# Coil amp rating for magnet wire

Discussion in 'General Electronics Chat' started by shortbus, Jun 20, 2012.

1. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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I'm in the process of building a DC electric motor(a SRM). I haven't been able to find information on selecting a wire gage for a given amperage in a coil. All of the ampacity charts are for wire in 'free air'. I know that the only source of heat from a DC coil is from the resistance of the coil/wire.

When you look at the windings in a motor or transformer, the gage of wire doesn't match the amp rating of the 'free air rating'. It is usually a smaller gage than the 'free air' would be. Can anyone point me to either literature or a chart that shows wire gages for coils?

2. ### #12 Expert

Nov 30, 2010
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I spent an hour on this and got squat. I can say that some power line frequency transformers use 100 circular mils per amp, but I can't prove it.

3. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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Thanks, I've spent many hours and got the same. Went to see a guy that rebuilds alternators and starter motors for cars and trucks to ask him and he just buys new stators or field coils. There has to be a chart some where. Theres one for every thing else.

4. ### WBahn Moderator

Mar 31, 2012
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I only spent a couple minutes, so was only able to come up with this:

http://en.wikipedia.org/wiki/Magnet_wire

Which gives a practical rule of thumb of 2.5A/mm^2 when isolated from free air (compared to 6A/mm^2 in free air).

I know I have seen standard wire tables that gave different ampacities depending on whether it was in free air, in a cable, in a plenum, and a few other instances. Those would probably give some useful guidance as well.

5. ### strantor AAC Fanatic!

Oct 3, 2010
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I think the magic numbers you're looking for are going to be the result of a bunch of equations having to do with wire gauge, space between wires, number of wires stacked on top of each other, amps, time, thermal resistance of the insulation, thermal resistance of the armature, thermal resistance of the motor housing, thermal resistance of air, surface area, air flow, and safety factors.

If I were the one building the motor, I would probably go to the ampacity table and pick the next AWG above what's recommended for my intended max current (or below, err.. ex. table says use 12awg, I would use 10awg) and call it good. Unless I'm reading the chart wrong, as the ambient temp goes up, the amp carrying capacity of the wire goes up, so I don't see the point in doing all the math when the safety factor is already on my side. I've never built a motor so take that for what it's worth.

6. ### WBahn Moderator

Mar 31, 2012
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Ask youself if that really makes sense.

I'm pretty sure what you are looking at is not the ambient temperature, but the maximum permissible wire temperature. The hotter you can let the wire get, the more current you can pump down it before reaching that temperature.

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7. ### strantor AAC Fanatic!

Oct 3, 2010
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DOH. Yeah it seemed a little counterintuitive but I looked at the chart 3X and came to the same conclusion each time. You could tell I was still a little unsure of it ("unless I'm reading the chart wrong"). I see now, it says at the top of the chart in fine print the ambient temp is 30C.
So for example 10AWG @ 30C, if it carres 47A the temp will be 80C. If it carries 75A, temp will be 200C.
Thanks for the correction.

Oh, yeah and then there's the correction factor chart for higher ambient temps. I look like a real genius now.

8. ### WBahn Moderator

Mar 31, 2012
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No, just like someone that hasn't dealt too much with the way information like this is often presented. That's how you learn.

These data tables frequently flood you with information and there is a very strong, natural tendency to look for the one piece of information that is of interest to you. Unfortunately, there are often tables are charts that look like how you would present that data if that was all that was of interest, but they are presenting data in a much broader context. So it's easy to get fooled. It is a very educational exercise to take a data sheet and really try to comprehend what every figure or table if telling you. You will probably still have some holes when you are done, but you will also probably be in a much better position to use the information there and to more quickly understand other datasheets down the road.

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9. ### #12 Expert

Nov 30, 2010
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2.5A/mm^2 calculates to 789.4 circular mils per amp.
That makes it look like my memory was off by a power of 10.
1000 circular mils per amp must be the number I was using in power transformers 35 years ago.

You are allowed to check my math!

10. ### strantor AAC Fanatic!

Oct 3, 2010
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that's what I get.
(2.5A/1mm^2) = (1A/.4mm^2)
(.4mm^2)(1973.5CMA) = 789.4CMA

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11. ### WBahn Moderator

Mar 31, 2012
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I also get 789.4 circular mils per amp, which would seem to compare quite reasonably with a rule of thumb of 1000 circular mils per amp. You would want to round up (to be conservative) and use an easy number if possible.

I can see the rules of thumb for power transformers being different than for magnet wire because of different assumed environments. But I can't convince myself whether I would expect the number to be larger or smaller, let along where something approaching an order of magnitude is plausible. Then again, we don't know how much of a safety factor that 2.5A/mm^2 number takes into account, so the nominal limit might be much closer to your memory than it appears on the surface.

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12. ### #12 Expert

Nov 30, 2010
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Just on gut instinct, a motor has a lot more internal air movement than a power transformer. That would make 789 seem like a better number than 1000.

and remember, it's only my opinion and sometimes I'm wrong.

13. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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Thank you to everyone for your help! Special thanks to #12 for the term "circular mills per amp", using that I came up with some results!

This chart - http://amasci.com/tesla/wire1.html gives both the standard and maximum rating for wire in a coil. Best information I've found so far.

This reprint of a transformer winding book also gave good in site - http://www.vias.org/eltransformers/lee_electronic_transformers_03_04.html

And where would we be with out a Wiki book. Although I never found this until the search for 'circular mills per amp'. - http://en.wikibooks.org/wiki/Electronics/Transformer_Design

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14. ### amilton542 Active Member

Nov 13, 2010
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If I were you, I'd conduct a few tests with your chosen wire gauge. I'd find the maximum current a single length in open air could tolerate then I'd build an identical 'dummy' coil and repeat the same process.

The current ampacity the wire gauge could cope with in free air is what the coil can be overloaded to for a very brief period (2 seconds at the most) then you must kill the current to the value your tests gave you for the 'dummy' coil.

15. ### #12 Expert

Nov 30, 2010
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I have that chart of wire gauges, diameters, circular mils and ohms per thousand feet nailed to the wall in my shop. A wire manufacturer provided a 2 foot by 3 foot chart for the power supply factory I worked at in 1975. When the factory was sold, I stole the chart. Too cool!

ps, This is an excellent example of why a forum works. I didn't think my post#2 had any value, but the fact that I used a different phrase sent shortbus searching in a different direction, and it worked. Also cool

Last edited: Jun 22, 2012
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