Transformers and Electrical Theory Flashcards
6 cards from real Journeyman Electrician’s Exam practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Transformers and Electrical Theory flashcards as text
Per NEC 450.21(A), dry-type transformers rated 600V or less must maintain a minimum separation of how far from combustible materials?
Answer: 12 inches
NEC 450.21(A) requires at least 12 inches of separation between a dry-type transformer and combustible material unless a fire-resistant barrier is installed.
What does a transformer's K-factor rating indicate?
Answer: Suitability for non-linear (harmonic) loads
A transformer's K-factor rating indicates how well it can handle harmonic currents generated by non-linear loads such as computers and VFDs.
In a 3-phase delta-wye transformer with a 480V secondary, what is the phase-to-neutral (phase) voltage on the wye secondary?
Answer: 277 volts
Phase voltage = 480 ÷ √3 = 277V; the 480V wye secondary is a 277/480V system used for commercial lighting and power.
What is the primary purpose of a buck-boost transformer?
Answer: Make small line voltage corrections of approximately 5–20%
Buck-boost transformers are used to raise (boost) or lower (buck) line voltage by small amounts—typically 5–20%—to correct for voltage drop or overvoltage.
Per NEC 210.9, an autotransformer used to supply branch circuits must have:
Answer: A common neutral conductor connected to the grounded circuit conductor
NEC 210.9 requires autotransformers supplying branch circuits to have a common neutral bonded to the grounded (neutral) conductor of the system.
Per NEC 250.30, where must the grounding electrode conductor and system bonding jumper for a separately derived system (such as a transformer secondary) be connected?
Answer: At the source of the separately derived system or its first means of disconnect
NEC 250.30 requires the grounding electrode conductor and bonding jumper for a separately derived system to be connected at the source or the first means of disconnect.