Lights blink as window unit air conditioner kicks on

[Ichinisan]

I rent this house. I moved to a different bedroom after a roommate moved-out several months ago. I noticed the lights blink whenever the window unit AC kicks on. The unit is about 1 year old. I worry these power flickers will cause problems with electronics and 3D printers that I use.

Is the issue more likely to be with the AC unit or with the house electrical wiring?

I suspect the electrical wiring in the house has problems because the house was hit by lightning in February 2019. Lots of equipment was damaged. Should I check behind the wall plates for melted insulation?

[edit]
I tried to see how many BTUs / amps the unit is (8000 / 6), and I also saw this on the label: "USE ON SINGLE OUTLET CIRCUIT ONLY"

I suppose that means it shouldn't even share a circuit with the lights. Maybe I'll need an electrician to run a dedicated power line for this outlet with the AC.

 

[VirtualLarry]

USE ON SINGLE OUTLET CIRCUIT ONLY

I wonder if my A/C unit says that.

I've got an 8000 BTU unit, in my bedroom, and the dedicated A/C circuit is in the living room. I prefer my A/C in the bedroom, to help me sleep. Unfortunately, even though this place has 20A circuits, the bedroom and living room outlets (other than the electric heat, and the ceiling lights) are all on the SAME circuit. That means, all three of my Gaming PCs, and the A/C, and the window fan (in the living room).

My Gaming PCs are on APC PureSine UPSes, as are my NAS units, but the NAS units are on a separate circuit, I think.

Often, during the day, when I have the PCs crunching, and the A/C going, the UPSes will click-clunk, and switch over. Every 5-10 minutes. Sometimes, if I have a third PC going, the breaker trips. Or if I turn on the A/C from a standstill.

 

[snoopy7548]

That's fairly normal. As the compressor kicks on, it draws a large surge of current. I don't know about window ACs, but central AC units have a start capacitor which helps.

My central AC does the same thing in my house even though it's on a dedicated breaker.

 

[Micrornd]

That's fairly normal. As the compressor kicks on, it draws a large surge of current. I don't know about window ACs, but central AC units have a start capacitor which helps.

My central AC does the same thing in my house even though it's on a dedicated breaker.

Trust me, that is definitely NOT normal for a central HVAC.

 

[jmagg]

My electrician says that's a result of using aluminum as the main into the panel. Which in my area is code compliant.

 

[Steltek]

Trust me, that is definitely NOT normal for a central HVAC.

It can be, depending upon the size of your central unit and the number of other homes sharing your transformer. I have a single 5 ton package unit, and the lights dim (or flicker a little, in the case of LEDs) when it comes on. My brother has a pair of 2 1/2 ton units (his house is two floors), so his don't tend to flicker unless both start at exactly the same time (which rarely if ever happens). My sister (who lives in a different state) also has a 5 ton unit that flickered badly until the start assist capacitor kit was added which mostly relieved the situation for her.

Many HVAC installers and techs won't install start assist capacitor kits unless you specifically ask for them as they are an extra expense.

 

[Steltek]

I wonder if my A/C unit says that.

I've got an 8000 BTU unit, in my bedroom, and the dedicated A/C circuit is in the living room. I prefer my A/C in the bedroom, to help me sleep. Unfortunately, even though this place has 20A circuits, the bedroom and living room outlets (other than the electric heat, and the ceiling lights) are all on the SAME circuit. That means, all three of my Gaming PCs, and the A/C, and the window fan (in the living room).

My Gaming PCs are on APC PureSine UPSes, as are my NAS units, but the NAS units are on a separate circuit, I think.

Often, during the day, when I have the PCs crunching, and the A/C going, the UPSes will click-clunk, and switch over. Every 5-10 minutes. Sometimes, if I have a third PC going, the breaker trips. Or if I turn on the A/C from a standstill.

Larry, if you own or are buying you might want to fix that.

 

[VirtualLarry]

Larry, if you own or are buying you might want to fix that.

No, I rent.

 

[PowerEngineer]

That's fairly normal. As the compressor kicks on, it draws a large surge of current. I don't know about window ACs, but central AC units have a start capacitor which helps.

My central AC does the same thing in my house even though it's on a dedicated breaker.

Pretty much this.

The compressor is an induction motor where the amount of current it draws is inversely related to its speed of rotation, so when starting up (from 0 RPM) it draws its peak current and then drops down to a lower rated value as it speeds up (note here that current and real power consumption are not the same thing). This initial surge is frequently called inrush current, and circuit breakers are set to ignore these momentary current spikes. Nonetheless, these momentary current spikes do bring increased drops in voltage along the circuit between your breaker box and the AC unit that might affect other devices that are sharing that same circuit (which is why you often have dedicated circuits for induction motor loads).

Without getting too technical, induction motors cannot start up using just a single phase of AC power (which is all that most all homes receive). The starting capacitor is engaged during startup to provide a slightly (angle) shifted second phase. Once the motor is spinning, the second phase isn't needed and the starting capacitor drops out.

Of course, the induction motor inrush current doesn't just originate at the breaker box, but is being drawn through the utility feeder you are connected to. Depending on the length and nature of your home's connection to the feeder (and the size of the inrush current), it is possible to get a noticeable voltage drop between the feeder and your breaker box that the whole house will see.

So to the OP's questions, the flicker is not an indication of melted insulation that you need to find and fix (not saying that this is proof that there isn't any either). I'm not sure about your specific electronics, but generally the AC-to-DC power supplies have some built-in tolerances for variations in AC voltage and the filtering capacitors on the DC side provide some (very) short-term ride through capability. High voltage spikes are the big concern. Low voltage spikes no so much.

As a wise man once said (more or less):

It's a balance. You simply cannot have 100% safety without rewiring your rental home.
Yes. I am willing to give up safety in favor of preserving my cash.

😊 I hope this helps....

 

[bigi]

My electrician says that's a result of using aluminum as the main into the panel. Which in my area is code compliant.

What does it have to do with anything? Unless, the length is really, really too long???

I'd change the electrician pronto.

 

[jmagg]

What does it have to do with anything? Unless, the length is really, really too long???

I'd change the electrician pronto.

"Aluminum has 61 percent of the conductivity of copper, but has only 30 percent of the weight of copper. That means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance. Aluminum is generally more inexpensive when compared to copper conductors."

Care to rescind your uninformed comment? There's much more drawdown with aluminum wire. My electrician is a master craftsman w/ 40 yrs btw.

 

[PowerEngineer]

"Aluminum has 61 percent of the conductivity of copper, but has only 30 percent of the weight of copper. That means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance. Aluminum is generally more inexpensive when compared to copper conductors."

Care to rescind your uninformed comment? There's much more drawdown with aluminum wire. My electrician is a master craftsman w/ 40 yrs btw.

It isn't quite that clear cut. You correctly observe that copper is a better conductor of electricity than aluminum (and gold is better than copper 😊), however resistance in a wire is also inversely proportional to the area of its cross section. It follows that a thicker aluminum wire can have the same (or even lower) resistance per unit length as a copper wire.

There are some older transmission lines still in service with copper conductors (i.e. wires) but the industry standard for decades has been aluminum (a complex trade-off of factors including cost, weight, and conductivity).

My recollection is that the real problem with aluminum wiring in homes has been in the connections to aluminum wires. Good (i.e. low resistance) and long lasting connections are harder to make with aluminum. Given the choice, I'd want to have copper wiring in my home (which I do).

 

[pcgeek11]

~snip~

My recollection is that the real problem with aluminum wiring in homes has been in the connections to aluminum wires. Good (i.e. low resistance) and long lasting connections are harder to make with aluminum. Given the choice, I'd want to have copper wiring in my home (which I do).

The biggest danger of using aluminum wiring I have seen is when someone mixes connectors rated for copper with aluminum wire. They expand at different rates which over time causes a loose connection and heat. Which is a severe fire hazard. I remember when they first started using aluminum and there were many house fires caused by these connections.

 

[Steltek]

The biggest danger of using aluminum wiring I have seen is when someone mixes connectors rated for copper with aluminum wire. They expand at different rates which over time causes a loose connection and heat. Which is a severe fire hazard. I remember when they first started using copper and there were many house fires caused by these connections.

It is actually worse than just heat expansion, though, as aluminum wire connected directly to copper is subject to galvanic corrosion in the presence of moisture - the humidity in the air can be more than enough to enable it over time. Copper coated aluminum wiring is a lot better in this regard, as long as the copper sheathing is not damaged, but is still subject to corrosion at the connections if done improperly.

My sister's house was built back in the 1950's and has a mixture of uncoated aluminum and copper wiring. Over time, prior owners have replaced outlets and fixtures on the old aluminum wiring with fixtures and outlets intended for copper wiring. The connections are now showing significant galvanic deterioration. I guess I'll have to rewire the entire house for her in the next year or two. Definitely not looking forward to that job for sure as I'm just getting too old to be crawling around in attics and crawl spaces....

 

[Greenman]

What does it have to do with anything? Unless, the length is really, really too long???

I'd change the electrician pronto.

"Aluminum has 61 percent of the conductivity of copper, but has only 30 percent of the weight of copper. That means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance. Aluminum is generally more inexpensive when compared to copper conductors."

Care to rescind your uninformed comment? There's much more drawdown with aluminum wire. My electrician is a master craftsman w/ 40 yrs btw.

It's not simply the material, it's length, load, and termination. When any of those three factors aren't properly addressed you'll have a problem no matter what material is used. I never use aluminium because copper is a more forgiving material, and in a residential setting the cost savings of aluminium vs copper isn't worth the trade off.

 

[jmagg]

My comment only pertains to the main service wire, attached to the grid, to the meter, then to the breaker panel. My first was a new house out of the ground, all new wiring, panel box. Recessed lights still showed drawdown when the window AC unit compressor kicked on. New service and wiring throughout this complete rehab house. Same condition with aluminum service wire.

 

[bigi]

"Aluminum has 61 percent of the conductivity of copper, but has only 30 percent of the weight of copper. That means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance. Aluminum is generally more inexpensive when compared to copper conductors."

Care to rescind your uninformed comment? There's much more drawdown with aluminum wire. My electrician is a master craftsman w/ 40 yrs btw.

Well, the post I was referring to was simply vague. No details were provided which then appeared in further posts.
It was as informed as the post it refereed to.

My electrician says that's a result of using aluminum as the main into the panel. Which in my area is code compliant.

There is not much there. One can draw range of conclusions based on this simple statement.

Once it happened it started to getting deeper with analytics and details.

 

[jmagg]

My electrician says that's a result of using aluminum as the main into the panel. Which in my area is code compliant. Reading is Fun-damental.

 

[Red Squirrel]

That's fairly normal. As the compressor kicks on, it draws a large surge of current. I don't know about window ACs, but central AC units have a start capacitor which helps.

My central AC does the same thing in my house even though it's on a dedicated breaker.

Same here. Washing machine too will do it. When an electric motor first kicks on it's practically a dead short that will draw a couple hundred amps for a very short period of time so it drops the voltage (less than a cycle) so it's normal to get a slight flicker due to voltage drop.

People often think breakers are magical devices that somehow split up everything into different electrical sources, but in reality everything is still riding on the same actual electrical source, which is the transformer (or an inverter, or generator, or whatever is feeding the building). If you want to get technical it's the power plant but the transformer can store a bit of energy to handle big startup loads. Electrically a breaker is just a switch, it just so happens to be designed to turn off if you draw too much current, other than that it does not do anything special to prevent voltage drop caused by other circuits.

That said, if the flicker is very apparent and not just a very slight dimming, then maybe there actually is a problem such as a loose connection in the panel on the incoming feeder. I would go check and make sure everything is tight. A good test as well is to notice if you get lot of flickering when it's windy. There could be something outside that's loose. Worth having the hydro company come and check the meter connections too as you won't have access to check those yourself.

Actually now I'm kinda curious to put setup my scope across the two hots to see what the wave form looks like when the A/C kicks on. I would need a differential probe to do that safely though, or to run it off batteries. You don't want to put the ground lead on one of the hots.

 

[PowerEngineer]

Well, pretty much correct. A non-rotating induction motor does look something like a dead short, but as it spins up to speed it produces an increasing "back EMF" that counters the applied voltage and thereby reduces the current draw. The reduction in current draw is therefore related to the time it takes the induction motor to spin up to its rated speed which may be considerably longer than a single 60 Hz cycle.

It is also important to remember that the impedance of a non-rotating induction motor is almost completely inductive, which means that much of the high initial inrush current is not delivering real power (delivering reactive power instead; it gets complicated...). Therefore the energy drawn to start up an induction motor is not nearly as eye popping as the inrush current.

While there is some energy stored in the electric and magnetic fields of power lines and transformers, the amount is negligibly small. Most of the stored energy in the electrical system is in what we call the "rotating masses" connected to it. These are all the generators and motors across the entire interconnection. Real power mismatches between inputs (generators) and outputs (loads) regardless of size all act to either accelerate (store excess energy) or decelerate (draw energy shortfall) the rotation of all these rotating masses. This shows up as constant jiggling of the system frequency around the nominal 60 Hz value. Keeping the electrical system balanced and operating around 60 Hz is a continuous challenge - a bit like balancing a broomstick in the palm of your hand.