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Re: ampacity question

Original poster: "Thomas McGahee by way of Terry Fritz <twftesla-at-uswest-dot-net>" <tom_mcgahee-at-sigmais-dot-com>

The problem in giving a "definitive" answer is that it really depends
on the environment in which the wire is being used.

An enamel coated wire surrounded only by ambient air will be able
to handle more current than the same wire with a thick pvc insulation.
Immerse the enamel coated wire in oil and it can handle a lot more
current because the oil helps remove the heat much better than
ambient air.

Now, take the wire and wrap it onto a bobbin with many layers, and the
heat problem is compounded because the wires that are surrounded
by other wires BUILD UP and retain the heat, because it is difficult
to get any airflow or oil flow over these inner windings.

So, you see, ampacity is dependent on MANY factors. My table tends
to be on the conservative side. Its ampacity values were arrived
at based on my perusal of dozens of other ampacity lists and
the fact that someone might use my tables to design things like
transformers and not just straight runs of wire, or single layer
coils. If you are designing for something where you can absolutely
guarantee that the wire will be sufficiently cooled, or only used
for short periods of time, then you can safely double my published
values. If not, then it is better to err on the safe side and not
end up burning up your home-made transformer.

An oil-cooled pig built to provide power for short runs, such as
Allen's home-brew pig page talks about can readily be pushed to
the values he records. But don't apply his ampacity ratings to
a transformer that is NOT oil cooled, and don't attempt to run
even the oil-cooled version for extended periods of time.
Keep in mind that on extended runs the inner wire will build up
heat that cannot be rapidly removed. You CAN fry even a pole
pig if you push it too hard or too long.

Fr. Tom McGahee

----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, February 10, 2001 9:17 AM
Subject: ampacity question

> Original poster: "bob golding by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> Hi all,
>     I have a question about current carrying capacity of magnet wire for
> a transformer secondary. I get different ratings from different web
> sites. Father toms table says the following.
> 28 awg=0.319A
> 30 awg=0.200A
> 32 awg=0.124A
> 34awg=0.0795A
> Allen's home brew pig web site says
> 28awg=761mA
> 30awg=477ma
> 32awg=304ma
> 34awg=187ma
> anybody got any definitive numbers for a transformer secondary running
> under oil.?
> cheers
> bob golding