1-Voltage, 2-Wire Secondary

[mivey]

Not true. The 2014 Handbook has explicit author's commentary following 240.4(F) that explicitly refers to these circumstances as "special cases."

Am I taking a little bit of creative freedom in going from "special case" to "rare exception?" Yes, but that is hardly a stretch.

Quite a bit actually. Nothing rare about the transformer at all.

Both phrases imply a degree of rarity.. and I maintain that the ability to wire any dual winding, multiple voltage output xfmr as a "2-wire (single voltage) secondary" (even if listed on the nameplate) is a direct contradiction to said rarity.

Not at all. It's just that most loads use 120/240 nowadays instead of straight 120 or straight 240.

You are way off base. Don't be so set that two terminals outside the tank means only one winding. You may pop the lid and find that the windings are jumpered internally and you still have two windings giving you a two-wire output the same as if you had three or four terminals outside the tank to look at and ponder over

It means nothing.

 

[mivey]

Both interpretations are absolutely plausible. Me, I'm going to err on the side of caution and assume this does NOT meet the parameters defined until I hear from a more official source.

Whatever makes you happy of course. But consider there are reasons (usually) behind the madness.

I design systems with multiple transformer configurations and varying protection schemes every day. The 2-wire rule has to do with how the secondary fault current is reflected on the primary side and what the primary protection will see.

You are trying to make it into something else. If the transformer needs additional protection, then that is a different topic. The transformer may have one of two windings fail or if it is a single winding it could have an internal failure of that winding. With either type you can have a transformer fail internally.

The 2-wire rule is about whether the transformer primary current can accurately reflect currents on the secondary circuit conductors, not about internal transformer failures.