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3 Extension power cords connected one after another

  1. Jul 18, 2016 #1
    Hi,
    I have a friend with whom I got into an argument of what happens when you multiple power cord extension connected.
    So here is the problem:
    - There are 3 extension power cords, each power cord has 3 plugs.
    - First extension power cord is connected to the wall outlet
    - Second one is connected to one of the plugs of the First extension
    - Third one is connected to one of the plugs of Second one

    So what my friend is basically saying that if there is high load on all 3 extension power cords, then the devices connected to the Third extension cord will get less power (or amperage)

    What I am saying is that it doesn't matter since its basically same thing as if all the connections are in parallel, so the power will evenly distribute if there is high load, so there is no difference.

    Here is a small demonstration:

    wall plug cable 1 <------------ Power cord A 1,2,3 <---------- Power cord B 1,2,3 <------------ Power cord 1,2,3
    wall plug cable 2 <------------ Power cord A 1,2,3 <---------- Power cord B 1,2,3 <------------ Power cord 1,2,3

    so basically the load is between

    Let me know what you guys think
     
  2. jcsd
  3. Jul 18, 2016 #2
    Unfortunately the question is not very clear in several respects.

    In a consumer context I have never seen power cables that split into multiple plugs at one end. Are you referring to power cables with multiple sockets at one end, known by many names such as power strip? I will guess this is the case.

    Are you describing a situation in which the first power strip is connected to the wall socket, the second power strip is connected to the first, and the third is connected to the second? If so, and you are only connecting devices to the third power strip, using the others as extension cables, then this is almost equivalent to using a single power strip. There will be a negligible additional drop across the added cable lengths and connectors. This assumes that all the power strips are properly rated for your intended current draw.

    If you are connecting multiple devices to this network of power strips, let alone fully populating it, you need to be very careful of the total draw. The cable to the wall must be rated for the total load and then some, as safety margin. Even if you are only powering what seem like low power digital devices, you should do the sum, especially as many of these consumer power strips combine flashy packaging and marketing with outrageously horrible and substandard construction. They are a typical causes of fire.
     
  4. Jul 18, 2016 #3

    Averagesupernova

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  5. Jul 18, 2016 #4
    I am aware of such products. Apparently, our interpretations of 'consumer context' differ. This is unimportant.
     
  6. Jul 18, 2016 #5

    CWatters

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    Pretty sure we don't have extension leads with multiple plugs here in the UK (only multiple sockets).

    Either way if you use a long extension cord it can have bad effects, particularly with devices like pressure washers. Some behave like constant power devices so if there is any voltage drop down the extension lead the current increases to compensate making the voltage drop worse. You can end up burning out the washer.
     
  7. Jul 18, 2016 #6
    Hi. First post here after my intro. Hopefully, I can help here.

    First - there is some terminology confusion over "plug" and "socket" (AKA "outlet", "receptacle"). Also referred to as male/female (plug/socket) connectors. An extension cord like the one pictured in post #3 has one "plug" (plugs into the wall), and multiple "sockets" (receptacles to accept additional plugs from devices). Multiple plugs on a cord is dangerous (you have live power on the exposed prongs when one is plugged in), and is also called a "suicide cord". These are sometimes used to connect a home generator output to a home socket, to power a circuit in a house. This is illegal, as it could send power down the line and kill a worker on the line. I guess that would be murder, or manslaughter, not suicide (wow, this got dark quickly!).

    On to the stated problem:

    So what my friend is basically saying that if there is high load on all 3 extension power cords, then the devices connected to the Third extension cord will get less power (or amperage)

    What I am saying is that it doesn't matter since its basically same thing as if all the connections are in parallel, so the power will evenly distribute if there is high load, so there is no difference.

    Your friend is correct in technical terms, but you are also mostly correct in looser, practical terms.

    What you are missing is that the extension cord and each connection has some small amount of resistance. So as the loads are pulling current, and there will be a small voltage drop along the cords and connectors. So the voltage at the end of the line will be lower than at the start of the line. So your friend is 100% correct.

    In your defense, these resistances are fairly small for proper extensions cords and loads, so are mostly ignored. Until you get into long cords and high loads, as pointed out in post #5 , and I'm sure you've seen power tools that specify a gauge and length limit for any extension cord used - this is why.

    This made me think - there really is almost no such thing as a true parallel circuit - there is always some relatively small, parasitic resistance between any two points. But if we drew all the parasitics in every schematic, it would be near impossible to read! So like your diagram, we generally ignore them until they become significant in the particular application (sometimes you will see the internal series-R modeled in a battery for instance).

    Maybe more than you wanted to know, but I hope this helps!
     
  8. Jul 19, 2016 #7
    Hi NTL2009,
    thank you for very detailed explanation.
    I agree that there is a very small resistance for each power cord cable but even if ignore that
    then still according to my friend there is going to be a power change on last cord,
    so the devices connected to that extension cord will not get enough. You can say that they are not as prioritized as devices connected to the first one. And this is complete nonsense to me. If the resistance is insignificant and you ignore it then there is no difference.
     
  9. Jul 19, 2016 #8

    Nugatory

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    And you'd be right, because all the loads are connected in parallel.
     
  10. Jul 19, 2016 #9
    Your friend is right, but so are you. The different power bricks are in series. In an ideal world, where you can ignore resistances, they would be in parallel, but in this case, he is right. The difference between them being in series vs. parallel is very small however, so what you have said is basically true, but he is correct in terms of how the circuit actually works.
     
  11. Jul 19, 2016 #10
    Yes, if you totally ignore any resistance in the wires and connectors, then the device loads are seen to be in true parallel, and would all see the same voltage, by definition. There is no 'prioritization' of devices under that view. You could ask your friend, if there is a lower voltage on the last device, but no resistance between them, where does the voltage drop go (or where did it come from)? Any basic circuit analysis book (well, OK, web page these days), will describe how all the voltages in a loop must add up to zero (loads offset sources). Your friend has to account for the voltage drop somewhere, and zero resistance has a zero voltage drop. It is inconsistent with everything I've ever learned.

    Just for 'fun', let's look at the real world. I grabbed a generic 6' extension cord typical of the ones you find in any store in North America (115V, 60 Hz, typical 15A fused outlets). The 6' cord is 16 AWG, and has 12' of wire, and tables tell me that has a resistance of .048 ohms. An NA 15A fused outlet is spec'd for 12A continuous I think, so let's consider 3 devices, drawing 4 A each:

    If a 4 A load was put at the end of each cord, you would have 12A on the first cord, 8A on the second, and 4 A on the 3rd cord

    R ------ A --- Vdrop
    0.048 --- 12 --- 0.576
    0.048 ---- 8 --- 0.384
    0.048 ---- 4 --- 0.192
    1.152 total drop at the end of the three cords.

    Roughly 1% of the 115V source, so that could be considered a small drop and ignored in many cases, but it is not ridiculously small either. Have we beaten this to death yet? :smile:

    -NTL2009
     
  12. Jul 19, 2016 #11
    hi,
    I understand that in real world when there is current there is resistance even if it is insignificant,
    Our argument was in ideal world, there was no mentioning of cable resistance from his side ever (in fact I specifically disregarded it)
    he just simply stated that its in series because they are connected one after another,
    and that the current will drop and not the voltage
     
  13. Jul 19, 2016 #12
    OK, it wasn't clear to me that you were looking for an 'idealized' answer, disregarding cable/contact resistance. Extension cords are a 'real world' thing. Idealized, I think it would have just been stated "Would three devices in parallel all have the same voltage across them?", and the answer, by definition, would be yes. It wouldn't even be question, really, but a statement of definition of parallel circuits. I guess that's what threw me.

    I'm also confused by your statement "its in series because they are connected one after another". The devices are in parallel. The cord conductors aren't really "in series" if you are ignoring their resistance, they are just connection points. I've always thought of components as being in either series, parallel, or combination. The connections just "are", and you don't really think of them as being "in series" until you start thinking of them in terms of their parasitics (resistance and/or inductance), or "in parallel" until you start considering their parasitic capacitance. And then you can take it further still, and consider the distributed reactance along the line.

    This is probably getting too far into semantics for you, but in technical discussions especially, words have meanings and it helps to use them as correctly - how far you take that depends on the context.

    Anyway, hope that helps! -NTL2009
     
  14. Jul 19, 2016 #13
    Hey, it's me, the friend.
    I think NTL2009 gets this perfectly. I said that they were in series because of what he said. I know that in the real world, the difference is very negligible, but nonetheless, it exists.
     
  15. Jul 19, 2016 #14
    Hi NTL2009,
    thank you for your answer,
    my friend just joined since he couldn't take the defeat properly lol

    To My Dear Friend:
    Things are connected in parallel, only you are in series (he will get the message)
     
  16. Jul 19, 2016 #15

    Averagesupernova

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    This is just about stupid. Either we ignore the series resistance of the conductors or we don't. Now if we really want to get nitpicky about who said what when, I can point out that if we put a regulated switching power supply that has a wide range of input voltage and with a constant load on it then does it receive less power when hooked out on the end of the line compared to the beginning? Typically a buck type convertor will draw more current from the source when the source voltage drops. So without specifying what type of load is on the 3 extension cords it is all somewhat irrelevant.
     
    Last edited: Jul 19, 2016
  17. Jul 19, 2016 #16
    As long as you are both still friends, that's what really counts! :smile:

    I like explaining somewhat basic things sometimes, it often kind of shakes up my own understanding of things I might take for granted, since I think I've known them for so long. But explaining really tests my assumptions, and I end up learning along the way as well.

    In this case, it made me think a bit more deeply about what we do and do not "take for granted" in a schematic. Parasitics, and/or layout details can really affect a circuit, and often in mysterious ways. But schematics don't always capture these details, which may be more physical in nature. A good schematic might take them into account, like a "star ground" (or "star common" might be a better term - I've seen some good discussions here about how ground and common are often used imprecisely).

    -NTL2009
     
  18. Jul 19, 2016 #17

    Averagesupernova

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    I recall having a discussion on this forum about the fact that schematics don't always tell the whole story. Some folk believe if they have a schematic they can simply copy the design and build something just as good as the original. Not so.
     
  19. Jul 20, 2016 #18

    CWatters

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    In the real world the resistance of an extension cord isn't always negligible. It depends on the load and the length/resistance of the cord. This is particularly true when the load is a relatively high power motor or pump.
     
  20. Jul 20, 2016 #19

    Averagesupernova

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    I have used a small air compressor on the end of a 100 foot cord that was apparently too light as it would start it on an empty tank and run it until it was full. But when the pressure dropped enough to start again it would not pull it.
     
  21. Jul 21, 2016 #20
    totally agree..... unimportant in the context
     
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