cause some voltage fluctuation no matter what you do.
All distribution transformers have a power loss called core loss (composed of hysterysis and eddy current losses) and it is proportional to the size of the transformer. The core loss is a power company load 24/7/52 whether you are using any power or not. Thus, the power company wants to stay with the smallest possible transformer. Even if you use 500 MCM dia feeders, there will still be the voltage drop in the transformer windings (economics again), hence you will always get a voltage dip unless you want to pay the power on a different rate schedule, provide your own substation, etc.
BTW, the light output of an incandescent varies as the 1.6 power of the voltage, so only a small voltage droop results in a visible light output variation.