Help understanding wire current carrying capacity

Posted by: @inq

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Based on the description, I'm assuming I have to use the former, but I'd like to understand the difference between these two use cases.  I don't expect the 3 amp maximum to occur often or for that long.  I expect the normal load to be under 1 amp.  I also plan on using a higher voltage (32V vs 12V) that should even further reduce my current draw, but I can't confirm that at the moment.  

Perhaps it relates to the power function P=I^2*R. You'd expect that R would be proportional to length (i.e. 1 ohm/decimetre).

Comparing short runs (like inside a small device) to transmission lines (long stretches) the same current would have a very large value and might generate enough heat to damage the wire.

Just a WAG.

@inq Look above the table at the section marked

Load Carrying Capacities (see table below)

and you will see

The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative.

If your max current is 3A the use size 16.

@inq Did you also use the calculator at the bottom? I tried with 12VDC 10ft 3A and get a 0.996V drop, is that 8.3% drop acceptable?

Hi Inq,

  Its too late in the day for me to look at this in detail, but I'll throw you some thoughts that you might like to comment on.

When specifying a wire to carry a current, you need to look at it from more than 1 angle. Basically you have more than 1 'master' to satisfy.

• The copper shouldn't melt
• The insulation shouldn't melt or deterioriate
• The wire shouldn't overheat what it is close to
• The voltage drop shouldn't be so large that the destination circuit is inadequately powered
• The cooling of the wire, which will determine the temperature it reaches, will depend on the environment surrounding the wire, including the ambient temperature and the ability of the surrounding surface to conduct the heat away

This list isn't comprehensive .. just a few random thoughts ... but hopefully you can see that there are several issues, but some will be more relevant than others.

e.g. copper not melting is usually the condition that applies to wire in a fuse ...

If the wire is insulated with a plastic, then this is likely to be the more important case, as plastic melts or decomposes at lower temperatures than the copper melts. If it is a 'plastic' insulation, then it depends which plastic. PVC starts to soften at relatively low temperatures .. I need to check the exact temperature, but above about 80C(?), then the copper wire starts to migrate through the plastic, possibly leading to a short after a period of time. By contrast, I would expect silicone insulation to withstand a much higher temperature.

If a wire is clipped to a cool wall that is reasonably good at conducting all the heat away, then this wll keep the wire temperature much lower than alternatives like free space or clipped to heat insulator.

Cables with two or more cores are also much more heat insulated, assuming all of the wires are carrying current. They may also have a second layer of plastic, further reducing the heat dissipation.

Very short wires can lose heat to their terminals ... again I forget exact numbers, and it depends on the wire size, but this is only really helpful for wires upi to few centimtres.

And the resistance of the wire means the voltage at the detination will be lower than at the input. Is this voltage drop acceptable? For longer wires, this can be more important than the wire getting too hot!

By the way, copper resistance is also quite temperature dependent .. so if the copper is hot, its resistance will be higher, and the voltage drops and temperatures will be higher still.

Sometimes a voltage regulator at the destination can enable you to increase the voltage at the source end, without applying too high a voltage to the load, and accept a higher voltage drop in the middle.

Sorry this is a quick ramble, but I hope it helps you to consider some issues, but it is not an answer ... I'll try to rethink it later. If you send any further description of the source, load, wire environment, wire type, etc. in the meantime, I'll take them into account in my second answer.

Best wishes, Dave

@davee @Inq To add to Dave's suggestions, read the wire chart, especially the 700mill rule, and use the calculator at the end.

The only disagreement I have with Dave (I think) is that fusing specifically is done to prevent the insulation from melting. That is a long way from the copper melting.

Maybe Dennis can share the application, I hope it's not solar-related, 22AWG is way too small for 10 ft, maybe 10AWG if this is the solar panel down feed.

Ron @zander,

  re: Dave (I think) is that fusing specifically is done to prevent the insulation from melting. 

Fuses are indeed used to break the circuit when the current is so dangerously high it risks insulation damage.

But that is not what I meant ... if you look in the inside of a simple glass fuse, you will see a wire, and for fuses of roughly 2A to 15A, this wire is probably copper (with some sort of plating to reduce corrosion).

The fuse acts when a part of the wire reaches at least melting point, and 'falls apart', breaking the circuit.

If the overload is 'modest', the middle of the wire will be furthest from cooling effect of the terminals, and hence will get the hottest, and will be the point of failure.

If the overload is 'massive' .. (say) a short circuit load .. then the whole wire heats up so rapidly it vapourises, and then condenses on the inside of the cooler glass.

Hence, some tables will give a 'fusing current' ..  you need to carefully check the context, but it can be the approximate current that causes the copper to literally melt. (I remember seeing this in a table on the net some years ago.)

Of course, the same or similar phrase to 'fusing current' might be the recommended fuse to protect that wire. However, as I mentioned in my note to @Inq above, the current a wire can carry is markedly affected by how and where the cable is mounted, so a table should normally show more than 1 rating for the same cable.

I wouldn't be surprised if you found examples of both meanings on the net... so check the context!!!

Best wishes, Dave

@will, @zander, @davee,

Thank you all.  I didn't want to bore everyone to death by my giving a long drawn out use case explanation and was mainly curious about the huge difference in current capability of the two columns of the chart.  The title and verbiage didn't quite click for me.  With zero EE background... well EE 101 they force all engineers to take... I figured this was some kind of national/international standard AWG, ASTM, ASME... type thing and someone here would know.  I'm sure there is some practical basis behind it per your all's comments, but I was mainly curious about the specification difference (22AWG  0.92A and 7.0A).  You have to admit, you can fit the Grand-Canyon in-between these!  https://www.powerstream.com/Wire_Size.htm

The Why

I want to build a bot to study AI... both using ANN and GA.  I've been following @thrandell thread on his progress.  He's got one actually working!!! https://forum.dronebotworkshop.com/neural-networks/genetic-evolution-of-a-neural-network-driven-robot/   Inqster is supposed to be using AI... it's primary purpose.  But, I had a D.A. moment that dah... it needs to run for hours... maybe days so it can learn.  Batteries to run such a big bot up and down the yard learning, running into creeks, off embankments, hitting trees at 20 clicks... is not showing signs of sanity. 

Anyhow, I need to build a more conservative (sane) bot (yet to be named or posted) that will be powered by an umbilical power cord.  I'll still use the WiFi for data transfer, so it will solely be a two wire deal.  So:

• I want it as thin as possible so the little light weight bot won't be affected by the inertial of the wire hanging from say... the ceiling.
• In my other diversion from the forum's norm, I want to use stepper motors.  The dirt-cheap 28BYJ-48 won't cut it (only 40 mm/sec) so I'll turn to the smallest Nema-17 I have.  They handle up to 1.5A... thus the 3A max for the bot.
• As they only draw that much under full acceleration, high-speed, I don't expect them to reach that current often or long.  
• I will start out with A4988 drivers, but they have proven to be under underperforming when driven with 3.3V logic (in a couple of other threads).  They only seem to go up to about 0.6A.  I also saw this manifest in Inqling Jr.  I'll be using some DRV8825 which are supposedly good up to 2.2A.
• I'll use a buck-converter on the bot to handle the voltage step-down for the ESP8266.
• The DRV8825 and steppers don't really care what the voltage is (between say 8 and 45V) but all I have is a bench power supply good up to 32VDC.  I'll nominally use this to reduce the current going over the wires.  
• The DRV8825 and buck converter doesn't really care what voltage it gets, so any voltage drop doesn't matter.  It is... what it is.  I'll live with whatever I get on the other end.
• I'll be evaluating this as soon as the drivers get in, so I'll have more concrete numbers, but I expect it to be higher than the 0.92A of one column and way lower than the 7A of the other column.
• I think @thrandell is getting away with AWG30 although his bot isn't as power hungry as mine will be.

See... I didn't want to bore you with the flotsam and jetsam to what I thought was just a standards question.  😉 😊  Again... thanks for responding.

VBR,

Inq

@davee Yes, and if you really want to get down in the weeds, there is, as I am sure you know, a time element. At the store, you will often find 'slo-blo' fuses. These fuses can take 10A for a few seconds or msecs but will blow if it's 1A for 3 seconds.

EDIT: I wanted to say that when designing a circuit that needs a fuse, the fuse selection needs to consider normal amperage, peak amperage, insulation temperature spec, and a lot of experience. Sometimes I use what I call a catastrophe fuse, and sometimes I use a fuse closer to whatever the normal draw times, some 'magic number' like 150%.

A lot of 'gut feel' is common practice.

Posted by: @inq

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I need to send DC power over a length of wire.  Just for round numbers, it shouldn't be over 10 feet long and no more than 3 amps.  I'm looking at this wiring chart https://www.powerstream.com/Wire_Size.htm .  I can figure out what voltage drop by the resistance of a wire candidate.  What I'm having trouble with is maximum amps.  There are two columns.  One say Maximum amps for power transmission (0.92A for 22AWG) which is a LOT lower than Maximum amps for chassis wiring (7A for 22AWG).  Based on the description, I'm assuming I have to use the former, but I'd like to understand the difference between these two use cases.  I don't expect the 3 amp maximum to occur often or for that long.  I expect the normal load to be under 1 amp.  I also plan on using a higher voltage (32V vs 12V) that should even further reduce my current draw, but I can't confirm that at the moment.  

I appreciate any help you might can shed on this.

VBR,

Inq

@Ing, I found this definition for Max Amps chassis vs Amps transmission:

"Chassis wiring assumes each wire is routed separately, and power wiring assumes they are wired in a bundle. In chassis wiring the cooling of the conductors is better, because they are all exposed directly to air. In a bundle, some of the wires are not in direct contact withe the air.This is why maximum current for chassis wiring will be more compared to an equivalent power transmission"

source: https://electronics.stackexchange.com/questions/338950/difference-between-maximum-amps-for-chassis-wiring-and-power-transmission

Hope this answers your question..

regards,

LouisR

Posted by: @inst-tech

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"Chassis wiring assumes each wire is routed separately, and power wiring assumes they are wired in a bundle. In chassis wiring the cooling of the conductors is better, because they are all exposed directly to air. In a bundle, some of the wires are not in direct contact withe the air.This is why maximum current for chassis wiring will be more compared to an equivalent power transmission"

source:> https://electronics.stackexchange.com/questions/338950/difference-between-maximum-amps-for-chassis-wiring-and-power-transmission <

Hope this answers your question..

regards,

LouisR

I see the title of that post was the exact wording.  I used general terms in my search and got everything from river water flow to high-tension power line limits.  😆 I should have my question in quotes!

That is what I was looking for!

Although what I want to use is simple two lead like thin lamp-chord, it sounds like because I'll be draping it through the air, I can use something closer to the chassis allowable than the bundled power transmission even though power transmission is what I'm actually doing.

Of course, I'll check it out under extreme conditions to verify.

Thank!

VBR,

Inq

Hi Ron @zander,

  Thanks for your contribution on fuses. I agree with your argument, albeit the part in the second sentence of "few seconds or msecs",  is somewhat ambiguous.

The total project will inevitably also benefit from some means to current limiting in the event of an overload, which could involve a fuse.

I was trying to explain that whilst the current carrying capacity of wire is obviously dependent on the wire size (cross-sectional copper area), it also depends on number of other factors, such as insulation and ability to transfer heat into the environment. I was using the 'simple' straight wire in a glass tube fuse as an extreme example of a wire carrying a current, and the effect of exceeding the 'safe' limit.

Best wishes and take care my friend, Dave

@davee My bad. I was trying to demonstrate the very wide range of these devices. Some slow blow fuses are designed to take an overcurrent for a few milliseconds, some for a few seconds.

Hi @inq,

   Part deux ... hopefully from my first effort, plus the other contributions, you now realise that with power electronics, the wire size (strictly cross-sectional area) is only one factor in the safe current carrying capacity.

It is also necessary to consider factors including, the ability of the conductor to lose its heat to the environment, the maximum temperature the conductor can reach without secondary problems like insulation deteriorating, the allowable voltage drop and the weather!

Of course there are tables of required wire sizes for specific circumstances ... e.g. in the UK this applies to fixed wiring in buildings, etc. and I am sure there are similar tables on your side of the Atlantic, albeit couched in archaic units of measure. 😉 

Hence, whilst I have not checked the numbers you quote, and a variation of 10:1, is surprisingly high but not implausible.

Plus, you will realise that some of the factors are very much dependent on the indivdual circumstances, so this will a case of taking an initial wag, and then checking it out in practice.

I am also curious as to whether you think it is necessary to take any precautions to prevent the wire becoming entangled like spaghetti or a "trip/strangulation" hazard to your robot.

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I'll start by summarising the known data, and making assumptions. The assumptions may be wrong, but hopefully the framework of the discussion can be upcycled with modified values later.

• Wire length 10' / 3m
• Transmission voltage 32V (from PSU)
• Load : 2 Nema stepper motors + electronics
• Load power control/conversion : buck converter for 3.3V; 2 * DRV8825 for stepper motors
  • I'll assume buck converter power consumption, including load will be (less than) 2W

Power/current consumption of motors is more tricky.

Based on Pololu website data, https://www.pololu.com/product/2133

The DRV8825 driver IC has a maximum current rating of 2.5 A per coil, but the current sense resistors further limit the maximum current to 2.2 A, and the actual current you can deliver depends on how well you can keep the IC cool. The carrier’s printed circuit board is designed to draw heat out of the IC, but to supply more than approximately 1.5 A per coil, a heat sink or other cooling method is required.

Thus, assuming you are using the sub-matchbox style boards from or cloned from Pololu, the maximum per coil current is 1.5A, if you want to minimise the risk of magic smoke.

Also we might expect to take effect of micro-stepping into account, but this is not an issue, due the mechanism deployed. This is an example spread sheet.

The resistance of the path through the DRV8825 board to the motor is assumed to be 0.5 Ohms, for each coil, and the coil resistance is assumed to be 5 Ohms. These are fictitious values, but the story is the same for any real values.

 The coil current has been set at 1.5 A per coil maximum.

When the DRV8825 is set to a full step position, 1 coil has 100% current, and the other coil 0% current.

The left hand group headed, Full Step, shows the current, power dissipation in the DRV chip and power dissipation for the motor coil, initially for coil A, then coil B, and then the sums for coil A and coil B.

The right hand group is the equivalent data for same DRV8825 chip and board current setting, and the same motor, but when the motor is being held at a Quarter Step position.

Note that for the Quarter Step position, the current for each coil is 1.061 A, making a total of of 2.121 A passing through the DRV8825 chip and the motor, about 41% more than for the Full Step position, but the total power dissipation values in both the DRV8255 and the motor coils are identical!!

Whilst this makes mathematical sense, to me, it is intuitively, really difficult to handle .. but nevertheless, I think it is correct!

I have included a copy of the spreadsheet so you can change the motor coil resistance to the value of your motor. The resistance of the DRV8825 board is also fictional, but at a rough guess, I might expect a board of that size to be able to dissipate 1 Watt, so hopefully it is close enough for the next step.

-------------------

Summarising, so far, the cable must supply 2 Watts for the buck converter and microcontroller, and two times the DRV8255 board and motor power (assuming two motors), which (as shown in the yellow shaded part of the table) sums to 26.75 Watts.

Assuming the cable is powered at 32 V, then 26.75 W would correspond to a current of  26.75 (W)/ 32 (V), which is about 0.84 Amps.

So let's assume average current through wire is 1 Amp.

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I think this is a good time for me to take a break ... Part trois, hopefully considering real wire, later!

Please feel free to post comments, corrections, questions etc.

Dave

@davee 

😳 You crazy guy you!  I remember the trig proportioning from our train elevator dealings.  I don't really think I'll be drag racing this or trying to do top-end speed runs, so I think the 1.5A per coil that the steppers can take will never be realized.  I was just using those for the baseline till I actually have these in hand and do the some testing.  The DRV8825 will have the heat-sync, but no forced induction, I'll adjust them for a maximum of 1.5A/coil.  They will be in free air, so I don't think I'll ever hit the 2.2A even with worse case acceleration and worse case micro-stepping.  

My custom bi-motor software driver does use micro-stepping and switches over to standard stepping like a transmission so I'll be hitting both coils at the same time. 

Although I wasn't really expecting to actually doing Math to see if everything was in bounds... You know my philosophy... "Buy in bulk, save some money and let the smoke be free at will."

Buck converter usage.  I think worse case usage of an ESP8266 draws about 250mA (at 3.3V) during boot up, but the steppers will never be running.  During runtime, it falls to about 70mA.

The main part, I can really do some Math on is this:  The motor/motor-driver combo doesn't really work on voltage.  It monitors current and PWM's whatever the supply voltage is to get up to that current.  So... although I know I can run the supply voltage anywhere from below 10V and upto the limits of the driver (I believe I saw 45V, but since I don't have above 32V, we'll say, more than I've got).  I understand how using a higher supply voltage should allow the transmission wire to carry less current, I don't really see how to calculate the current the bot's going to pull... even knowing that it will run up to 1.5A, I don't know what effective PWM voltage that might be (or watts) to back calculate the what current the trans wires at 32V will be pulling.  Am I make sense or just rambling?