Install heavy load breaker first in panel to avoid lights dimming

[Nuber]

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

With respect - You have the right to interpret anything you wish to in any manner. I have explained this theory using electrical principles. Not sure that there is any more that I can do.

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.

You are analyzing the system over seconds of time, if not hours. I am analyzing moments in time closer to the scale of milliseconds. An alternating current system in the US is constantly changing (60 full cycles per second). I am talking about the difference between the moment the AC contactor is open compared to the very next moment when the AC contactor is closed. RMS Voltage is constant in these two moments.

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.

OK

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

Not exactly and not fully. Where does the electron come from that first travels to your neighbor’s AC unit? How about the next million electrons? Are you trying to tell me that 100% of those electrons come exclusively from the utility in the exact moment after the contactor closes? The electrons for the AC circuit just skip right on by every other circuit in the panel and only come from the utility wiring without affecting anything else along the way? Kirchoff disagrees with you.

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

True. Not my point at all. I am not analyzing the voltage drop on the bus, nor the voltage drop at any point of the utility. I am analyzing the severe shock that a 100 amp load instantaneously introduced on the panel bus causes to the system overall.

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.

Cool. I would be interested in hearing or reading more about this if you have any links to provide. I fully concede the point that light flicker is highly susceptible to perception capabilities of the viewer.

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.

I would suggest this - We are not having an issue with me not understanding electrical theory. We are having an issue about discussing different things (I am speaking oranges, you are speaking apples). I am talking about current flow over a time period of milliseconds across the entirety of an electrical panel and their associated circuits. You are talking about voltage drop across a bus bar.

I am saying that whatever voltage drop on the bus bar that exists is negligible to this discussion, that current flow on a millisecond time scale is the factor we are addressing here. Where do those electrons come from in the instant that new 100 amp load is introduced to the bus?

 

[Jon456]

Not exactly and not fully. Where does the electron come from that first travels to your neighbor’s AC unit? How about the next million electrons? Are you trying to tell me that 100% of those electrons come exclusively from the utility in the exact moment after the contactor closes? The electrons for the AC circuit just skip right on by every other circuit in the panel and only come from the utility wiring without affecting anything else along the way? Kirchoff disagrees with you.

Except the electrons are barely moving at all. The drift velocity of electrons is very slow, about 360 millimeters per HOUR. But what we consider electric current is not the drift velocity of individual electrons, but rather the signal velocity of the electromagnetic wave, which in a copper wire is roughly equal to 95% of the speed of light in a vacuum (~300,000,000,000 millimeters per SECOND). That means electric current in a copper wire can cross the United States coast-to-coast in about 16.9 milliseconds.

So how long do you think it takes electric current to travel the length of a distribution panel bus bar? Do you think your eyes could see the effect?

 

[infinity]

I say install the breakers in a way that makes you feel all warm and fuzzy just realize that how you arrange them doesn't matter one bit.

 

[ActionDave]

I say install the breakers in a way that makes you feel all warm and fuzzy just realize that how you arrange them doesn't matter one bit.

:thumbsup:
That's what I do. The older I get the less things that don't matter matter to me.

 

[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?

Sorry guys, I thought it might be obvious I was being facetious by putting the A/C first in my priorities. I forgot the smilie.

 

[StarCat]

Suggestions

Suggestions

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.

My suggestion for Single Phase residential condensing units is to install the correct " OEM Hard Start " package for said model. If you have a recording meter such as the Fluke 87 with current clamp, you will notice a significant difference in the inrush on startup. A better quality and higher rated contactor is also useful for reliability and keeping all this in trim. Not all compressors and systems are created equal. There are myriad design factors on the mechanical side which can influence the starting characteristics. Some are simply better Engineered and better built, but we live in the age of low quality and high cheapness currently.
The sustem charge needs to be spot on, and any other " install quality " issues can affect things as well. Typical newer systems are generally protected, but older systems need to be retrofitted to 5 minute delay on break setup, which can easily be done with a modern thermostat. Battery operated thermostats are junk.
You need a good brand Stat like Honeywell, with common 24VAC run to the sub base. Programmable thermostats are also useless and 99% of the time just create service calls. Keep it simple.

 

[gar]

190703-1407 EDT

Nuber:

You are analyzing the system over seconds of time, if not hours. I am analyzing moments in time closer to the scale of milliseconds. An alternating current system in the US is constantly changing (60 full cycles per second). I am talking about the difference between the moment the AC contactor is open compared to the very next moment when the AC contactor is closed. RMS Voltage is constant in these two moments.

Change RMS voltage to instantaneous voltage and your statement is correct if no current was flowing at that instant.

An RMS measurement involves an integration over time (summation of small increments is integration), and then averaging over said time. This is usually done over many cycles, or else an integral number of cycles, if you want a meaningful measurement.

A tungsten incandescent light bulb or a transformer has a very short duration, usually in the region of 1/120 second, turn on current transient. See my plots P3, P4, and P6, P7 for examples at
http://beta-a2.com/EE-photos.html

A typical induction motor has a starting current higher than its running current for many half cycles. See my plots and posts at
https://forums.mikeholt.com/showthread.php?t=174880&highlight=motor+starting+current+plots

See my plot PE1 at
http://beta-a2.com/energy.html
for a 24 hour plot of my main panel voltage with a resolution and averaging time of 1 second. Obviously this is a single 120 V phase.

.

 

[romex jockey]

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.

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.

So then, why does everey PV guy that hires me insist on their 'panel interface' ocpd being the loowest and furthest away from the incoming mains/mlo?

~RJ~

 

[gar]

190703-727 EDT

I got my RMS post in the wrong thread. So a copy is now in this thread as follows:

The following discussion is possibly useful to understand what RMS is. But with no background in calculus it is possibly just greek.

https://www.raeng.org.uk/publications/other/8-rms

AC meters are not necessarily RMS reading, and with very peaked waveforms may have errors from limited dynamic range. Also may not read correctly when an added DC component is present.

If you have no calculus background, then take equally spaced samples of the signal over one full cycle, and then divide by the number of samples.

In the RMS calculation we first square each sample, next take the average of all the samples, and last take the square root of the average.

.

 

[MAC702]

So then, why does every PV guy that hires me insist on ...

Primadonnas.

 

[ptonsparky]

Sorry guys, I thought it might be obvious I was being facetious by putting the A/C first in my priorities. I forgot the smilie.

It was obvious, at least to me.

 

[kwired]

Sorry guys, I thought it might be obvious I was being facetious by putting the A/C first in my priorities. I forgot the smilie.

That is where my priorities might be

I can see some of the small town athletic fields that I have done work at - concession stand maybe has an AC, or at least a fan running because it is hotter than you know what in there, then at some point they turn on the field lights, which is a bigger load than the concession is, and concession voltage drops because service voltage dropped with the added load.

 

[wwhitney]

So then, why does everey PV guy that hires me insist on their 'panel interface' ocpd being the loowest and furthest away from the incoming mains/mlo?

That's a requirement for the most common method of protecting the panel bus from overload when the bus has two sources of power, e.g. utility and PV. See section 705.12.

Cheers, Wayne

 

[romex jockey]

Yeah i know it's code to do so Wayne....no,i don't understand why....:?~RJ~

 

[gar]

190756-0757 EDT

The reason might be this for example:

1. A 200 A main panel with a 200 A main breaker.

2. Main panel bus bars are designed for a maximum of 200 A.

3. A 200 A breaker from a PV system feeding the panel.

4. Both the power company and PV can simultaneously each supply 200 A to the panel.

5. Thus, the panel can be loaded to 400 A without tripping the two 200 A breakers.

6. If both breakers are at the same end, then at least at that end 400 A is flowing thru the bus.

7. If these two power source breakers are at opposite ends of the bus bars, then maximum bus bar current may be below 400 A. The maximum bus bar current somewhere on the bus will depend upon how the bus bar loads, circuits using power, are distributed.

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[sameguy]

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

In your neighbors panel.

 

[infinity]

So then, why does everey PV guy that hires me insist on their 'panel interface' ocpd being the loowest and furthest away from the incoming mains/mlo?

~RJ~

There are reasons for that one of which gar mentioned however those reasons have nothing to do with the questions asked in the OP.

 

[jaggedben]

190756-0757 EDT



7. If these two power source breakers are at opposite ends of the bus bars, then maximum bus bar current may be below 400 A. The maximum bus bar current somewhere on the bus will depend upon how the bus bar loads, circuits using power, are distributed.

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If the sources are at opposite ends, them the maximum bus bar current is theoretically only 200A, per Kirchoff's law, unless an individual load is violating the equipment listing. (Have you ever seen an OCPD that is listed to attach to a busbar with a lower rating than the OCPD?)

But on a 200A panelboard the code rule actually limits us to only 240A from the combined sources in any case. The CMP wanted to be extra extra conservative due to the unknowns of thermal dissipation in panelboards.

This has almost nothing to do with the OPs question.

 

[kwired]

But on a 200A panelboard the code rule actually limits us to only 240A from the combined sources in any case.

Correct, and 200 amp panelboard shouldn't have anymore than 200 amps (calculated) load connected to it anyway, or even as low as 160 amps if all the load is continuous load.

 

[gar]

190706-1424 EDT

My post #35 had nothing to do with code, or any likely condition, or the original post.

My post #35 was in response to romex jockey's question in post #34 as to why. I used simple values that you could juggle in your head to try to illustrate the problem.

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