RE: [CAD_CAM_EDM_DRO] Re: Rewinding microwave transformers

The 20 Amp rating for 12AWG wiring is based on the wire being laid out
"straight", not wound onto itself hundreds of times.
Specifically, the table (in the Ontario Electrical Safety Code) that gives 20
amps on 12AWG is titled "Allowable Ampacities for Not More Than 3 Copper
Conductors in Raceway of Cable" i.e. it only applies to 12/2 and 12/3 cable, or
up to 3 loose #12 wires in a conduit etc.
A subtitle of the table says "Based on ambient temperature of 30 degrees C".
The 90 deg. C rating is actually the temperature the *insulation* can stand
without damage.

In other words, the table says that if you have up to 3 #12 wires carrying
20amps each in a cable in 30C air, the insulation temperature will remain below
90C.

In a transformer, each wire is surrounded by many other windings each one
generating the same amount of heat. The ambient temperature would be well above
30C. As a result the wire's current capacity must be derated.

By the way, this is why thay say you should always unwind and spread out your
extension cords for power tools, even if you do not need their full length.
Otherwise the coiled portion could overheat.
-Kevin

-----Original Message-----
From: stirlinguy [mailto:Gary.Rose@...]
I'm a little confused by the calculations on this page you
referenced.

When calculating the current carrying capacity of the secondary
winding, we are told to find a wire diameter that has enough circular
mils for the current we want (750 or 1000 circ. mils per amp). 12 AWG
has 80.8^2 = 6529 circular mils which gives us 8.7 or 6.5 amps
depending on if we use 750 or 1000 circ. mils per amp.

Now, 12 AWG wire is used for house wiring up to 20 amps. Why such a
big discrepancy? I realize that you wouldn't want a transformer
operating up to 90 deg. C (like house wiring is rated) but the results
are still too far away to make sense.

What say ye transformer guys?