Extension Cord Question

Use the bigest first, it cant hurn nuttin

John T

• Hobby farm

Now, if you are going to put a tap with a load in the middle between them, yes, you want the heaviest cord first, as it will be carrying the most current, then the smaller cord will pass only what is being drawn thru it.

Charles

You are correct. In the mechanical world we sometimes use electrical circuits to simplify the analysis of mechanical piping systems. It's called using an electrical analogy. In your case it is conecting two resistances in series with the same current draw in each. The total voltage drop is the sum of the two voltage drops, no matter which one is first. The same is true of a piping system that is connected similarly. Gpm is analogous to amps and pressure drops are analogous to voltage drops. If two pipes are connected in series, the total flow is the same in both and the total pressure drop is the sum of the individual pressure drops. It doesn't make any difference if the larger pipe is first in line or last. The assumption is, of course, as others have indicated - that there is no intermediate load or flow take-off at the junction of the two cords or pipes.

Rod

An orifice in a hydraulic or pneumatic system is analogous to a resistor in an electrical system. It inserts a resistance in the circuit large enough that the sizes of the pipe (or wire) in the rest of the circuit becomes relatively insignificant. In the case of an orifice, its effect on pressure drop is very large relative to its length due to the turbulent flow through it. So even though the orifice is maybe an eighth of an inch long, it creates greater restriction than several feet of hose. Likewise, if you have corrosion on your electrical outlet contacts, it may not matter how long or large your wires are.

Now, does it matter where in the circuit the orifice is? In the steady state, no. The same flow exists anywhere in the circuit, so the pressure drop is the same. HOWEVER, the initial transient response will be much different depending on where you put the orifice and where you turn on the flow. Using your torch example, if I put an orifice in the torch and turn on the valve at the torch, I will see a constant flow from the instant I open the valve. On the other hand, if insert the orifice back at the regulator end of the hose, the gas flow will be very high when I first open the valve and will taper off as the pressure drops in the hose. In this case, the hose is acting similar to capacitance in an electrical circuit, and is storing energy that gets released when I open the torch valve. (We don't see this phenomenum in our extension cord example because the capacitance of the cord in negligible. Also, the extension cord is normally transmitting AC power which is a whole 'nother thing.)

We should learn how electricity works on its own rather than depend on the old water flow stuff that often makes things more confusing.

As an example, when a resistor is placed in a wire the voltage after the resistor drops and the current in the WHOLE wire drops.

With a water pipe if a resistor is placed in the pipe the water flow decreases after the resistance but the pressure INCREASES before the resistance. This is provided that there is less restriction after the resistance.
So electricity and water flow are not always comparable.

The thing to keep in mind with electricity is that the current is ALWAYS the same in the whole circuit loop no matter where the resistance is.
All series resistances add together and it does not matter where in the loop they are.

Regards
Gary

Thanks for your comments. I do have to nitpick on one point:

"With a water pipe if a resistor is placed in the pipe the water flow decreases after the resistance but the pressure INCREASES before the resistance."

Actually you will see the same behavior in an electrical circuit. As the resistance increase, the current through the conductor decreases, causing the voltage ahead of the resistance to increase due to reduced voltage drop.

But your point is well taken that trying to take analogies too far isn't productive. It is more important, in my opinion, to understand the math required to solve the problem at hand. Once that's done, the math is identical for electrical, mechanical and fluid systems. And it is the mathematical aspects of the analogy that are most often overlooked.

Yes you are correct again. This adds to my point about easily causing confusion when trying to compare water flow to electrical flow. In Rod's examples he is assuming a constant pressure fluid source and you are assuming a non regulated pressure source and or non constant voltage source.

Both work fine as long as the parameters are fully defined. But to my point, what is usually used as an example in the voltage side is a constant voltage source i.e. battery or wall outlet.

A water source is often thought of as coming from a variable pressure source such as a pump that has limited flow capabilities. Close the valve and pump pressure goes higher, open it and pressure drops at the pump. This can make things look awfully confusing if one has a basic understanding of how the pump works and tries to compare that to an electrical circuit directly.

I can't remember any examples over the years that I have seen the water flow to electricity flow comparisons where it was defined that the water source was a constant pressure source.

This is my nit. :>)

Regards
Gary

I'll also add another thought regarding the mention of an orifice restriction and a resistor. Yes, it is an analogous situation in general, but one has to be careful about how it is represented in any mathematical model. You cannot directly equate the pressure drop that occurs immediately across (upstream and downstream) the orifice to the voltage drop that occurs across a resistor. The high adjacent pressure drop across the orifice is a very localized occurrence. Several pipe diameters downstream of the orifice, much of that pressure drop is recovered by the conversion of local velocity head to static head. The "global" orifice pressure drop in the system that would be analogous to the voltage drop across the resistor needs to be used instead of the local pressure drop. The proper term for this is "permanent pressure loss of the orifice". It is very commonly used in metering systems for both compressible and incompressible flow and is usually significantly less that the localized pressure drop.

Anyway, I hope you all will excuse me for now bugging out of this discussion because 1) I am getting tired, 2) It is getting far down on the page and I suspect most readers are no longer interested, 3) It has become far removed from Anthony's original question, which I think he got properly answered a long time ago and 4) It has turned into the "float your boat" debate that Stan has mentioned. I am sure Anthony's eyes have glazed over if he is still reading posts here.

Rod

Regards
Gary

I assume you're referring to a "stick" welder, since it's not practical to add cable to a TIG or MIG welder. Stick welders are considered to constant current sources; that is regardless of the load, the output current will be constant. (The same is true of TIG welders, while MIG welders are considered to be constant voltage sources.) This brings up a valid question: "Why do I need crank up the current on the welder when I add cable, if it's supposed to be a constant current source?" The answer is that the welder is not a true constant current source. If it was, the stinger would start arcing all around the room as soon as you turned on the welder, because a true constant current source would put out a bazillion volts or whatever was required to start the arc going. Actually, the welder is neither a true constant voltage nor a true constant current source. When you add cable, you effectively change the output impedance of the welder, which requires you to adjust the current setting to get the same current as before.

I could go into further detail, but it's really not necessary. The main point is that from an electrical point of view, it doesn't matter if you put the small cable at the stinger end or the welder end of the cable. You will get the same current and voltage at the stinger in either case.

Gary is correct about the current being constant between the welding machine and the stinger. You don't "lose" any current along the way any more than you "lose" any water flow in the series pipe analogy. Larger welding cables are used to keep the cable temperature within workable limits when using high welding currents and to minimize the voltage drop between source and load. The exact same reason why larger wires are used in the higher current circuits in your house wiring. The resistance (ohms) in a wire or cable is dependent on temperature so the voltage drop changes as the wire or cable heats up for any constant current flow. This is where we are getting into the transients that MarkB talks about and that only serves to confuse the situation more. Lets just leave it that there will be a steady state condition where the temperature (and the voltage drop) stabilizes. But that doesn't change the fact that at any instant in time, the current at the stinger is the same as the current at the welding machine, no matter what size cables are used in between, no matter which is connected first and no matter if they are aluminum or copper. That requires, of course, a constant condition across the arc itself - the "load" if you will. You need to maintain the arc length constant or we are into another one of those transients that are not necessary to delve into for the purposes of this thread. Again, it doesn't invalidate the analogy at any instant in time. Mathematical models can be developed using the analogy that will take such transients into consideration but they are not needed for the simple purposes of this thread.

When you talk about using larger wires first, yes, there's a reason for it - but it is not relevant to this thread. It's the same reason as using larger pipes first: there are branch connections downstream of the source. Again, that is a very common, necessary practice that can be seen most anywhere you care to look. It's the reason why the cable from the utility's electric meter to your house breaker panel is larger than the wire from that panel to one of your wall outlets. And the reason why the water main coming into your house is larger than the pipe feeding your bathroom sink.

I cannot convince you to believe in the validity of the mechanical analogy of water flow in pipes to electric current if you choose not to do so. But if you would think about it a little bit in this instance, it might help you to understand that you don't "lose" any electrons (current) in that welding circuit any more than you "lose" any water (flow) in the pipe circuit. You don't have to take my word for it. Just plug the words "electrical analogy" into Google or any other search engine of your choice for more professional testimonials than you'll care to read about the time-proven validity of the concept and the widespread use of it.

Rod

The only reason that you would want to put a smaller cable at the stinger end would be so you have more flexibility. Remember that the welding cables are a series circuit. The current flows from the welder, thru the cable that feeds the stinger, into the work you are welding and then back to the welder thru the ground cable.

If you insert a higher resistance (small size cable) ANYWHERE in that loop you will decrease the voltage available at your work. Decreased voltage at your work will decrease the amps available for the arc.

The welder itself is sort of voltage regulated and tends to hold the voltage up as current is drawn when you weld. But that voltage is only regulated at the welder output terminals. Any resistance you add after the welders terminals starts to kill the regulation as it lets the voltage drop more at your work with a smaller, higher resistance, cable. Of course there is also resistance in the windings of the coils in the welder so regulation is not perfect to begin with but that gives you an idea of how it works.

Always remember that any electrical circuit is a series circuit. That is, there must be a supply line and a return line or nothing happens. The same exact amount of current flows in both the supply line and in the return line.

If you add any resistance anywhere in either line it reduces the current equally everywhere in both lines.

All cables have resistance. The smaller the cable the more resistance it will have.

If ohms Law is learned and remembered you can figure out most any electrical problem with relative ease or at least see why things happen as they do.

Here is an easy way to remember ohms law if my drawing works ok. You arrange the letters E I R in a pie configuration.

E
----- ----- -- I R
E is voltage (Electromotive force) in volts.
I is current in amps.
R is resistance in ohms.

In the little pie above you place your thumb over the one that you want to know and it tells you how to find it from the other two.

You want voltage (E) you would then multiply I x R.
You want current (I) you would divide voltage (E) by R.
You want resistance (R) you would divide voltage (E) by current (I)

Remembering this little crutch will let you figure out most any electrical problem. If you forget just remember that E is always on top.

Ohms law will tell you the relationship in a circuit between voltage, current and resistance. Using simple numbers you will be able to show what happens to one as you change one of the others.

Regards
Gary

As an example, when a resistor is placed in a wire the voltage after the resistor drops and the current in the WHOLE wire drops.

And the same is true when placing a restriction in a pipe with a flowing fluid having a constant pressure source. But the subject of this thread is not about adding things to an existing system. It is about locating the same things differently in the same system.

With a water pipe if a resistor is placed in the pipe the water flow decreases after the resistance but the pressure INCREASES before the resistance. This is provided that there is less restriction after the resistance.

Ditto above. A new restriction is not being added for this thread, just existing restrictions ( the two cords) relocated with respect to each other. It is important to note that for this thread, the source pressure must be a constant - otherwise the analogy is not correctly modelled. No matter what the originator does with his extension cords, he will not increase (or decrease) the supply voltage available at the outlet. So any kind of a mechanical analogy has to include a constant pressure, variable flow source. The same with the load. In this case It has to be a variable flow, variable pressure device. A hydraulically powered drill or saw could be such a device. If all the pieces of the pie are not modelled correctly, then yes, the whole thing falls apart.

The thing to keep in mind with electricity is that the current is ALWAYS the same in the whole circuit loop no matter where the resistance is.

And the same thing is true for fluid flow in pipes. Except in the transients that MarkB mentions. But that's true for both mechanical and electric circuits. That situation can be modelled also but is more complex.

All series resistances add together and it does not matter where in the loop they are.

Again, the same thing is true for fluid flow in pipes. Neither the source nor the load, if they are properly modelled, know or care where the restrictions are located between them. They are affected the same regardless. With the exception of the transients mentiond by MarkB, of course. But transients are not particularly relevant to the subject of this thread.

I am not aware of any significant usage of mechanical analogies in the consideration of electric circuits but it is very common to utilize electrical analogies in the analysis of complex mechanical systems. The concept is often used by professionals in both the heat transfer and fluid flow fields in order to simplify answers to problems. At least it was until hard analysis was abandoned in favor of blindly punching input into a predefined computer program and accepting without question somebody elses thinking on how it should be done. The model has to be set up correctly for it to yield valid results. A good example is quantifying the individual flows in a district water distribution system consisting of miles of many different sizes of pipes, with a couple of hundred interconnecting nodes spread throughout. With multiple sources (water storage tanks) and many different loads (end user consumption). Such a flow network can be and is set up as an electric network model that simplifies the work necessary to produce usable results. If you ever have a chance to review the dirty "innards" of any computer program set up to evaluate such a network, I suspect you'll find electrical analogies at work behind the scenes. As with anything, to yield valid results it has to be used properly. Is such a deal perfect in all instances? Of course not. Nothing is in the real world. But such an analogy, used for calculation and design purposes, is far more accurate than the assumptions needed to quantify the many concurrent loads in the first place.

Rod

Without disputing anything you've said, I'd like to address a couple of the points you made.

"... but the analogy quickly falls apart in many cases so the water flow can not be used."

Analogies are only good until they aren't good anymore. An analogy can't be perfect because then it wouldn't be an analogy---it would be a description of the same situation in differnet words. In plane geometry there's an element of proof called identity. It says that we can accept a certain thing as true because it is identical to a given; for instance, angle x equals angle y because angle x IS angle y (possibly by way of a previous proof.)

"We should learn how electricity works on its own rather than depend on the old water flow stuff that often makes things more confusing."

But often makes things less confusing, you'll have to agree. We make analogies to things we can imagine or things we are familiar with to help us understand unfamiliar things or things we can't imagine. The more removed from the everyday an explanation is, the fewer people will be able to understand it. Words are also maddeningly inexact in describing processes and conditions in many areas of science; for the sake of clarity it would often be better if we didn't try to use words but dealt only in math. In which case only a tiny fraction of one percent of the general population would ever be able to understand anything more complicated than how to put bread in a toaster.

There are individuals on this site who possess high levels of technical expertise, and they're perfectly free to communicate with each other at whatever level of complexity floats their boats. But, for obvious reasons, they aren't as likely to be the ones asking the questions in their areas of expertise. An answer which is incomprehensible to the person who asked the question is really no more use than no answer at all.

All the best, Stan

Here's my thoughts:

Certain size pipes only allow so much maximum flow

True.

If you loose the flow or amperage in the beginning, how do you get it back?

I'm not sure what you mean by "loose the flow or amperage in the beginning". As far as flow goes, what goes in one end comes out the other. The same is true for current. You don't get losses (pressure drops or voltage drops) back. At least not in any normal sense. In a piping system, the pressure drops can be compensated for by installing a booster pump somewhere between the beginning and the end. But you'd have to add energy into the system to run the pump. I'm not sure how you'd do something similar in an electrical circuit. Perhaps with a series battery - at least in a DC circuit

for a cutting torch that you have to put the bigger hose first and then you can run a smaller size so it is easier to handle

Ease of handling is the only reason I can see for that. I do it only for that reason. The same is done with compressed air hoses. I generally run 3/8 air hose but for many smaller tools and a spot spray gun, I use a short (6 ft) length of 1/4 hose at the end for easier handling, especially in tight quarters like an engine compartment.

Once you restrict the oriface size you limit the maximum flow. You couldn't put another hose of bigger size on the end of the tip and then get more flow than that tip allows

True. No matter what pipe sizes you hook together in series, what goes in at the beginning (gpm) comes out at the end (gpm). The conservation of mass demands it. Assuming no leakage at joints, of course.

Rod

Good explanation. Almost every day that I'm on the planet I learn that I don't actually know something I thought I knew. I hope that I've got enough of those things left that I can break exactly even just as I croak. That would be sublime.

All the best, Stan

I always thought that it was as Pooh Bear said, but recently in answering a question asking basically the same thing (concerning the order to run 8, 6, and 4 gauge wire from the breaker box to an outbuilding) I was informed that my answer applied to water pressure in mixed size plumping pipes but not to electrical current. The person told me that the voltage drop is determined by the amount of cumulative drop caused by each run of different sized wire and that the order makes no difference. I can't quite get my mind around it in a way that let's me visualize the process, but I'm going to consider that the correct answer unless or until I hear differently.

All the best, Stan

Pooh Bear