Ampacity Question

• posted

Happy New Year everyone.

I've got all these wire tables and the numbers are not in agreement... apparently. I'll provide more data if needed.

NEC defines ampacity as the maximum current a conductor can carry continuously... I'm wondering who else may have redefined it :)

Copper wire, BTW, and everything is in Amps

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*.htm: Wire Parameter Calculator 32 AWG Single Wire Ampacity: 0.847633078162764

12 AWG Single Wire Ampacity: 23.05743240716959

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---------- wire table.htm:

32 AWG Max Amps: .0213 Ampacity: 164

32 AWG Max Amps: 2.1700 Ampacity: 23 **

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** I can beleive 23A since 12 AWG goes with a 20A circuit. That's about 85% derating. The other ones look right too. It's when you get below 18 AWG that the table goes to hell.

The Ampacity column of the table looks like a concave curve with a minimum of 8.1A for 18 AWG. Extremes are 312 for 4/0 and 1049 for

40 AWG and it doesn't look like a decimal place foul up.

Please tell me why those 32 AWG ampacities are different and if you see any other glaring probs.

I'm going to use some thin stuff between 20 and 32 AWG for a solenoid (the actuator kind) and want to know which table(s) / calc to trust. It will be energized for only 1 second at a time at a max Duty Cycle of 10% (each 10 seconds) I'm going max 1800 Ampere-turns and I haven't got the rest cause I haven't picked the wire yet.

I don't want to fuse the coil :)

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*.htm: 32 AWG Fusing Amps: 7.19

12 AWG Fusing Amps: 235

that I can understand and beleive. Can you?

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----------- Then I've got this other table. I'll post the post :) it came from:

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---------------------------------------------------------------- awg.txt:

32 AWG NormalAmps: 0.1264 MaxAmps: 0.1441

12 AWG NormalAmps: 13.060 MaxAmps: 14.892

but NEC defines ampacity as max...

Normal and Max? is that like saying very orange or not quite apple?

Max to get a warm, fuzzy feeling?

I see the defs here, now I know why I saved this table. Would you all agree with this table? I mean, does it look correct? :)

The guy was posting about building a bobbin... He meant a coil on a bobbin. Here's the answer he got. I hope this posts ok for you. It's wrapped but looks readable here.

------------------------------------------------------------- complete awg.txt:

AWG Wire Table for BARE COPPER Wire Compiled by a program written by Fr. Tom McGahee

Compiled by Fr. Tom McGahee tom_mcgahee@*****.com

AWG = American Wire Gauge size Dia-mils = Diameter in mils (1 mil = .001 inch) TPI = Turns Per Inch (Ignoring thickness of unknown insulation) Dia-mm = Diameter in millimeters (For comparison with non-USA coilers) Circ-mils = Circular Mils. (circular mils = diameter in mils squared) Ohms/Kft = Ohms Per 1,000 Feet Ft/Ohm = Feet Per Ohm Ft/Lb = Feet Per Pound Ohms/Lb = Ohms Per Pound Lb/Kft = Pounds Per 1,000 Feet NormAmps = Normal Average Amp Capacity based on 500 circular mils per Amp MaxAmps = Maximum recommended Average Amp Capacity in Open Air based on 438.489 circular mils per Amp

Actual Amp capacity of a wire depends on form factor and method of cooling! MaxAmps assumes free flow of air around wire. Do NOT exceed this maximum without cooling! Wire wrapped in a coil or without any form of cooling may over-heat at MaxAmps! Many factors govern the ACTUAL Max Amps you can pass through a wire continuously. Be careful!

AWG Dia-mils TPI Dia-mm Circ-mils Ohms/Kft Ft/Ohm Ft/Lb Ohms/Lb Lb/Kft NormAmps MaxAmps

0000 459.99 2.1740 11.684 211592 0.0490 20402 1.5613 0.0001 640.48 423.18 482.55 000 409.63 2.4412 10.405 167800 0.0618 16180 1.9688 0.0001 507.93 335.60 382.68 00 364.79 2.7413 9.2657 133072 0.0779 12831 2.4826 0.0002 402.80 266.14 303.48

AWG Dia-mils TPI Dia-mm Circ-mils Ohms/Kft Ft/Ohm Ft/Lb Ohms/Lb Lb/Kft NormAmps MaxAmps

0 324.85 3.0783 8.2513 105531 0.0983 10175 3.1305 0.0003 319.44 211.06 240.67 1 289.29 3.4567 7.3480 83690 0.1239 8069.5 3.9475 0.0005 253.33 167.38 190.86 2 257.62 3.8817 6.5436 66369 0.1563 6399.4 4.9777 0.0008 200.90 132.74 151.36 3 229.42 4.3588 5.8272 52633 0.1970 5075.0 6.2767 0.0012 159.32 105.27 120.03 4 204.30 4.8947 5.1893 41740 0.2485 4024.7 7.9148 0.0020 126.35 83.480 95.190 5 181.94 5.4964 4.6212 33101 0.3133 3191.7 9.9804 0.0031 100.20 66.203 75.489 6 162.02 6.1721 4.1153 26251 0.3951 2531.1 12.585 0.0050 79.460 52.501 59.866 7 144.28 6.9308 3.6648 20818 0.4982 2007.3 15.869 0.0079 63.014 41.635 47.476 8 128.49 7.7828 3.2636 16509 0.6282 1591.8 20.011 0.0126 49.973 33.018 37.650 9 114.42 8.7396 2.9063 13092 0.7921 1262.4 25.233 0.0200 39.630 26.185 29.858

AWG Dia-mils TPI Dia-mm Circ-mils Ohms/Kft Ft/Ohm Ft/Lb Ohms/Lb Lb/Kft NormAmps MaxAmps

10 101.90 9.8140 2.5881 10383 0.9989 1001.1 31.819 0.0318 31.428 20.765 23.678 11 90.741 11.020 2.3048 8233.9 1.2596 793.93 40.122 0.0505 24.924 16.468 18.778 12 80.807 12.375 2.0525 6529.8 1.5883 629.61 50.593 0.0804 19.765 13.060 14.892 13 71.961 13.896 1.8278 5178.3 2.0028 499.31 63.797 0.1278 15.675 10.357 11.810 14 64.083 15.605 1.6277 4106.6 2.5255 395.97 80.447 0.2031 12.431 8.2132 9.3654 15 57.067 17.523 1.4495 3256.7 3.1845 314.02 101.44 0.3230 9.8579 6.5134 7.4271 16 50.820 19.677 1.2908 2582.7 4.0156 249.03 127.91 0.5136 7.8177 5.1654 5.8900 17 45.257 22.096 1.1495 2048.2 5.0636 197.49 161.30 0.8167 6.1997 4.0963 4.6709 18 40.302 24.813 1.0237 1624.3 6.3851 156.62 203.39 1.2986 4.9166 3.2485 3.7042 19 35.890 27.863 0.9116 1288.1 8.0514 124.20 256.47 2.0648 3.8991 2.5762 2.9376

AWG Dia-mils TPI Dia-mm Circ-mils Ohms/Kft Ft/Ohm Ft/Lb Ohms/Lb Lb/Kft NormAmps MaxAmps

20 31.961 31.288 0.8118 1021.5 10.153 98.496 323.41 3.2832 3.0921 2.0430 2.3296 21 28.462 35.134 0.7229 810.10 12.802 78.111 407.81 5.2205 2.4521 1.6202 1.8475 22 25.346 39.453 0.6438 642.44 16.143 61.945 514.23 8.3009 1.9446 1.2849 1.4651 23 22.572 44.304 0.5733 509.48 20.356 49.125 648.44 13.199 1.5422 1.0190 1.1619 24 20.101 49.750 0.5106 404.03 25.669 38.958 817.66 20.987 1.2230 0.8081 0.9214 25 17.900 55.866 0.4547 320.41 32.368 30.895 1031.1 33.371 0.9699 0.6408 0.7307 26 15.940 62.733 0.4049 254.10 40.815 24.501 1300.1 53.061 0.7692 0.5082 0.5795 27 14.195 70.445 0.3606 201.51 51.467 19.430 1639.4 84.371 0.6100 0.4030 0.4596 28 12.641 79.105 0.3211 159.80 64.898 15.409 2067.3 134.15 0.4837 0.3196 0.3644 29 11.257 88.830 0.2859 126.73 81.835 12.220 2606.8 213.31 0.3836 0.2535 0.2890

AWG Dia-mils TPI Dia-mm Circ-mils Ohms/Kft Ft/Ohm Ft/Lb Ohms/Lb Lb/Kft NormAmps MaxAmps

30 10.025 99.750 0.2546 100.50 103.19 9.6906 3287.1 339.18 0.3042 0.2010 0.2292 31 8.9276 112.01 0.2268 79.702 130.12 7.6850 4145.0 539.32 0.2413 0.1594 0.1818 32 7.9503 125.78 0.2019 63.207 164.08 6.0945 5226.7 857.55 0.1913 0.1264 0.1441 33 7.0799 141.24 0.1798 50.125 206.90 4.8332 6590.8 1363.6 0.1517 0.1003 0.1143 34 6.3048 158.61 0.1601 39.751 260.90 3.8329 8310.8 2168.1 0.1203 0.0795 0.0907 35 5.6146 178.11 0.1426 31.524 328.99 3.0396 10480 3447.5 0.0954 0.0630 0.0719 36 5.0000 200.00 0.1270 25.000 414.85 2.4105 13215 5481.7 0.0757 0.0500 0.0570 37 4.4526 224.59 0.1131 19.826 523.11 1.9116 16663 8716.2 0.0600 0.0397 0.0452 38 3.9652 252.20 0.1007 15.723 659.63 1.5160 21012 13859 0.0476 0.0314 0.0359 39 3.5311 283.20 0.0897 12.469 831.78 1.2022 26496 22037 0.0377 0.0249 0.0284

AWG Dia-mils TPI Dia-mm Circ-mils Ohms/Kft Ft/Ohm Ft/Lb Ohms/Lb Lb/Kft NormAmps MaxAmps

40 3.1445 318.01 0.0799 9.8880 1048.9 0.9534 33410 35040 0.0299 0.0198 0.0226
• posted

On Thu, 01 Jan 2004 15:08:11 GMT, Active8 Gave us:

The ratings are defined such that they describe the maximum a wire can handle without an appreciable temperature rise in the wire, and are to keep wires from getting so hot that their insulation gets compromised, thereby allowing for shorts in power wiring situations.

Transformers have high temp mag wire insulations on solid wire. The ratings for mag wire account for the environment which it encounters. It may or should be the same though as I believe the NEC rating is for bare wire temp rise. It is also de-rated for elevated ambient temperature environments. It is ll about stability and safety.

The ratings for the NEC are based on the resistance of copper, and keep temperature rise below a certain factor at all times, regardless of the insulation which was utilized in the wire manufacture in question. That keeps all electrical power installations safe from fire.

The NFPA was the original founder of the NEC.

• posted

On Thu, 01 Jan 2004 15:08:11 GMT, Active8 Gave us:

A single, 26Ga mag wire can handle an amp. Anything more will get hot.

Don't design anything using any numbers even close to fusing amps.

Normal amps is more like it.

• posted

Given certain ambient conditions, installed in certain configurations. Don't try to stretch the NEC ampacity figures to cover situations for which they were not intended.

Many people have, to suit their particular application. [snip]

This is beyond the scope of the NEC ampacity tables.

You are going to have to calculate the I^2R heat generated by that coil and then do a thermal analysis to ensure that this heat can be conducted out of the coil to the outside ambient without the hottest spot in the coil's interior exceeding the maximum ratings of the coil's materials (conductors, insulation, etc.). This is not a trivial problem, although there may some good analysis programs one could plug numbers into. You'll need to know the coil and surrounding devices dimensions and thermal properties of all the materials used in its construction.

• posted

Yup. More than anything, I was hoping to find out why that one particular ampacity figure for the 32 AWG was so high compared to the fractional amp from the calculator.

If I can't find a free app (hints?) do you think I could use the Neher-McGrath Calculation if I integrate small segments or better yet, thin cylinders? That's the closest I've come to a possible way to attack the prob. I anticipated this :) I mean :( or :( and :) ;)

The DC will be so low, and the overall use so infrequent, there might not be much cumulative thermal effect - \$20 phrase for heat rise. ;)

• posted

Ampacity is determined by the Fourier Heat transfer equation in this form: TC-TA = I*I*(RDC)*RCA Where TC is maximum allowable conductor operating temperature TA is ambient temperature I is amperes RDC is DC resistance RCA is thermal resistance The tables in the NEC are derived using variations of this equation. An excellent reference is the book, Rating of Electric Power Cables by Dr. George J. Anders available from amazon.com

Have fun!

• posted

I don't know. I didn't see enough in the original post to understand the assumptions being made for the various figures. It didn't look like NEC table stuff, since I don't see wire sizes down below 18 AWG in there.

Something like that might do. The Neher-McGrath Calculations are just a particular interpretation of general heat loss formulas. Take the NEC

310.15(C):

I = sqrt( (Tc - (Ta + dTd))/(Rdc(1 + Yc)Rca) )

and juggle the terms around (eliminating dTd and 1+Yc for the DC case) and you have a simple temp rise across thermal resistance Rca equals the I^2R loss:

I^2 * Rdc = (Tc - Ta)/Rca

You'll have to make this fit the appropriate volume integral. In addition, the thermal resistance above, Rca, is a total figure between the conductor and ambient. Your problem may have to take separate components of the thermal resistance into consideration discretely.

• posted

Thanks for the clues below. This should be cake.

I'll post the thing on A.B.S.E. under "Who wrote this table?"

I typo'd, also:

*.htm: Wire Parameter Calculator 32 AWG Single Wire Ampacity: 0.847633078162764 > You are going to have to calculate the I^2R heat generated by that coil
• posted

The problem with using the handbooks is knowing where the handbooks apply.

NEC (and CSA) electrical code tables are for building wiring, not for coils.

What sort of insulation will be on the wire? What's the maximum temperature rise you can tolerate?

How many coils are you planning on making? Wind the coil, try it out and see how well it performs; at worst you lose a coil, but it may not be worth the time and effort to solve for the temperature rise using more sophisticated methods. If you're really gung-ho you can measure the cold coil resistance, apply your typical operating current for a while and measure the hot resistance, and work out the average coil temperature rise from the change in resistance; this would give you some idea as to the margin of stability of the winding insulation, though again all the lab work may not be worth the trouble if you're only doing a one-off for a single personal project.

Of course if you're designing something that will go into 100,000 washing machines, you'll want to spend more effort at this - but then you'd have more reference material at hand.

The Neher-McGrath calculation is for buried underground cables, not for solenoids, and the application conditions and heat flow problems are quite different than in a winding. Neher-McGrath is essentially a

1-dimensional heat flow calculation and no practical solenoid coil will be a good approximation to that condition. Neher-McGrath isn't even very good for duct banks if they are short compared with their burial depth.

It's all very well and perfectly accurate to give the formula relating temperature rise, current and thermal resistance of the surroundings - but the whole problem is *calculating* the thermal resistance.

The ARRL Handbook gives 700 circular mils per amp as the middle of the range of current density for transformer windings for amateur radio service - this criterion is probably more relevant to solenoid coil design than the building wiring tables. The handbook says 32 AWG will carry 0.090 amps using this criterion. The table does admit that values between 500 and 1000 circular mils per amp are used. With your very low duty cycle you probably can use a higher current density in this range. If I were winding a one-off solenoid for a private experiment with a small budget, this is where I'd start, and work my way out from there.

Bill

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.....

.....

• posted

Yeah, and I'd pilfer ever scrapped solenoid I could get my hands on. I did find one in a junk box, too. It won't work :(

Yes. Now that I think of it, the fact that the coil wires are not in contact over their entire surface area makes this a thermo nightmre though it might serve as a guesstimate if I knew the fudge factor, which would best be found from measured data IMO.

Thanks. Now I can kick myself for not buying my own ARRL Handbook when they were only \$25. But really, Thanks. That is the best guide line I've seen at this point and I don't have to derive a new equation :)

You mentioned a table. What year and pages is all that on? Maybe our local library has it, albeit unlikely.

• posted

relating

surroundings -

I have the 1991 edition of the ARRL Handbook, which did sell for \$25 when it was new. The table I'm looking at is Table 11 in the "component data" chapter, chapter 35, page 35-6. Anyone interested in electronics as a hobby would be well advised to comb the used book stores and hamfests looking for old ARRL Handbooks, or even breaking down and buying one brand new. Though I haven't seen a 21st Century edition - perhaps they've been "improved" past the point of usefulness? ( Now that I think of it the 1991 edition had a switch-mode power supply project in it which talks about winding inductors, and the problems of heat loss in core and coil - not quite the same problem as an actuator solenoid since the frequencies are much higher, but some geometrical similarities.)

Heat flow problems are generally bread and butter items for computer calculation but you're not going to buy and learn a \$10,000 finite element program just to wind one coil. Empirical trial and error still has a place - after all, Edison had no scientific basis to guide him when he was looking for lamp filament materials and tried literally hundreds of different items before he found one that worked.

Another massively useful book for the hobbyist is "Reference Data for Radio Engineers", 5th edition, by ATT. Look for older editions. I recall seeing a much more recent sucessor to this book and thinking that it was not as well suited as older editions were for hobbyist use. I bought mine in 1974 out of my paper-route money, but it's probably the only thing that I still have from that year. A good investment. Gulp. That's 30 years ago! Where *did* the time go?

Bill

• posted

Yeah. Found one in KC (Independance) MO a few years ago.

Fine with me. The one I found wasn't hobby-mart either. At the time, I was quite happy with the author's treatment of multipath propagation. Unfortunately, that library wouldn't lend out reference books, so I never got take it home and make notes, but I got a few photocopies.

Thanks.

I rig the bobbins to a rotating machine and "spray" the wire on :) Kinda sloppy, sometimes. I'll have to slow it down this time. It'll most likely be multiple parallel windings to get the amp-turns in the volume I'm shooting for.

• posted

NEC is the name of NFPA 70, one of the many standards written by NFPA.

Sincerely,

Donald L. Phillips, Jr., P.E. Worthington Engineering, Inc.

145 Greenglade Avenue Worthington, OH 43085-2264

snipped-for-privacy@worthingtonNSengineering.com (remove NS to use the address)

614.937.0463 voice 208.975.1011 fax

• posted

On Fri, 02 Jan 2004 22:39:28 GMT, "Don Phillips" Gave us:

The proper term to use by me would have been author.

• posted

message

Actually, the NFPA was not the "original" founder of the NEC. The National Fire Protection Association has acted as sponsor of the National Electrical Code since 1911. The original Code document was developed in 1897 as a result of the united efforts of various insurance, electrical, architectural, and allied interests.

• posted

A little history:

1896: Representatives from a variety of organizations meet at the New York City headquarters of the American Society of Mechanical Engineers. The committee recognizes that the five existing codes should collectively be used as a basis for a single, comprehensive document. In the first known instance of international harmonization, the group also refers to the German "safety rules for electrical power installations," the code of the British Board of Trade, and the Phoenix Rules of England