### Author Topic: Wire ampacity...  (Read 4780 times)

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#### chipwitch

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##### Wire ampacity...
« on: March 09, 2016, 07:21:23 pm »
In advance, I apologize for asking this... I'm sure it's been asked a thousand times before, but damned if I can find it.

I'm looking for some kind of table that will tell me the accepted ampacities of small gauge wires.  All my searches seem to turn up are references for residential/commercial/Industrial wiring.  What I'm looking for specifically is for small gauges, down to 30 AWG or so.  Varnish insulation.  I have some small solenoids and motors with unknown voltages.  It would be nice to be able to know the ampacity limit so I can estimate the maximum voltages.

Thanks.
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#### Neilm

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##### Re: Wire ampacity...
« Reply #1 on: March 09, 2016, 07:43:55 pm »
The losses in a wire are due to the resistance. This produces heat, so it isn't just a case of saying wire gauge x can take y Amps. It depends how hot you are able to run the wire. Usually this is determined by by the insulation of the wire and temperature of where ever that wire is.

I use a programme called Miscel http://www.miscel.dk/MiscEl/miscel.html as a guide for wire ratings. It also can calculate track widths and many other useful things.

Alternatively, if you have an Android phone there is an app called ElectroDroid that has this information as well.
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#### chipwitch

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##### Re: Wire ampacity...
« Reply #2 on: March 09, 2016, 08:39:29 pm »

The android ap is pretty cool.  Lots of good info, but I can't find anything to help me decide how many amps will burn up a solenoid.  I understand there are lots of variables.  Temperature, length, insulation etc.  I see a trace calculator and large (16 AWG and up) but nothing I can use.  I'm a noob, so maybe I'm overlooking it.

The other program I downloaded... looks like you need an engineering degree to read it.  I found the wire section, but when I type in "1 inch" for a length, it fills the yards, meters and feet values... and they certainly don't equal 1 inch.  (25.4 m, for example).  Maybe just a misplaced decimal.
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#### Ian.M

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##### Re: Wire ampacity...
« Reply #3 on: March 09, 2016, 09:32:27 pm »
Its all down to its thermal resistance to ambient, the total power dissipated and the max working temperature of the solenoid's insulation.  If its wound on a plastic former, then the outer surface will be the main heat loss path as the thermal resistance through the former will be quite high.  If its wound directly on a tape insulated core, the heat loss through the core may dominate.   If you need a pulsed power rating, you have to consider the pulse duration.  For very short pulses, the limit is based on the difference between the current temperature of the hottest part of the winding, the max insulation temperature and the specific heat capacity of copper.

#### suicidaleggroll

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##### Re: Wire ampacity...
« Reply #4 on: March 09, 2016, 09:59:55 pm »
There are plenty of charts like what you're looking for, eg:
http://www.powerstream.com/Wire_Size.htm

But they're useless for your application.  As Ian.M mentioned, what matters is thermal resistance to ambient.  When you have a 30 awg wire in more-or-less open air, you can follow charts like that which suggest limiting it to <.75 amps or so.  As soon as you coil it up into a solid mass, things change.  Your inner wires can no longer shed their heat to ambient, they have to conduct it to the wires next to them, the wires next to them, and so on until it reaches ambient.  The wires on the inside of your spool (assuming they can't conduct to the inner surface) will be running significantly hotter than those on the outside of the spool.

You're not going to find a convenient chart, as it's all VERY application-specific.  You need to know the thermal resistance and thickness of the insulation, the winding thickness, the former material, etc., and then do a full thermal simulation.  Or you can sacrifice one or two for the greater good and find the limit experimentally.

#### chipwitch

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##### Re: Wire ampacity...
« Reply #5 on: March 09, 2016, 11:04:29 pm »
There are plenty of charts like what you're looking for, eg:
http://www.powerstream.com/Wire_Size.htm

But they're useless for your application.  As Ian.M mentioned, what matters is thermal resistance to ambient.  When you have a 30 awg wire in more-or-less open air, you can follow charts like that which suggest limiting it to <.75 amps or so.  As soon as you coil it up into a solid mass, things change.  Your inner wires can no longer shed their heat to ambient, they have to conduct it to the wires next to them, the wires next to them, and so on until it reaches ambient.  The wires on the inside of your spool (assuming they can't conduct to the inner surface) will be running significantly hotter than those on the outside of the spool.

You're not going to find a convenient chart, as it's all VERY application-specific.  You need to know the thermal resistance and thickness of the insulation, the winding thickness, the former material, etc., and then do a full thermal simulation.  Or you can sacrifice one or two for the greater good and find the limit experimentally.

I understand the complexity... that's why I was looking for a chart.  I assumed there had to be something somewhere.  Your link is exactly what I was looking for.  It's all just a jumping off point... I won't be designing any life dependent systems or anything.  I promise

Besides, the site you provided looks VERY conservative.  10 AWG is listed as 15A for power transmission.  The US NEC limits it to 30A in most cases, but depending on insulation, ambient, bundling, and temperature rating of equipment it's attached to, can go as high as 45A iirc.  May be even higher today.  It's been a while since I've opened a code book.
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#### ajb

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##### Re: Wire ampacity...
« Reply #6 on: March 09, 2016, 11:13:26 pm »
Transmission means long-ish distance which means that voltage drop is going to be the limiting factor. You probably want to start from a chassis wiring table if you want a ballpark idea of accepted ampacity, but for things like solenoids it's the temperature that you care about.  To figure that out, all you really need to know about the wire is its resistance, which you can simply measure.  If you know the DC resistance of the coil, you can easily determine the power dissipated in steady state conditions at a given voltage or current.  Then it's a matter or thermal modeling. Or, you could apply a known amount of power and measure the temperature which will give you a ballpark thermal resistance.

#### slaterk93

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##### Re: Wire ampacity...
« Reply #7 on: March 09, 2016, 11:35:23 pm »
and they certainly don't equal 1 inch.  (25.4 m, for example).
25.4 millimeters equal one inch. I guess the default is meters

#### chipwitch

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##### Re: Wire ampacity...
« Reply #8 on: March 10, 2016, 02:02:17 am »
I know how to convert metric.  It says 25.4 m... thus my reason for saying a decimal place was probably off.  Sounds like you may not be familiar with the program to which I was referring?
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#### dwpatter53

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##### Re: Wire ampacity...
« Reply #9 on: March 13, 2016, 03:08:22 am »
There are plenty of charts like what you're looking for, eg:
http://www.powerstream.com/Wire_Size.htm

But they're useless for your application.  As Ian.M mentioned, what matters is thermal resistance to ambient.  When you have a 30 awg wire in more-or-less open air, you can follow charts like that which suggest limiting it to <.75 amps or so.  As soon as you coil it up into a solid mass, things change.  Your inner wires can no longer shed their heat to ambient, they have to conduct it to the wires next to them, the wires next to them, and so on until it reaches ambient.  The wires on the inside of your spool (assuming they can't conduct to the inner surface) will be running significantly hotter than those on the outside of the spool.

You're not going to find a convenient chart, as it's all VERY application-specific.  You need to know the thermal resistance and thickness of the insulation, the winding thickness, the former material, etc., and then do a full thermal simulation.  Or you can sacrifice one or two for the greater good and find the limit experimentally.

The Electrical codes have tables to de-rate conductors based on number of conductors in a conduit or cable tray.
So depending on how many levels of wire in that coil; you might start with 10% of normal current for a single conductor.

Do you have an IR temp gun you can use to monitor energized temp?

Good luck

I understand the complexity... that's why I was looking for a chart.  I assumed there had to be something somewhere.  Your link is exactly what I was looking for.  It's all just a jumping off point... I won't be designing any life dependent systems or anything.  I promise

Besides, the site you provided looks VERY conservative.  10 AWG is listed as 15A for power transmission.  The US NEC limits it to 30A in most cases, but depending on insulation, ambient, bundling, and temperature rating of equipment it's attached to, can go as high as 45A iirc.  May be even higher today.  It's been a while since I've opened a code book.
Dave P.Eng. BScEE
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#### SeanB

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##### Re: Wire ampacity...
« Reply #10 on: March 13, 2016, 08:12:34 am »
Best is to make a sample coil and include in it a sacrificial type K thermocouple ( the ultra cheap welded bead ones available either free with a Horror Fright discount coupon meter, or actually bought for the purpose) placed on the solenoid winding former, and insulated with a single turn of kapton tape. Then wind your solenoid with the wire you want to use, or a similar gauge ( 2 sizes up or down will not really matter except in the resistance of the coil and number of turns you can physically fit on it) and then try it on the desired voltage, leaving it powered for a half hour to reach a stable temperature inside the core. If it is under 100C you are fine for long term reliability with most wire designed for survival at 130C, above around 110C you are getting close to it being unreliable long term, or usable only in pulsed applications. Over 130C it will only be usable at a very low duty cycle for short term operation.

#### Seekonk

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##### Re: Wire ampacity...
« Reply #11 on: March 13, 2016, 12:34:19 pm »
No need for thermocouple.  Just measure coil resistance when cold and hot. Then calculate temperature for change in resistance for copper.  This is accepted practice for relays.

#### SeanB

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##### Re: Wire ampacity...
« Reply #12 on: March 13, 2016, 05:48:33 pm »
Yes, but using a thermocouple is good as well, as it will allow confirmation of the temperature increase in the centre, while the hot resistance will tend to average out the whole coil, not showing you what the hottest part in the centre is. Next time you have more confidence in the winding resistance measurement, so do not need a temperature reading to see if the increase in temperature is low enough to stay under the insulation failure point.

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##### Re: Wire ampacity...
« Reply #13 on: March 14, 2016, 06:24:22 am »
You can also opt to use high temp (155 deg C) wire. It's (much) more difficult to strip but far better at high temp than polyurethane coated wire.
Also, some bobbins are available in fibreglass (as apposed to nylon) which can handle higher temps.
Similarly Kapton (polyimide) tape is far superior to the normal yellow polyester tape.
The thermocouple suggestion I believe is the way to go if you want an accurate temp reading and not just a guestimate.

Smf