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wire gauge amp rating
By rpcancun, Section Mechanical Posted on Tue May 01, 2007 at 08:26:27 AM MST
.

http://www.powerstream.com/Wire_Size.htm
I went to this site but kinda confused under the 16 gauge area
it had 2 amp capacities
Maximum amps for chassis wiring: 22
Maximum amps for power transmission: 3.7
is there a simple chart for magnet wire capacity?
12 13 14 15 16 gauge?
help!!!

wire gauge amp rating | 8 comments (8 topical)

Re: wire gauge amp rating (3.00 / 0) (#1)
by Flux on Tue May 01, 2007 at 03:07:56 AM MST

No.

Wire used for wiring and electrical power supplies has definite ratings based on volt drop and temperature rise ( different ratings for open wiring and wire in conduit).

Magnet wire has no rating. It depends entirely on the application. The only rating is the maximum temperature rating of the coating which you must not exceed for reasonable life. All except polyurethane will die slowly and may survive significant overload in low voltage devices for a long time. Polyurethane disintegrates spectacularly on overload, gives off a stink of poisonous TDI and leaves you with virtually bare copper and a nice short circuit.

Most common wire grades are good for 160 deg C. Better 2 coat systems are rated to 200C.

Possibly as a rough guide, small transformers often run the wire at 3000A/sq inch, welders and other short time rated things often push things to 5000A/sq inch. Wind with reasonable cooling and a somewhat intermittent rated output may let you run near this figure. I have a dreadful feeling that many are pushing the wire well beyond this point. With any source other than the rather fluctuating power out of wind the life would probably be a matter of hours.

Flux

Re: wire gauge amp rating (3.00 / 0) (#3)
by rpcancun on Tue May 01, 2007 at 12:04:09 PM MST

"Magnet wire has no rating. It depends entirely on the application. The only rating is the maximum temperature rating of the coating which you must not exceed for reasonable life. All except polyurethane will die slowly and may survive significant overload in low voltage devices for a long time. Polyurethane disintegrates spectacularly on overload, gives off a stink of poisonous TDI and leaves you with virtually bare copper and a nice short circuit."

Flux, they gave me a rating of 12.3a, but that bare wire so for magnet
wire now with its coating, is 12.3 still safe?
Or does it drop after it's coated?
What would be a safe (cool) ampacity for 16, 14, 13, 12,......

Thx

[ Parent ]

Re: wire gauge amp rating (3.00 / 0) (#4)
by Flux on Tue May 01, 2007 at 01:05:05 PM MST

I stick to what I said originally, the rating of wire in an induction motor, wound on an iron core with a monster aluminium finned heatsink on the outside is not going to be the same as in the armature of a dc machine with only indirect air cooling and even less will it be similar to a coil potted in resin with no direct metallic conduction of heat to the outside world.

I took the trouble to download that link thing, I don't believe it and it has been deleted, how anyone can be so definite about something so variable is beyond me.

The best guide is to take advice of Dan's recent alternators, he pushes them hard, don't even think of trying for more or you will fry it.

It is difficult even to know how much wind cooling there is on these machines I suspect there is far less wind at the centre of a prop than commonly suspected, I have some reasons to suspect that very little of the wind passes through the central third of the rotor.

Even if you can determine what the wire can stand then the thermal rating long term of fibreglass resin is questionable. Polyester in wire terms is about class F, that is certainly as hot as I would want to run any of these things without lots of experimental investigation. The occasional 50A at start of furling would probably equate to less than 20A average. That is largely why these machines survive the high ratings that people expect. When you have it furled hard in a 70 mph wind then that 50A may come nearer to 40A average and the dissipation goes up as current squared.

Flux

[ Parent ]

Re: wire gauge amp rating (3.00 / 0) (#5)
by dinges on Tue May 01, 2007 at 01:42:51 PM MST

Thanks Flux, your values for current density are almost exactly the same as the ones I was taught:

4 A /mm^2 for transformers. (you: 3000A/sq.inch = 4.65 A/mm^2)

A few days ago I repaired a transformer welding machine. Gave it a few testruns. The machine can do 100A at 10% duty cycle (don't laugh...); the wire is 4mm diameter. This gives a current density (at 100 A) of 8 A/mm^2. After about a minute the secondary of the transformer gets HOT. The 8 A/mm^2 correlates perfectly to your 5000A/sq.inch (7.75 A/mm^2)

10% duty cycle means 1 minute welding followed by 9 minutes of cooling...

I have tried to reverse calculate the current densities for some axial fluxes based on figures published on this board. 'Normal' current density at rated power seems to be near 20A/mm^2. Peaks of 30A/mm^2.

Insane, IMHO. I consider it a miracle that there are so (relatively) little failures of stators burning out. As you say, the fluctuating nature of windpower may give a lower average current density (and power dissipation). The cooling effect of the wind (as opposed to e.g. a transformer in a closed case) may explain the relatively low failure rates of stators as well.

But, if furling mechanism doesn't function properly, stator burnout is virtually guaranteed.

Personally, I like conservative designs that don't push things close to or over their rating.

[ Parent ]

Re: wire gauge amp rating (3.00 / 0) (#2)
by wind pirate on Tue May 01, 2007 at 07:22:14 AM MST

Go to http://wiretron.com and download their calculator software. Its a nice little tool for what you are asking.

"Safe current capacity"

ga 31.2A .001589 ohms/ft
ga 24.7A .002001 ohms/ft
ga 19.6A .002524 ohms/ft
ga 15.5A .003181 ohms/ft
ga 12.3A .004019 ohms/ft

Re: wire gauge amp rating (3.00 / 0) (#6)
by Devo on Wed May 02, 2007 at 04:54:44 PM MST

The wire tron chart is quite low for wind turbines as there is a good cooling effect.

My genny has 15 gauge 2 in hand wired in star.It will run my ammeter at 50 amps all day long no problem, that's 25 amps for 15 Gauge , well above the chart & that's at 12 volts after 80 to 100 feet of cable.

At 60 amps in heavy winds one day I noticed some slight darkening of the outer stator. I would think 16 gauge in a well ventilated genny would do 20 amps. I wouldn't gaurantee it but I wouldn't be afraid to push it to 20 here....

Devin

Re: wire gauge amp rating (3.00 / 0) (#7)
by Seaspray0 on Wed May 02, 2007 at 08:28:29 PM MST

Are you stating the DC current going to the batteries or the current of one of the phases in you generator? They are different animals.

[ Parent ]

Re: wire gauge amp rating (3.00 / 0) (#8)
by Flux on Wed May 02, 2007 at 11:56:56 PM MST

They are stating the dc current to the battery.

The rms ac current of the alternator will be about .76 of this, so that does help quite a lot.

That brings Devo's current density down to about 7000A/sq inch. That is probably a useful figure to work on with the good cooling and the fact that wind never delivers a constant current for very long.

These figures are higher than I would want to go but if it gives reasonable service life then it is a good guide.

Don't try it for anything driven by other than wind power.

Flux

[ Parent ]

wire gauge amp rating | 8 comments (8 topical)

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