This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
The leads coming from most Apple II power supplies have a low resistance- - much less than 1 Ohm. (Thanks to Michael Mahon for driving home this point!) Even so, as a user piles on peripherals and the current load increases, a wire lead's resistance may prove to be too high to allow maintaining proper operating voltages and low system noise-- e.g. you routinely get "FATAL SYSTEM" errors. A good indicator of power problems is a noticeable drop in voltage on the +5V line as measured on the motherboard. Instead of the 4.9V - 5.0V typical for a light load, it will be 4.7V or lower. Measurements and experiments with standard IIgs power supplies indicate that the actual DC voltage drop through the 18 gauge +5V and Ground leads is only (approximately) a total of 0.04V at 3Amps, which is what a moderately "loaded" IIgs system will draw. The explanation for getting a drop of 0.2V - 0.4V or greater appears to be power supply regulation error. Some standard (and "heavy duty") Apple II power supplies with 18 gauge leads will hold at- motherboard voltage to around 4.9V at 3-4 Amps and some won't. It is easy to see that a system designed to work at 5 Volts will eventually begin to malfunction as the available voltage drops by half a volt or more. In fact, any actual computer system would be likely to experience crashes long before the average, measured at-motherboard voltage got down to 4.5 Volts. A voltmeter reading at the motherboard does not show instantaneous spike or "noise" voltages. Each time a circuit switches, there is a change in current drain. Quite a few circuits are switched with each main system clock transition; so, the change in current can be substantial at 1 x and 2 x main clock frequency. Other events, like turning ON a disk drive, can also produce brief up or down shifts in current drain. Either way, you have brief changes in voltage across the power supply and its leads. The brief voltage changes are called "spikes" because they are VERY brief. The larger the current shift and the greater the effective resistance of the power supply plus its leads, the higher the spike voltage generated. Since these spikes are in series with the circuits connected to the PS and since they are difficult to eliminate via bypass capacitors, they propagate throughout the system. Even worse, as current draw increases and spike voltage increases, at- motherboard supply voltage decreases. So, you have a 'double whammy': the lowered supply voltage reduces IC noise immunity just when you need it most. At some point, noise spikes appear which cause latches, memory IC's, etc. to switch state. If the latch is on a RAMfast, you may get a disk read error. If a memory chip is affected, data will be corrupted, program instructions may change, ....; in short, your computer is likely to malfunction. All of which is bad enough; but, there may be another negative affect when noticeable system noise appears 'across' the power supply. How many csa2 posts complain about GS power supplies that crater "for no reason" after just a couple months? How many users seem to be on eternal quests for a solution to PS woes? Reducing the noise may significantly extend the life of your Apple II power supply. Power supplies with noticeable regulation error often benefit greatly from heavier leads, especially for +5V and Ground. Reducing the actual drop through the lines reduces the resulting error; and, at-motherboard voltage is back to 4.8V or better under high loads. The heavier leads also reduce noise. Whether or not tighter regulating power supplies benefit significantly from swapping in heavier leads is an open question. At high switching frequencies the power supply's leads will have a higher effective resistance and the spike voltages appearing across the power supply output will be larger. If you notice otherwise 'mysterious' system glitches despite having a good, solid looking power supply and/or that power supplies tend to crater when connected to your Apple II, swapping in heavier leads may be a good idea. Our current GS power supply is the one which came with the computer when it was purchased in the Fall of 1986. Yes, our GS was plugged into a System Saver IIgs fairly early on; but, then, line deglitchers have been a nearly universal accessory since the mid-1980's. The main difference between our power supply and the piles of blown units is that fattened +5V, Ground, and +12V leads were added back when we upgraded to an accelerator board. Despite having also added a Focus Hard Card drive, RamFAST SCSI interface, and a couple other boards, system crashes are very rare-- perhaps one every 8-10 sessions (and those usually relate to a fault in the software). The 'bottom line' is that, so long as the power supply is not actually defective, real world experience supports fattening at least the +5V and Ground power supply leads as a major step in curing crashes due to system noise. (See also Marvin Miller's GS WorldView article: "I Did the IIgs PS Mod, and the glitches are gone!" at http://apple2.org.za/gswv/a2zine/Sel/IdidThePSmod.html .) By: Rubywand, Marvin Miller, Michael Mahon