Install heavy load breaker first in panel to avoid lights dimming

[Stevenfyeager]

My instructor years ago told me to keep house lights from dimming when the AC turns on, to install the AC rwo pole breaker first or next to the main breaker, upstream from the lighting breakers. Is this what you also suggest ? Thank you.

 

[PaulMmn]

I dunno-- I'd "assume" that putting the lighting circuits next to the main breaker would mean the lights would get first crack at the electrons, and let the A/C get the leavings.

 

[ptonsparky]

We need someone to tell us the VD on 2" of busbar, say 35 amps then 3 amps an inch later. For this instance ignore the 100' of overhead #2Al, the 30'of 4 cu, and the 10KVA pot with 2.3% impedance feeding the three adjoining neighborhood homes. Non of that matters.

 

[Jraef]

If your installation is so weak that the microvolt drop that might take place in 1" of bus material is significant, then you have MAJOR problems.

No... it was BS.

 

[LarryFine]

I prefer landing 2p breakers on one side (away from neutral bus if only one), highest to lowest, and 1p breakers on the other (neutral bus) side.




I especially try to avoid placing 2p breakers head to head.

 

[infinity]

If your installation is so weak that the microvolt drop that might take place in 1" of bus material is significant, then you have MAJOR problems.

No... it was BS.

I agree, the bus is typically the same ampacity of the conductors feeding the panel so a few inches when calculated against the entire length of the conductors back to the POCO transformer is nothing.

 

[Jraef]

Stevenfyeager,
If you want to test his hypothesis on your own, connect your voltmeter leads to a live panel bus, one at the top and one at the bottom of the same bus (wearing appropriate PPE of course). If you read a voltage, that's the voltage drop across that distance. Make sure it is on the millivolt scale to have any possible chance of reading anything... which I doubt.

 

[Nuber]

Yes, this is a mitigating technique with minimal effectiveness. In other words, better is not best. Some have argued it is a placebo effect; I cannot and have never scientifically proven this theory, but I have used it. And I explain it to customers when I do use it (thus the placebo argument).

Proper sizing of the branch circuit conductors accounting for voltage drop will be a far more effective solution, although costs will rise.

Take a 29 amp MCA AC unit with a 50 amp MOCP. The LRA is about 100 amps. (These are rounded numbers off a brand name single phase 240V unit)

You could size the circuit with #10 copper and be code compliant. The voltage drop on start up would be significant compared to other options (and the light flicker would be measurable).
You could size the circuit for the MOCP and use #8 copper. The voltage drop would be less (and the flicker).
You could size the circuit for the LRA and use #3 copper. The voltage drop would be closer to non existent, but costs and termination issues would arise.

OR

You could understand that Kirchoff has a current law, and that his law is static (fixed in time and must be true at all milliseconds of time). The moment that the AC contactor pulls in, current flows to a new 100 amp load in an instant. Each electron comprising the flow of current does not instantaneously travel from the utility to your condenser exclusively, but takes some time to travel (although we are speaking about milliseconds).

So in the instant that the contactor makes, electrons are "stolen" from every connected circuit on the bus in the panel, with the system equalizing in very short order. The more circuits between the new load and the source, the more circuits are affected. The systems affected that our human eyes can see are lighting circuits - otherwise known as flicker.

Because the electrons want to come from the source, if you put your AC breaker closer to the main feeder, fewer electrons will be "stolen" from the other circuits and tend to come from the source.

Let me be clear - this does not solve the problem, it only slightly mitigates it at best.

 

[LarryFine]

You could size the circuit with #10 copper and be code compliant. The voltage drop on start up would be significant compared to other options (and the light flicker would be measurable).
You could size the circuit for the MOCP and use #8 copper. The voltage drop would be less (and the flicker).
You could size the circuit for the LRA and use #3 copper. The voltage drop would be closer to non existent, but costs and termination issues would arise.

This would only apply to any lights on the same circuit as the AC (which there normally would not be).

Actually, increasing the AC conductors' size would cause more starting-current-induced voltage drop on the conductors that the AC and light share; i.e., ahead of the panel.

Reduced starting voltage caused by excess voltage drop on the AC circuit would decrease the effect of voltage drop on the shared service/feeder conductors, reducing flicker.

 

[MAC702]

I don't want to dim my air conditioner while turning lights on. Now I'm totally confused. Where should I put the breakers?

 

[LarryFine]

I don't want to dim my air conditioner while turning lights on. Now I'm totally confused. Where should I put the breakers?

As I said in post 5: 2p, highest to lowest, down one side, and 1p, high to low, down the other side.

If there's only one neutral bus, put the 1p breakers on that side. Usually suits KOs, too.

Terminate all grounds first, neutrals second. That's why my panels almost look empty.

 

[don_resqcapt19]

I don't want to dim my air conditioner while turning lights on. Now I'm totally confused. Where should I put the breakers?

It doesn't matter even a little bit where they are placed in the panel. In most dwelling units the biggest source of voltage drop when large loads are started is the utility transformer.

 

[Stevenfyeager]

Ok, thank you for your input. Here is another part of my situation. I have an 200 amp outside meter /disconnect with 8 (or 16) spaces in it. Then 4/0 cable to a panel inside the home 24 feet away. Would it be best to power the AC from the outside panel (closest to the meter, source) ? Does it make any difference ? It is just about as easy to feed it from either panel. Thanks again.

 

[LarryFine]

I would use the outside panel for anything outside. That makes one less hole through the wall.

It also means less light flicker when the compressor starts.

 

[ptonsparky]

Yes, outside items are fed from an outside panel, especially when it’s closer.

 

[Nuber]

This would only apply to any lights on the same circuit as the AC (which there normally would not be).

And using Kirchoff's theory the bus is a node, thus the lights and everything else in the panel are interconnected. All current into the node must equal all current out. There is no restriction or limitation on where that current comes from because there is nothing to stop it in an AC system from flowing in any direction within the confines of the interconnected bus.

Actually, increasing the AC conductors' size would cause more starting-current-induced voltage drop on the conductors that the AC and light share; i.e., ahead of the panel.

Starting current does not induce voltage drop. Voltage drop is a side effect of conductor size. The amount of voltage drop varies solely based on the size of the conductor and it's associated impedance. Because the impedance of the load is fixed, different wire sizes change the measured voltage at the AC unit (in my presented scenarios, the #3 wire would have the highest measured voltage at the AC terminals and the lowest voltage drop). In comparison to different wire sizes, the starting current must change because of Ohm's law.

Reduced starting voltage caused by excess voltage drop on the AC circuit would decrease the effect of voltage drop on the shared service/feeder conductors, reducing flicker.

Yes, that was the first point I was making. I suspect that you are moving past the initial moment without analyzing the initial moment. The training that I received discussed these issues within time constants. Time constants are merely different points in time (in this case I am looking at cycle level time, or 1/60th of a second increments or so).

Initially, the AC contactor is not made, what is system voltage? Constant. At the same time there is a measurable voltage drop at the AC contactor, which does not affect voltage measured at the panel. Voltage drop at the AC contactor is not a part of the discussion I am presenting for the breaker location.

In the next time constant the contactor makes, what is system voltage at the panel? Constant (system voltage is maintained by the Utility prime mover, wherever that may be). System impedance drops causing an increase in current flow. Ohm's law is maintained. The question is where does the current come from? Kirchoff says the node balance must be maintained at the bus. What stops some electrons from flowing out of the lighting circuit upstream (in bus terms) in this particular moment? Are they not closer and easier to be utilized by the new AC load? The additional current flow from the source is not instantaneous. So if some electrons are taken from the lighting circuit (and others), what now happens to power consumption in that circuit? Light flicker, perhaps, while the utility power source is trying to deliver the additional electrons?

I am merely arguing that the theory to locate a large load breaker closer to the power source is sound, and at the same time has a minimal effect at best.

The best solution to minimize light flicker from an AC unit is to over size the circuit conductors in the first place.

 

[Nuber]

I don't want to dim my air conditioner while turning lights on. Now I'm totally confused. Where should I put the breakers?

You can put the breakers anywhere in the panel. Your instructor and I share an opinion that placing the breaker closer to the panel feeder/service conductors is best, others claim it does not matter at all.

There is no code violation, no fear of the house burning down, no adverse affect on the lights or the AC unit, so do as you will.

 

[Tony S]

Mounting higher rated breakers nearest the incomer is common place over here despite the busbars being fully rated over their entire length. Trying to convince people to intersperse high and low ratings to aid heat dissipation is mission impossible!

 

[gar]

190703-0823 EDT

Tony S:

Your comment on power dissipation makes good sense. Separate the breakers that dissipate high power.


Nuber:

My interpretation of your discussion implies that you fail to understand circuit theory.

The voltage at the input to your main panel is not a constant even if the power company primary supply voltage is a constant, which it is not.

My next door neighbor does not get his power from my main panel, but we are the only ones, except for two street lights, that share the same 50 kVA transformer.

When my neighbor's air conditioner kicks in my lights slightly flicker. The service wire and transformer impedances are the cause.

For any load I place on my main panel the voltage drop along the bus bars is nil compared to power company line drop.

With an incandescent bulb I have to concentrate and have "signal known exactly" to detect a 1 V change in applied voltage to the bulb. 2 V is more likely to catch my attention without the "signal known exactly". With "signal known exactly" I may be able to detect a light flicker that I really don't know that I saw. I have been a subject in such psychophysical experiments, back in 1953.

From the Internet

Psychophysics is the scientific study of the relationship between stimuli (specified in physical terms) and the sensations and perceptions evoked by these stimuli. The term psychophysics is used to denote both the substantive study of stimulus-response relationships and the methodologies used for this study.

Put a 100 A load at the far end of a main panel bus bar and measure the voltage drop along the bus. I did a test with a 10 A load change and possibly saw a 1 millivolt change along a 200 A bus bar. Or at 200 A this would be about 20 millivolts. Almost certainly not worse than this. You won't see light flicker from a 20 millivolt change.

.



.

 

[kwired]

Yes, this is a mitigating technique with minimal effectiveness. In other words, better is not best. Some have argued it is a placebo effect; I cannot and have never scientifically proven this theory, but I have used it. And I explain it to customers when I do use it (thus the placebo argument).

Proper sizing of the branch circuit conductors accounting for voltage drop will be a far more effective solution, although costs will rise.

Take a 29 amp MCA AC unit with a 50 amp MOCP. The LRA is about 100 amps. (These are rounded numbers off a brand name single phase 240V unit)

You could size the circuit with #10 copper and be code compliant. The voltage drop on start up would be significant compared to other options (and the light flicker would be measurable).
You could size the circuit for the MOCP and use #8 copper. The voltage drop would be less (and the flicker).
You could size the circuit for the LRA and use #3 copper. The voltage drop would be closer to non existent, but costs and termination issues would arise.

OR

You could understand that Kirchoff has a current law, and that his law is static (fixed in time and must be true at all milliseconds of time). The moment that the AC contactor pulls in, current flows to a new 100 amp load in an instant. Each electron comprising the flow of current does not instantaneously travel from the utility to your condenser exclusively, but takes some time to travel (although we are speaking about milliseconds).

So in the instant that the contactor makes, electrons are "stolen" from every connected circuit on the bus in the panel, with the system equalizing in very short order. The more circuits between the new load and the source, the more circuits are affected. The systems affected that our human eyes can see are lighting circuits - otherwise known as flicker.

Because the electrons want to come from the source, if you put your AC breaker closer to the main feeder, fewer electrons will be "stolen" from the other circuits and tend to come from the source.

Let me be clear - this does not solve the problem, it only slightly mitigates it at best.

The voltage drop that is mitigated is probably in the microvolt ranges. Most the VD when the AC starts is likely in the feeder or service conductors or even in the supply transformer itself.

It doesn't matter even a little bit where they are placed in the panel. In most dwelling units the biggest source of voltage drop when large loads are started is the utility transformer.

Or the service conductors, especially if you have a transformer at say far end of the block, some distribution conductors (may supply others) running part of the way then a drop cable to your premises.

Ok, thank you for your input. Here is another part of my situation. I have an 200 amp outside meter /disconnect with 8 (or 16) spaces in it. Then 4/0 cable to a panel inside the home 24 feet away. Would it be best to power the AC from the outside panel (closest to the meter, source) ? Does it make any difference ? It is just about as easy to feed it from either panel. Thanks again.

Bet your lights dimming results will be about the same either way, most the significant VD will be in the service and not in premises wiring.