Quick Hits

EPISODE 7 — 265.9

Autotransformers

SLIDE 1 — 265.9

CIRCUITS DERIVED FROM AUTOTRANSFORMERS

Article 265 places a restriction on deriving branch circuits from autotransformers.

Generally, the supplied circuit must have a grounded conductor electrically connected to a grounded conductor of the system supplying the autotransformer.

SLIDE 2 — WHY THIS MATTERS

This isn't just an isolated transformer rule.

The NEC is concerned with the relationship between:

SUPPLYING SYSTEM

⬇️

AUTOTRANSFORMER

⬇️

BRANCH CIRCUIT

Pay attention to the grounded-conductor connection.

SLIDE 3 — EXCEPTION

There's an exception involving a:

REDUCED-VOLTAGE AUTOTRANSFORMER STARTER

The autotransformer can supply the branch circuit during motor starting until the bypass contactor engages.

This is one of those exceptions worth recognizing for an exam.

SLIDE 4 — REVIEW ⚡

A branch circuit is derived from an autotransformer.

What does 265.9 generally require?

A. No grounded-conductor relationship is needed

B. The supplied circuit must have a grounded conductor electrically connected to the supplying system's grounded conductor

C. The transformer must always be isolated

D. The branch circuit must be DC

ANSWER: B

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