Ampacity Question

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.

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.

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.

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. ;)

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!

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

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.

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:

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

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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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.

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?

Good luck with your winding.

Bill

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.

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.

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

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

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

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.

A little history:

On Tue, 6 Jan 2004 15:44:40 -0800, "deanmk" Gave us:

Thank you for the fine reference and snippet.