# Short duration amperage vs. wire gauge

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Hi all,

I'm currently doing some work trying to optimize coils (basically a large center hole solenoid) for a series of experiments.

Two of the variables I'm going to be able to adjust are wire gauge of the coil and amperage through the coil.

Online, I have found 'rule of thumb' amperage limits for continuous use vs. wire gauge. The tests I will be running will not be running for long periods of time (maybe 10-15 minutes tops), and the amperage limits I've looked up online seem to be very limiting.

I'm wondering if anyone knows where I could find equations or rules-of-thumb for short duration loads vs. wire gauge.

We're probably going to have to do some failure tests to be sure, but I'd like to get close before we start winding the coils up (which will be a relatively lengthy process).

## Answers and Replies

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According to my stupidly simple analysis Heating = k . F^2 / A
where F is the field and A is the solenoid cross section area -- i.e guage does not matter.

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Heating is a result of power dissipation - the resistance of the wire and the amperage through the wire. Smaller wires have higher resistances, so their heating rates will be higher.

add

smaller guages allow more turns per area hence more field what are you trying to get??
I doubt that the standards quoted apply to your circumstance you may ( heaven forbid) have to estimate it yourself, my suggestion was a back of the envelope calc i'm not claiming 100% accuracy but it's better than your reply.

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chroot
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Enigma,

10-15 minutes is long enough to be considered continuous duty. The wire will heat up and reach 90% of its nominal operating temperature in probably a minute or two. If I were you, I would just go by the continuous current ratings. They are indeed conservative. You could probably double the ratings without any real danger, but you'll be losing the safety factor that is necessary for, say, UL certification.

- Warren

Cliff_J
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how many circular mils/amp are these rules-of-thumb? I've seen 200mils or 300mils/amp used as acceptable limits, but no mention of margins of safety.

sounds like time for some empirical data? burning up stuff is almost as fun as blowing stuff up or crushing it... And once you get a few windings thick so the heat dissipation into the air isn't much of a factor for the inner windings....

Cliff

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chroot said:
You could probably double the ratings without any real danger, but you'll be losing the safety factor that is necessary for, say, UL certification.
Thanks. That's what I was looking for.

I coded an optimization program. When sizing for minimum power dissipation, the result is a trivial: maximum layers, maximum wire gauge.

I am getting non-trivial results when sizing for minimum solenoid mass assuming that the required field is kicked up fairly high.

rayjohn01 said:
my suggestion was a back of the envelope calc i'm not claiming 100% accuracy but it's better than your reply.
Sorry about that, rayjohn.

I hadn't typed the code up yet, so it just wasn't jiving with what I was expecting. I thought that maybe I wasn't clear with what I was asking, and I was sort of thinking out loud (so to speak). Now that I've played around with it a bit, I get what you were trying to say.