real world a/c resistance

[iwire]

Go to college before you kill someone playing around with you’re meter. The internet is no place to learn electrical theory.

:thumbsdown:

Tony, that is out of line and is not the position of this forum. If you do not want to help someone just don't post.

 

[ADub]

Go to college before you kill someone playing around with you’re meter. The internet is no place to learn electrical theory.

Your*


Sent from my iPhone using Tapatalk

 

[junkhound]

Did not see it mentioned to the OP, but he has to consider power factor unless he can set up a pure resistive load like baseboard heaters or incandescent light bulbs.

THEN OP can measure amps and volts at one end and volts at the other end and get an ac 'resistance'. That is, assuming the poco incoming has unity power factor, which is unlikely.

After that, OP does need a little more math, an understanding of vectors, and even a little trig. And how to measure phase difference between voltage and current at the point of voltage and current measurements.

Or one can go online and download a FEA circuits program (finite element analysis) such a LT spice and it will do the math for you by working backwards.

 

[gar]

150804-0023 EDT

rambojoe:

For most wiring circuits at 60 Hz, and closely spaced wires (think Romex), and you can neglect reactance. Also at these frequencies small diameter wire does not exhibit much skin effect. Thus, DC resistance is close to AC resistance and inductance can usually be neglected. Therefore, a DC resistance measurement would be useful. Temperature of the wire may be important in some cases.

If you measure the voltage drop along a wire, and the current thru the wire (the wire is nearly a pure resistance), then the phase shift of the current thru the load is of no importance, because the voltage across the wire is in phase with the current thru the wire. This you know because we have assumed the wire is a pure resistance.

A convenient test arrangement for 120 V circuits is a nominal 1500 W heater, an AC voltmeter that can resolve 0.1 V at 120 V, and an ammeter. With this and a little experience you can estimate the number of feet of wire between two measurement locations. For example, two wall outlets. You can get a reasonable result within a foot or two. The same size wire needs to be in the wire path, and you need to know the resistance per foot. The resistance per foot you can measure using a sample of wire.

You can use this method to check the quality of a wired circuit. For example: looking for loose connections, bad breakers, bad switches, or sockets.
.

 

[rambojoe]

gar- you nailed it, testing the installed quality of a circuit. I have read each of your posts many times and my initial searches failed because I was missing key terms like inductive and capacitive reactance among others. and holy cow, that's what I was missing. do I need to know these things to set a top hat on gear? heavens no- but I want to better understand it all and everyone helped. interesting how the question of resistance brought up neat things like leading and lagging I & E reactance in different circuits. I even found yet another play on words "ELI the ICE man"- "L" being inductive reactance and "C" being capacitive.....so I lags E in,say motor circuits and so on. awesome! confusing because I found it all on the internet. I guess its not just for porn . oh, and another way to kill someone with a meter- you can strangle them with the leads. genuine thanks to all of you, I found at least some of my answers! I at least have the tools to answer my questions now.

 

[kwired]

gar- you nailed it, testing the installed quality of a circuit. I have read each of your posts many times and my initial searches failed because I was missing key terms like inductive and capacitive reactance among others. and holy cow, that's what I was missing. do I need to know these things to set a top hat on gear? heavens no- but I want to better understand it all and everyone helped. interesting how the question of resistance brought up neat things like leading and lagging I & E reactance in different circuits. I even found yet another play on words "ELI the ICE man"- "L" being inductive reactance and "C" being capacitive.....so I lags E in,say motor circuits and so on. awesome! confusing because I found it all on the internet. I guess its not just for porn . oh, and another way to kill someone with a meter- you can strangle them with the leads. genuine thanks to all of you, I found at least some of my answers! I at least have the tools to answer my questions now.

Finding new or forgotten information online about electrical theory is porn for electrical professionals

 

[Strathead]

gar- you nailed it, testing the installed quality of a circuit. I have read each of your posts many times and my initial searches failed because I was missing key terms like inductive and capacitive reactance among others. and holy cow, that's what I was missing. do I need to know these things to set a top hat on gear? heavens no- but I want to better understand it all and everyone helped. interesting how the question of resistance brought up neat things like leading and lagging I & E reactance in different circuits. I even found yet another play on words "ELI the ICE man"- "L" being inductive reactance and "C" being capacitive.....so I lags E in,say motor circuits and so on. awesome! confusing because I found it all on the internet. I guess its not just for porn . oh, and another way to kill someone with a meter- you can strangle them with the leads. genuine thanks to all of you, I found at least some of my answers! I at least have the tools to answer my questions now.

Good luck with that. Personally, I think you would be best off taking a course in basic AC theory so that you can be taught in a structured manner.

 

[kwired]

Good luck with that. Personally, I think you would be best off taking a course in basic AC theory so that you can be taught in a structured manner.

I think that depends on what you already know and how easily you do pick up new stuff.

If you don't already have much basic fundamentals down the reading materials won't help you much more either way, but the labs and practical exercises in real education program can make a big difference in learning some of this stuff for the first time.

Inductance and reactance are sort of basic fundamentals in a way though. Not quite as basic as learning what volts, current, basic resistance is, but really only a step or so past those elements. Learning about power factor, which inductance and reactance are a major part of, is a few more steps into more advanced theory though. But isn't so complex that you couldn't learn more about it just reading books or internet articles if you have most other basic theory handled.

 

[Strathead]

I think that depends on what you already know and how easily you do pick up new stuff.

If you don't already have much basic fundamentals down the reading materials won't help you much more either way, but the labs and practical exercises in real education program can make a big difference in learning some of this stuff for the first time.

Inductance and reactance are sort of basic fundamentals in a way though. Not quite as basic as learning what volts, current, basic resistance is, but really only a step or so past those elements. Learning about power factor, which inductance and reactance are a major part of, is a few more steps into more advanced theory though. But isn't so complex that you couldn't learn more about it just reading books or internet articles if you have most other basic theory handled.

With all respect to the OP, having a light bulb moment in regards to reactance is part of the reason I stated what I did. It indicates that he is trying to better himself, but shows how very fundamental parts can be easily overlooked.
AC theory was far harder for me to grasp than DC. Well, once I got over the fact that everything is talked about from electron flow, but all of the "rules", like the right hand rule are based on conventional flow. There is far more voodoo in AC theory than DC. I just think that being taught is a structured, step by step manner is more productive than reading whatever you come across. Even then, the internet research is necessary to fill in the gaps.

 

[kwired]

With all respect to the OP, having a light bulb moment in regards to reactance is part of the reason I stated what I did. It indicates that he is trying to better himself, but shows how very fundamental parts can be easily overlooked.
AC theory was far harder for me to grasp than DC. Well, once I got over the fact that everything is talked about from electron flow, but all of the "rules", like the right hand rule are based on conventional flow. There is far more voodoo in AC theory than DC. I just think that being taught is a structured, step by step manner is more productive than reading whatever you come across. Even then, the internet research is necessary to fill in the gaps.

Everyone learns a little differently as well, but I do agree that the structured environment still is probably the easiest way to learn for most any person. But if you already have most basic theory down, those more complex things are easier to learn on your own. Even if you once learned something but don't use it a lot that basic knowledge make it easier to "relearn" what you forgot. When I do power factor calculations I need to get out some reference materials and refresh my memory some, as I don't do it often enough that it is all there and accurate, but just lingering around on a site like this seems to keep that retained information more up to date as well.

 

[Strathead]

Everyone learns a little differently as well, but I do agree that the structured environment still is probably the easiest way to learn for most any person. But if you already have most basic theory down, those more complex things are easier to learn on your own. Even if you once learned something but don't use it a lot that basic knowledge make it easier to "relearn" what you forgot. When I do power factor calculations I need to get out some reference materials and refresh my memory some, as I don't do it often enough that it is all there and accurate, but just lingering around on a site like this seems to keep that retained information more up to date as well.

Absolutely. I spent tons of time pouring over documents and being instructed in the effect of harmonics before I had a light bulb moment about the actual problem. I think that most of the "instructors" didn't understand what they were actually teaching, especially the parroting of "triplen harmonics" and why they were the "problem".

 

[gar]

150804-1251 EDT

rambojoe:

I have two duplex receptacles spaced 5 ft apart on the wall in my kitchen eating area. When I built my home 49 to 50 years ago I did the wiring. The receptacles are Hubbell, the wire #12 copper from Anaconda, the breakers, main panel, and sub-panels are Sq-D QO.

For the above receptacles the approximate wire length from receptacle terminal to terminal is about 7 to 8 ft. DC copper wire, #12, at 20 C. is about 1.588 ohms per 1000 ft.

I have various test cords with 3 prong plugs, and #16 wire to banana plugs. One of these has one hot and one neutral lead. I also have a Kill-A-Watt with a 6 ft extension and a straight plug.

I will make a measurement on the hot wire between the two receptacles by connecting the meter, in this case a Fluke 27, between the two hot slots. AC voltage range has a resolution of 1 mV. The mV range provides resolution of 0.1 mV. With no current flow on the hot wire the residual noise pickup of the 1 turn loop of the measurement circuit is about 0.5 mV, which reads 1 mV on the auto-range AC position.

A 1500 W heater is used as a load to provide a current thru the hot wire. This is connected to the receptacle furthest from the main panel. The Kill-A-Watt is used for the current measurement. The current varies quite a bit until the heater element stablizes.

My measured results were 84 mV at 7 A. This calculates to 12 milliohms. At 1.588 ohms per 1000 ft the length calculates to 12/1.588 = 7.6 ft. Good agreement with expected length and thus circuit is probably good. The same test cam be run on the neutral. if either or both current and voltage jumped around, then you might expect a poor connection.

The above measurement pretty much provides a voltage measurement that eliminates the voltage drop within the receptacle between the plug to the load and the spring contact in the receptacle. But it does include the contact resistance between the wire and the receptacle screw terminal.

There are other similar tests that can be performed, but this is enough for you think about for now.

.

 

[rambojoe]

You are good man Gar, thanks for the homework! I never thought romex and brain flexing could be so closely related! Out of all the responses this one will take careful review. Thank you.

 

[gar]

150805-0902 EDT

rambojoe:

This morning I performed the same measurement on the neutral between the two receptacles. The measured voltage at 7 A was 8.6 mV. Close to the hot wire measurement as wold be expected.

If I performed the measurement on the EGC the resistance would be higher because in 1965 the EGC in #12 Romex was smaller than the current carrying conductors. Some would say it is dangerous to apply current to the EGC. It is not dangerous if done correctly. A safe procedure is to use a low source voltage, like 1 V.

For you to play with these ideas mount two open, not in a box, duplex receptacles on a board or something. Then take several lengths of #14, #12, and #10 and connect the test wire between the two receptacles and perform different measurements. Completely open receptacles allows you to easily change the test wire, and to make voltage drop measurements at different points. One such experiment should be directly on the wire.

See photos P19 thru P21 at my web site http://beta-a2.com/EE-photos.html on four terminal resistors.

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