Cap banks - the pre-emptive answer

[gtrcguru]

As far as I know, Doc, AWG refers only to the total circular mil area of the conductor and is the same for all conductors, AC or DC. The diameter in the chart is likely for a single solid conductor or one of fewer strands, which would have a larger diameter. A multi stranded conductor like we use in boating will have a larger diameter because of the air gaps between strands, but still has the same cross section of copper. Even with AC wiring, some wire strippers will recommend for example to use the 12 AWG hole to strip 14 AWG stranded. Also, the ampacity of a conductor is primarily limited by heat, so while the #10 with common THHN(90 deg C) insulation that we use in buildings that has an ampacity of 35 amps(but only to be fused at 30), can be bought with much higher temp insulation which gives it a rating of 70 amps. Most of the wire we are using in boats is rated for very high temps, I'll have to look in my code book and see what a 200 deg insulated #10 is good for....
Acceptable voltage drop is typically figured as a percentage of the voltage, so yes, you have far less room for loss with lower voltages, as the resistance is still the same per foot regaurdless of the voltage applied.

For the sake of discussion, in the case of your toaster, the heating element is really just a pure resistor. The 2500 watts is what it is rated to consume at 110v, making it essentially a 4.85 ohm resistor, pulling 22.7 amps at 110v. If you only apply 25v, it's only going to draw 5.15 amps, and you aren't making much toast at 128 watts, never mind melting the wires.
But, in your hypothetical situation, I thought if the wire only carried a fixed 22.7 amps, it likely wouldn't heat up any more at 25v DC than at 110v AC, could you explain to me why you are saying otherwise? I am familiar with the term "I sqared R losses", but it's been a long time since I've been in a theory class, and it's not something that regularly applies to my work, please refresh my memory, is it strictly a DC thing? I am not familiar with ratings of "wattage capacity for DC cables", I am interested to understand more....
In my work, most of our wire insulation is rated for 600v, the ampacity ratings do not change weather it's 24v or 480v, and I did't think it changed for AC or DC either, but I don't deal with much big DC. Looks like I'll be flipping though the code book tomorrow.
BTW, that sounds like a serious toaster!

+1

http://www.southwire.com/support/voltage-drop-calculator.htm

[drwayne]

+1

http://www.southwire.com/support/vol...calculator.htm

AC relevance only without further reference to cable lengths, current loadings etc.
But it does give a nice dimension of cable rating
But the pvc insulation is not for us .

[NativePaul]

Diddely squat? I have heard that electrons migrate to the circumference of the wires so multiple smaller strands are better than a single bar as it has more circumference per square mm, but is that only true of high voltages and AC currents?

[kevinpratt823]

Diddely squat? I have heard that electrons migrate to the circumference of the wires so multiple smaller strands are better than a single bar as it has more circumference per square mm, but is that only true of high voltages and AC currents?

This is true, and it's referred to as "skin effect", and applies far more to AC. It allows for a VERY slightly lower resistance value to be used for voltage drop in AC vs. DC(your talking a few hundredths of an ohm difference in 1000' I believe in smaller size wires). In higher voltage and higher frequencies it becomes more prevalent, as the back voltage induced in the center of the conductor which forces the electrons out is greater.

Amps are still amps for all intents and purpose. The standard ampacity ratings of the AWG are conservative to begin with, and the ratings for 6' appliance cords is slightly more lenient but should have it as at least a #12. The only other benefit to the larger surface area of multi-strand conductors that I am aware of other than flexibility is heat dissipation, but it still isn't nearly enough to assume that a 6' cord, (which is already a multi-strand for flexibility), would all of a sudden melt because the amps were DC. FWIW, with AC, a 22.7 amp reading is kind of an average(.707) of what the peak amperage actually is, that current is actually peaking at over 32 amps at the top and bottom of the sine wave(same goes for the voltage). You do not need special wire to achieve the same ampacity rating just because it is DC, especially in a purely resistive circuit. The only difference will be that the voltage drop becomes more significant(at lower voltages), but that is not what the discussion is about. For example, I ran the 12v dc winch motor on my old driveway snow plow on the same #10 THHN I use in buildings, well over 6' long, through my(hot) engine compartment. I repeatedly lifted and dropped that plow back to back, drawing well over 35 amps, for hours at a time, and it was perfectly fine. Granted, it's not a continuous load, but the current was definitely higher than the rating of the wire. The 7' leads on the cheap 12v/10 amp car battery charger in my garage are 16 gauge and look like a cord on a cheap table lamp, and I also just looked at the halogen headlights in my truck, they are a regular 16 gauge wire, I have no idea why Aus. has those specs.
Just because a circuit doesn't achieve maximum efficiency according to one's standards doesn't mean it burns up, sometimes it's just less efficient.

Inductive and capacitive dc theory and circuitry may get complicated, but simple DC theory in a purely resistive circuit is not. All the wire charts for sizing in DC give footages primarily to reduce voltage drop, and for 6' you can easily carry 22.7 amps on a #12 @25v, which is likely what the cord of the toaster would be, with no special strand count or insulation noted and probably even maintain an acceptable voltage drop. There will be a voltage drop and thus a reduced wattage at the load, making it less efficient, but amps are amps(coulombs of electrons being transferred across the open spots in the valence electron ring of the conductor's atoms, just so happens that copper has a lot of open spots). You can get as technical as you want, but the cord would not melt. We may get away with carrying more current, more efficiently, through wires because of the fine multi-strands and high temp insulation, but voltage drop aside, a wire that safely carries a given current of AC will safely carry that same current in DC.

Now some of these factors may become more significant when you are, say, trying to squeeze 300 amps through a #10 in a PWM circuit full of capacitance and inductance with very high frequencies at play, but this is not the case here.

Because I am confident in my knowledge and experience on this one, but respect yours as well, I'm now trying to think of what purely resistive appliance I might have that I can hook up to a fully charged 6 cell lipo as an experiment, unless you have a less than vague reply to back up your original statement.

And as far as doing homework and self learning Doc, you ruminated over the term "ampacity" for 10 days, when all you had to do was google "definition of ampacity". Somebody's slacking in the homework dept........

[ray schrauwen]

Buuuuttttt, once you solder all those nice individual strands, the current travels only on the exterior as if it were solid, which it is..... for our purposes, yes, no?

Diddely squat? I have heard that electrons migrate to the circumference of the wires so multiple smaller strands are better than a single bar as it has more circumference per square mm, but is that only true of high voltages and AC currents?

[kevinpratt823]

Buuuuttttt, once you solder all those nice individual strands, the current travels only on the exterior as if it were solid, which it is..... for our purposes, yes, no?

A valid observation, but nonetheless the circuit still likely benefits a bit along the way as the electrons still travel all the strands then regroup at the connector for another inevitable point of resistance and voltage drop.

And for previous reference, here is a chart that says I can go 8.8' with 20 amps @24V DC on a #12 wire while maintaining a 3% voltage drop, I'm pretty sure the extra 2.7 amps combined with 2.4' less wire isn't going to cause a meltdown.....

http://www.theresourcestore.ca/toolbox-wiring.php

[kevinpratt823]

I'm sorry if it went off on a tangent. I certainly respect your background, but some people take your word as gospel on these matters, with obvious good reason, but you gave an example with a very exaggerated result presented as fact, which IMO presented DC as some kind of black magic that cannot be harnessed by ordinary wire, when in reality it was just a case of inefficiency.
There was no broad leading question really, I asked "could you explain to me why you are saying otherwise?", just an inquiry to a couple statements I believed to be inaccurate coupled with the reasons I believed them inaccurate. It was your replies that were broad leading("keep looking", "that's your homework", etc.). They opened the door for me to share what I think I know and what I found in my "homework"; none of it supported the validity of the original examples, so I was left wondering why they were not clearly elaborated on when challenged.
Forgive me if I am passionate about discussing/debating things that intrigue me.

Re-"I do agree, Fluid."
Fluid hasn't posted here, so on first look should I assume that is some kind of passive aggressive implication towards something you are discussing outside of this thread?

Carry on

[drwayne]

Keep this thread on DC application of cap banks ..
I do understand people's frustration paddling through the irrelevant flotsam to find DC electrical safety.
An audit/edit/trim/pruning is in order .........

Adieu !

[Cooper]

Wow , I've been reading over this and a lot of misguided theory,,,, I believe it's safe to say that the bottom line is your batteries and esc signals will be much happier with capacitors. (Rated for correct voltage). And one thing that it seems that is confusing some is that dc and ac current act different in transmission. Best transmission of electrical signal mown to man is gold cable frozen to as close to absolute zero as possible. And only reason for a stranded wire is flexibility. Solid is more efficient. Bottom line is our toys will be more efficient from less restrictions and more direct sourcing. (Reaction source (battery) to transmission of work (motor) and I firmly believe capacitors are not necessary for all applications but for what most are dealing with here they become a necessity for longevity of your equipment. This is a pretty good and educational thread :)

[drwayne]

Thanks Cooper

Skin effect re current transfer
Skin depth is sqrt((resistivity of conductor)/(pi*frequency*magnetic permeability of conductor*permeability of free space))
For our Brushless esc/motor loadings of >4S of 2200Kv, that skin effect is <0.12mm due high freq of draw.
Single strand copper conductor do not translate same current as multistrand same cross section when at high frequency.....
more skin = more transfer

that calculation is irrelevant under 22.126KHz in pure copper ... from where conductivity is almost static over solid or multi stranded wire.

Capacitors can reduce/clip most induced frequency prior to power being fed to the esc.. but the flow from battery to caps is where these rules prevail.

chuckle..Ive corrected from 'Ray' to 'Jay' in previous post.... apologies.

[kevinpratt823]

The AWG charts do not specify AC/DC, because they apply to both. The only reason you have separate charts for DC is primarily because of more critical voltage drops.
The RMS(root means squared, essentially .707 of the actual peaks on the sine wave) rating of 22.7 amps, creates the same heat as it's DC counterpart at the same reading. Here is one of many discussions I found when I googled "do awg ratings apply to dc?", regarding AC/DC using the same AWG charts
It basically applies directly to the scenario we're discussing.https://answers.yahoo.com/question/i...6093115AACP5Hx


As soon as the toaster was shown to be irrelevant, I began speaking of a hypothetical 22.7 amp load. Your original statement that 22.7 amps @25vdc would melt the cord is false, and grossly exaggerated from a real world perspective, and you have failed to prove otherwise.
When challenged on this, you sent us on a wild goose chase about surface area of stranded conductors, but now you are acknowledging that this is only relevant at very high frequency.


Let's cut to the chase then, and go back to the original statement-
"However an ac cable suitable to power my 2500W toaster oven on 110AC ( 22.7A) with length of say 6' from the power socket.. would over heat and melt on 25VDC at same 22.7Amp .( just 567W )... "

In the real world it definitely will not.

If you don't believe me, talk to somebody who wires DC power in marine applications(my father for example). Non critical systems are allowed a 10% voltage drop, and a #12 can be run 20', protected at 25 amps. That is at 12v. That wire does not melt, just more voltage drop.
Go ahead and pull the lever to block me, it won't make the statement any more true. Again, I respect your knowledge and background, and I have an ego too(obviously), so I can relate, but here is something to lighten things up a bit.......https://www.youtube.com/watch?v=ubIpoPjBUds

[tlandauer]

Hey, The boats I wire go BOTH Over and Under----that is only when something goes wrong!
Now seriously I have a question: on 10 or 8 ga. wires that come with some ESC, if I were to solder a cap bank onto the wires near the ESC, the stripped insulation spot need to be properly pre-tinned. It takes quite a bit of heat to achieve that, my soldering technique is average to slightly above average, how much risk do I have before damaging any soldering joints up stream and also electronic components. I hold the wire with cold/wet towel---no dripping water to wet the ESC, of course.
I hope my question does not lower the value of this thread. I feel as if my question is like the ugliest house on the block, lol....

[Cooper]

Thanks Cooper

Skin effect re current transfer
Skin depth is sqrt((resistivity of conductor)/(pi*frequency*magnetic permeability of conductor*permeability of free space))
For our Brushless esc/motor loadings of >4S of 2200Kv, that skin effect is <0.12mm due high freq of draw.
Single strand copper conductor do not translate same current as multistrand same cross section when at high frequency.....
more skin = more transfer

that calculation is irrelevant under 22.126KHz in pure copper ... from where conductivity is almost static over solid or multi stranded wire.

Capacitors can reduce/clip most induced frequency prior to power being fed to the esc.. but the flow from battery to caps is where these rules prevail.

chuckle..Ive corrected from 'Ray' to 'Jay' in previous post.... apologies.

.

I don't like to be the bad guy but your wrong on the skin effect. I was assuming you would realize the discrepancy in solid 10g wire vs multi strand. It has nothing to do with the space between the strands, skin effect is rather specific to size and frequency. It's the relationship of a signal and percentage of it through the cross section of a conductor. Some specific uses would be a cardboard tube clad in copper would carry a better signal than any multi strand wire at a very specific diameter and frequency. And a lot of times even engineers think multi strand wire is more conductive of electrons (as we are talking of current not signal pwk) than solid conductor. It's one of those real world applications and manufacturing curves that has to be acknowledged. It is impractical to use solid wire by means of installation and by non conforming, non flexible. If solid is bent that location will not be as conductive, kinda like when you bend a tin can it becomes weaker at that stress point. So to keep it relevant the more skin= more transfer is just not the case. Think of it like these battery to motor electrons are rather stupid, they don't know to assemble into anything resembling a sine, just do a flat out sprint to the finish.

[kevinpratt823]

Regardless, the heat generated by 22.7 amps is the same whether its 480vac, 110vac, 25vdc, or even 12dc, that is why the charts don't specify voltage. The heat resistance is not going to run away exponentially in this circumstance, maybe at a far higher current, or a much smaller wire, but not here. The wire is not going to melt. The wire was rated to withstand the current and heat safely regardless of voltage, it would barely get warm in the first place, that is why it is rated(by the US coast guard) to be run 20' and carry 25 amps in marine application, it is being done in the real world on a regular basis. The ratings are all more than conservative, and I deal with conductors working at their ampacity limit on a regular basis. When within the ratings, they barely get warm, there will be no thermal runaway, there will not be enough heat to begin the cycle. Thermal runaway doesn't happen until you are WAY out of the suggested ratings etc. It will just warm up to a safe operating temperature and level off there, dissipating the heat of 22.7 amps like it is rated to do. That is why when wires are confined in a pipe, grouped together, there are de-rating factors, you upsize the conductors to account for the lack of the ability to dissipate heat. In this case, the wire is presumed to be in open air, at ambient temperature, dissipating the heat like it is rated to do.

Do you actually run the branch circuit wires for these boats at work, or do you just build circuit boards and components? I ask because you weren't even familiar with the term "ampacity" 2 days ago, a term that is common vocabulary for most people who concern themselves with appropriate conductor sizes, and it didn't even occur to you that wires with different strand counts would have different diameters, which is irrelevant to AWG sizes anyway.

You also mentioned that I couldn't run 2400 watts on #12 for more than 33'. I have temporary power wiring on my job right now maxing out 20 amp/120v circuits for extended periods running well over 100'(that's 200' if you're counting the return). I know there is significant inefficiency, but the lights are on, tools are working, and nothing bad happens. I also have 15 runs of (5) 400w metal halide lights on 120v, well over 100' to the first, and another 100' to the last, that's 2000 watts, they have been lit on 3 huge floors for 6 months, the wire looks brand new........

Thanks for making me do the research to back up what I thought I knew though. I will point out one mistake I made that maybe nobody picked up on, now that you bring up heat resistance. When I gave the example of your toaster actually being hooked up to 25vdc, I calculated the resistance based on the current at 110v, and used that at 25vdc, but this would not be the case. The toaster,(much like you're describing the wire) may be somewhat of a non linear load. It will probably read as a dead short if you put an ohmmeter on it, the resistance is created when the element heats up after the voltage is applied, so I think the resistance would change at a different voltage.



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[drwayne]

.

I don't like to be the bad guy but your wrong on the skin effect. I was assuming you would realize the discrepancy in solid 10g wire vs multi strand. It has nothing to do with the space between the strands, skin effect is rather specific to size and frequency. It's the relationship of a signal and percentage of it through the cross section of a conductor. Some specific uses would be a cardboard tube clad in copper would carry a better signal than any multi strand wire at a very specific diameter and frequency. And a lot of times even engineers think multi strand wire is more conductive of electrons (as we are talking of current not signal pwk) than solid conductor. It's one of those real world applications and manufacturing curves that has to be acknowledged. It is impractical to use solid wire by means of installation and by non conforming, non flexible. If solid is bent that location will not be as conductive, kinda like when you bend a tin can it becomes weaker at that stress point. So to keep it relevant the more skin= more transfer is just not the case. Think of it like these battery to motor electrons are rather stupid, they don't know to assemble into anything resembling a sine, just do a flat out sprint to the finish.

Correct.
It is based upon total surface area .. ergo your 'cardboard tube surrounded by copper' has larger surface area.
Read my words again.
Thanks