Transformers and Special Occupancies
Transformers and Special Occupancies
This domain covers Article 450 transformer protection and Articles 500–517, 680, and 690 for hazardous locations, health care, pools, and solar. The exam tests both transformer calculations and location-specific restrictions.
Core rules
- Transformer primary and secondary OCPD sizing follows 450.3(B); 125% is common for primary protection, with next-standard-size allowances.
- Transformers need ventilation, accessible disconnects, and grounding and bonding per Article 250.
- Hazardous locations use Class/Division or Zone systems; equipment must be listed for the area.
- Health care facilities have critical-branch and essential-system rules; patient-care areas restrict receptacles.
- Pool and spa installations require equipotential bonding and GFCI protection.
Worked example
A 45 kVA, 480 V to 208/120 V, 3-phase transformer: primary FLA = 45,000/(480 × 1.732) = 54.1 A. Primary OCPD at 125% = 67.6 A; use a 70 A standard device. Secondary FLA = 45,000/(208 × 1.732) = 124.9 A; secondary OCPD at 125% = 156 A; the next standard size is 175 A, subject to 450.3(B) and conductor protection.
Traps
- Using single-phase formulas for three-phase transformers.
- Applying 125% to both primary and secondary without checking 450.3(B) exceptions.
- Treating a hazardous location as an ordinary area because the equipment is "sealed."
- Omitting equipotential bonding around pools.