[Amps] Filament Xfmr

[Manfred Mornhinweg]

Pete,

> When I received the transformer all looked great to me except for the 
> secondary pigtail wires.  The tube requires 6vac at 10amps, the pigtails 
> provided are about five inches long and are a #18 well insulated wire. 

For the primary that wire size is plenty, but for the secondary it's 
rather cheap.

> Being concerned with the wire size it put the transformer under load and 
> drawing 10amps  left it that way for about an hour.  The wire did not over 
> heat in that time but hey how about hours of operating. 

It won't change between one hour, and continuous use. The thermal mass 
of single thin wires in air is low enough that the temperature will 
stabilize after several minutes.

The transformer proper will take longer to stabilize. One hour might not 
have driven it to its final temperature.

> Should I be concerned with the wire size. 

If it survived the 1 hour test, and if the voltage delivered to the 
filament is enough, I would say you can use it without much worry.

 > I have looked for information on how to
> figure how much current various wire sizes can carry and at various voltages 
> and have not found anything.

Wire tables are widely available on the web. Many of them list the 
current carrying capacity in different applications, per wire size, so 
you don't need to do the math yourself. But a quick and useful rule is this:

In thick bundles, like in transformer windings, you can use up to about 
3A per square millimeter. With very large transformers it's a little 
less, with very small ones you can go to 4A per mm^2.

In bundles of a few wires, such as in house wiring, about twice as much 
current per area is fine and safe.

For single wires in free air, it can be even a bit higher, as long as 
you don't get too much voltage drop.

#18 is about 0.82mm^2. So it's fine for 2.3A in transformer windings, 
roughly 5A in bundles, and at least 7 or 8A for a single wire in free 
air. It won't melt or anything at 10A, but it might get noticeably warm.

I would have used #14 for those pigtails, and #11 or 12 for the winding.

Manfred

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