Transformer Wire Current Capacity

That table looks wrong. According to them 10 gauge is only good for 3.4 amps.

What does ampacity mean btw ?

Graham

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Although the ampere numbers are wrong, they might be internally consistent; ie, 4 strands of #24 is right, to equal #18. Wire size doubles (fairly closely) when guage decreases by 3, so #18 has 4 times the area of #24.

ampere capacity, I think

-jiw

According to the NEC: "The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."

Don't know if "ampacity" means precisely that as used in the table at the url Rich posted.

Ed

Be skeptical of any tables or calculators that give wire ampacity without specifying ambient temperature, maximum conductor/insulation temperature, thermal resistance or at least specifications of the conductor's installation, etc. Be doubly skeptical of a table that begins with the word 'Gauge' misspelled.

Yes. In a transformer, the rating is going to be heavily dependent on cooling which depends on construction.

Graham

On Fri, 05 May 2006 00:41:16 GMT, Rich Grise Gave us:

Ampacity of mag wire doubles every three gauges since circular area does as well.

On Fri, 05 May 2006 01:57:20 +0100, Pooh Bear Gave us:

You're kidding right?

On Thu, 04 May 2006 21:32:28 -0700, "Paul Hovnanian P.E." Gave us:

Yes. EACH wire out there is not alike.

A "single strength" mag wire cannot handle the same temp as a "double strength" mag wire, or a High temp" mag wire.

There is a raw copper ampacity table, and from there on, as you have said, there are a LOT of factors that go into a given wire installation that determine what it can or should pass through it on a continuous duty basis.

It's purely a US term.

Graham

On Fri, 05 May 2006 07:51:07 +0100, Pooh Bear Gave us:

Mmmm... OK...

The chart on my website doesn't list current ratings because there are too many variables like insulation type, and how its used. For transformers you have to decide how many circular mils per amp, depending on how its to be used.

My rule of thumb for line-frequency power transformers is "about" 500 circular mils per amp. As someone else posted, the rating depends on transformer construction, wire insulation temperature rating, maximum operating temperature, cooling options, ... But 500CM/A should be a reasonable starting point. 18AWG is 1624CM; 19 is 1288. So 2.5A sounds at least reasonable.

It's possible to measure the average temperature of the wire in operation by measuring the wire DC resistance. You can either compare that with the resistance measured at some known temperature (e.g. room temp), applying the known temperature coefficient of resistance for copper, or put the transformer (non-operating) into an oven and raise the temperature until you reach the same resistance. The latter method is more difficult but more accurate. That won't tell you the peak temperature in the winding, though, which will be somewhat higher than the average.

Some good things to know about AWG numbers:

-- 3 gauges doubles (or halves) the cross-sectional area

-- Therefore, 6 gauges doubles (or halves) the diameter (This is approx., so you'll be slightly off if you try to use it to go from AWG0000 to AWG48!)

-- To a good approximation, copper wire resistance at room temperature is 10^(0.1*AWG# - 1) ohms per 1000 feet. (So 10AWG is 1 ohm per 1000 ft, 20AWG is 10 ohms, ...). The approx is almost right on at 10AWG, 5% low at 40AWG.

-- Exactly, diameter(mils) = 5 * 92^((36-AWG#)/39)

You'll notice that the gauge# behaves like dB: 3dB doubles the power;

Cheers, Tom

Probably not in this table - it starts increasing at #18, which doesn't make any sense at all.

Thanks, Rich

From the charts I am seeing it looks like you are right on with the wires. The chart specifics 700 cm/amp, but states that from 500 to 1000 are being used. The #18 is listed for 2.3 amps using 700 cm/amp.

I would guess that it depends on what the transformer is used for. If for long term use such as in a VCR or computer monitor probably the larger wire would be used and if for a short term use like a printer where it may use a small amount of current all the time but large amounts for a very short time the smaller wire may be used.

At 60 Hz only up to about #2/0 (assuming copper). At higher frequencies the transition gauge is smaller, above 1GHz it is very thin indeed.

It's only out by an order of magnitude. In some fields, that's pretty good.

It's interesting to see that lighter guages are not rated to carry any current. I wonder what they are good for?

I don't mind spelling mistakes, just so long as they're consistent.

RL

Ratings for power transmission conductors are grossly underrated. The reason for it is not only fire safety, but also the fact that lighter gauge means more voltage drop, which means cost to consumer.

Without conservatve code requirements, builders would skimp on wire, causing more expense to the consumer in the long run. The code protects electricity consumers by demanding heavier gauge wire that could be gotten away with.

This does not apply to microwaves.

There, there, John, we all know "gage" is a noun. :-)

Cheers! Rich