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Stevenfyeager:
I am not clear on whether you are talking about your own existing home, a new one you are building for yourself, or some other new or older home.
To minimize flicker you want a large pole transformer, large and short distribution wires, two panels with separate main breakers are limitedly useful, and adding impedance in series with the motor load will reduce peak starting current, but lengthen starting current duration.
The number of other customers on your transformer, and the loading on the transformer at the time of your large load turning on, has nothing to do with the magnitude of your flicker. In reality more customers on one transformer might mean a larger transformer, and therefore, less flicker from your load change.
Your flicker magnitude is dependent upon all the source impedance that is common to both the load change, and to your flickering light. Do whatever you can to minimize this common impedance. You can do nothing about the transformer except get it changed if you can. The distribution wires from the transformer to your meter may also be out of your control. After the meter do whatever you can to separate the wiring and other components that are common to the lights and large load.
Small impedance changes, such as the position of a breaker on a bus bar, will have little effect on flicker. Having two separate main panels might make a small difference. This would depend on how much you can separate the common impedance. Two panels gets rid of the main breaker voltage drop, and some possible input wire drop.
If the large load is 120 V, then put the lighting load on a different 120 phase.
If you are concerned with an existing installation and there is a circuit problem, then you need to find the problem.
In my basement shop on my bench I have two 120 V phases 180 degrees apart, and a common neutral. Also a separate EGC.
An experiment with a CREE 9.5 W LED bulb and a 15 W Sylvania incandescent in parallel by my perception showed no difference in flicker. Both bulbs were side by side and powered at about 123 or 124 V with whatever continuous load is on the house and bench.
A 1/3 HP 120 V single phase induction motor was the changing load. Peak current is about 60 A (42 A RMS). This starting current lasts for about 5 cycles (almost 1/10 second), and the peaks are close to the the same value. At 5.5 cycles the current has started to diminish. After the centrifugal switch opens it takes a short time to reach steady state running current. This is a mechanically unloaded motor, and no additional inertia load, not even a pulley.
At the bench the loaded phase, hot to neutral, drops from 176 (124.4 V RMS) to 163 V, or an RMS change of 13*0.707 = 9 V measured with a scope.
Using a Fluke 27 in min-max mode the following measurements were obtained:
Call phase A the one with the motor load, and B the 180 degree opposite phase that shares the same neutral,
The Fluke has a longer averaging time than 1/10 second, and thus does not display as much voltage change as can be seen on the scope.
Using the Fluke phase A shows a change of of -2.3 V on motor start up. Phase B shows a +0.9 V change. Thus, for this startup duration the Fluke showed only 26% of the actual change. These measurements include both the hot and neutral voltage drops. The hot drop is greater than the neutral because there are three breakers and one fuse in the hot line path. I will let you analysis why one change is a reduction and the other an increase.
With a 240 V load change and viewing a 120 V light you will only see the change in voltage of the single common hot circuit path. There is no neutral change.
Even though a Fluke may not give an accurate min-max it can still be a good useful relative measuring tool.
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