DIYbanter - View Single Post - Blew another damn transformer on my Trane XB80

Don Klipstein

In , wrote:
On Tue, 12 Apr 2011 17:37:15 +1000, "Phil Allison"
wrote:

Got a ****ing clue how much current it takes to vaporise copper wires like
that ?

Obviously ****ing not.

How about "more than 15 amps"?

And it didn't blow the circuit breaker or fuse.
So the current came from somewhere other than the mains?

I don't remember any V-A rating stated for the tranny in question since
the number of posts in the 2 threads is huge. Though, I have the
impression it's around 40 or 50 or so.

A 120V one this size usually has its primary around AWG 24 or 26, and a
240V one or a dual-primary 120/240V one usually has primary winding size
around AWG 28. Properly connected for 120V, that means 2 roughly-AWG-28
strands in parallel, effectively around AWG 25.

A transformer winding this size can gradually get charring hot at as
little as 1 or 1.5 amps or so.

Leads of this size get charring hot within a minute at several amps.

The CRC Handbook says fusing current for copper wire is 20 amps for AWG
26, 30 amps for AWG 24. Though these figures are only approximate, I
usually hear that these fusing current charts are for bare wire in free air.
Fusing current will be less for a wire next to another turn of overheating
wire.

Meanwhile, I doubt a 15 amp slow-blow fuse or slow-acting breaker will
keep a 26 or 24 AWG wire from fusing. One thing to keep in mind is that a
15 amp fuse or breaker normally passes 15 amps without opening, and may
take a few seconds or more to open at 20 amps.

I think the overcurrent that caused charring here came from the power
line without opening the breaker/fuse. Back EMF won't do that, though it
could cause insulation failure. However, I doubt that explains 3 blown
transformers and nothing else blown.

--
- Don Klipstein )