RE: RE: Re: Rewinding microwave transformers

Thanks Kevin!

That makes a lot of sense now. Nice to get the answer from a fellow
Canadian, too.

Gary

> Date: Wed, 13 Feb 2002 13:35:03 -0500
> From: "Kevin P. Martin" <kpmartin@...>
> Subject: RE: 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?