Why manufactures don´t burn in their amps and ...

A voltage drop on the power cord is caused by two things: the gauge of the wire and the quality of the connections.

If your power cord is heating up at the connectors, here is an example of why things like the Porterport or medical-grade connectors can make a difference!

The wire itself will heat up if it is inadequate gauge.

If it is built improperly, it can resist the high current/high frequency surges required at the peak of the waveform.

I think its a good idea to have a shield too.

IMO, building a cable carefully with good connections at either end and a heavy gauge will hit about 95% of what is important. We built a few power cords just for fun and they turned out quite well. Some of our gear does draw some power and they seemed will up to the task. After you build a few and add up the costs, you start to see why some power cords cost what they do. Not all though :)

First, there are NO high frequency 'surges' in a power cord.
If a power cord measures as highly reactive, you will have voltage and current not in perfect phase.
Just for a trivial example for which I have measures.
A 40 watt fluorescent light draws about .32 amp after warmup. The power factor (PF) is about .8, so the lamp really draws 40VA.
If the PC is such a load, along with the powersupply of the equipment in question, that can be the source of some bad effects.
The electric company really hates low PF loads, and at least in industrial applications, charges a premium.

As an aside, the same thing can be said of the amp/speaker relationship. A speaker with huge phase angles can suck the life out of an amp while having only a fraction of the power delivered to the load. Add low impedance and the problem compounds.

Magfan -- Despite the claims that some cable manufacturers may make in their marketing literature, I don't think that a power cord can have a significant power factor, because its inductive reactance and capacitive reactance at 60Hz will be completely negligible. Although I agree that the power factor of the load can certainly be significant, which would presumably be inductive due to the power transformer.

Using this inductance calculator, the inductance of say a 72 inch power cord of any reasonable gauge is in the range of 2 to 3 microHenries. That is roughly a milliohm (0.001 ohms) of inductive reactance at 60Hz, which is negligible both in absolute terms and in relation to the load (and undoubtedly also in relation to the house wiring inside the walls, as well!).

Capacitance will vary widely with the power cord design, but as a very worst case guess let's assume 1000 pf/ft. At 60 Hz, for a 6 foot cord, that would be a capacitive reactance (in parallel; therefore the higher the better) of about 500,000 ohms, again totally negligible both in absolute terms and in relation to the load impedance.

Regards,
-- Al

Magfan and Al, you want to keep in mind that the power surges are the ones where the power supply rectifiers commutate (that is to say they turn off and on) at the peaks of the AC waveform. As soon as the AC waveform drops below the value where the caps are charged in the power supply, the rectifiers turn off- please refer to my earlier comment regarding this.

The bottom line is that there are indeed HF power surges occurring in the power cord (yes, 60 times/second), unless you have another way of describing a current spike a few milliseconds wide :)

The rectifier commutation is often responsible for a great degree of radiated HF noise. They have to be properly bypassed to reduce it. However, the power transformer core has its own reaction to the ON/OFF load! If you don't believe me Google 'spark coil' and look at the principle of operation.

A fair amount of this noise manifests on the primary side of the transformer. That is why I think its a good idea to have a shielded cord :)

Excellent points, Ralph. Thanks!

Re Googling "spark coil," I think that also Googling the term "inductive kickback" would further reinforce your recommendation about shielding.

To make sure it's clear to the others, inductance (which is present to a considerable degree in a transformer) resists abrupt changes in the current flowing through it. If the current flowing through it is forced to change abruptly, such as by the rectifier commutation Ralph describes, the result can be extremely large voltage spikes, resulting in high frequency noise that can radiate through the air as well as couple through circuit paths.

Regards,
-- Al