In ,
bud-- typed:
Twayne wrote:

A "Class 2" xfmr is simply a double-insulated transformer
without an earth ground on its plug, nor a polarized plug.
A "Class 1" xfmr requires a third wire, the earth ground
pin on its plug. The "Class of xfmr is a UL/CSA et al
classification which approves the xfmr and nothing else as
safety approved.

The primary characteristic of class 1, 2, and 3 power
sources is that they are power limited. There is a
specified maximum voltage, current and power you can get
from the source. For a class 2 24V transformer the max
current is 8A - the maximum current you get if you short
circuit the transformer.

No. You're mixing up NEC/Safety agencies. UL and CSA and all like agencies
only have classes 1 and 2, no class 3, and they deal with the safety aspects
of a power transformer and its container, and must be so tested/listed in
order to be sold legally anywhere in the US or Canada. You are trying to
apply NEC wiring classses to the safety classes, incorrectly. The
definitions are exactly as I stated.

NEC Class 1, 2, and 3 circuits are classified as remote-control, signaling,
and power-limited circuits in the National Electrical Code (NEC).
Transformers are NOT required to meet ANY of those requirements. The 8A is a
wiring issue and part of the NEC, NOT part of safety. Look up UL 1459 and
you'll see what I mean. A transformer/power supply does not HAVE to be power
limited and in fact most are not, and if it's NOT power limited, that
section does not apply. Most transformers are NOT power limited but per
Safety agencies can never, under ANY load including locked rotors,
solenoids, short ckts, etc., can never present a fire or safety hazard to
anyone or anything.
A class 1 OR class 2 24V transformer sold in North America is required to
have UL or CSA or equivalent markings and submissions or be listed as a
component. INSTALLATION of same is where NEC comes in, and may or may not
apply; usually not. If it does not claim to be a class 3 installation
device, then it's a moot point; other parts of the NEC will apply.
HTH,

Twayne`

Because the power is limited, the downstream circuit does
not have to meet the general wiring requirements of the
NEC. Article 725 is used instead.

Secondary ckts attached to the output
of a xfmr are not subject to UL/CSA et al safety testing
unless a voltage in it equals or is more than 42VDC or 42V
ac p-p. The original spec, UL1459 (now superceded but not
changed in this area) is online for those who wish to read
it. I -think- the CSA spec was CS-03. Whatever class 1
& 2 means ckt-wise, I don't know; it's not a UL/CSA et al type spec but
might be NEC, NFPA or otherwise.

HTH,

Twayne`