Transformer Sizing Formulas
Single-phase: kVA = (V × I) ÷ 1,000
Three-phase: kVA = (V × I × 1.732) ÷ 1,000
Find secondary current (3φ):
Isecondary = kVA × 1,000 ÷ (Vsec × 1.732)
Turns ratio: NP/NS = VP/VS
Motor Branch Circuit Sizing (Art. 430)
| Component | Sizing Rule |
|---|
| Branch circuit conductor | FLA × 125% |
| Inverse-time breaker (OCPD) | FLA × 250% max |
| Instantaneous trip breaker | FLA × 800% max (listed) |
| Dual-element fuse | FLA × 175% max |
| Overload protection (SF ≥ 1.15) | FLA × 115% |
| Overload protection (all other) | FLA × 125% |
| Disconnecting means | HP rated, within sight of motor and controller |
Service Sizing — Commercial (Art. 220)
Feeder demand factors for lighting (Table 220.42):
| Portion of Load | Demand Factor |
|---|
| First 3,000 VA | 100% |
| Next 117,000 VA (3,001 – 120,000) | 35% |
| Over 120,000 VA | 25% |
Optional method for dwelling units ≥ 10 kW:
Apply 40% demand factor to total load at or above 10 kW.
Three-Phase Voltage and Power
| Formula | Description |
|---|
| VL-L = VL-N × 1.732 | Line-to-line from line-to-neutral (wye) |
| IL = Iφ (wye) | Line current = phase current in wye |
| VL = Vφ (delta) | Line voltage = phase voltage in delta |
| IL = Iφ × 1.732 (delta) | Line current in delta |
| P = V × I × 1.732 × PF | Three-phase real power |
| kW = kVA × PF | Real vs. apparent power |
Conductor Ampacity — Copper (Table 310.12)
| AWG / kcmil | 75°C Ampacity | 90°C Ampacity |
|---|
| 4/0 AWG | 230 A | 260 A |
| 250 kcmil | 255 A | 290 A |
| 350 kcmil | 310 A | 350 A |
| 500 kcmil | 380 A | 430 A |
| 600 kcmil | 420 A | 475 A |
| 750 kcmil | 475 A | 535 A |
Use 75°C column for terminals rated 75°C. 90°C column only for derating calculations.
Transformer OCPD Sizing (Art. 450.3)
| Type | Primary OCPD | Secondary OCPD |
|---|
| 600V and below — primary ≥ 9A | 125% of primary FLA | 125% of secondary FLA |
| 600V and below — primary < 9A but ≥ 2A | 167% | 167% |
| 600V and below — primary < 2A | 300% | 250% |
Service Entrance Design — Key Rules (Art. 230)
| Rule | Requirement |
|---|
| Number of services | Only one service per building (with exceptions for large buildings, fire pumps, different voltages) |
| Disconnecting means | Maximum 6 disconnects grouped together at service entrance |
| Service clearances (overhead) | 3 ft minimum from windows, doors, porches; 10 ft above finished grade at entrance |
| Drip loop | Required at service entrance head to prevent water entry |
| Main bonding jumper | Required at service equipment to bond neutral to equipment ground |
| Surge protection | Required at service equipment for new installations (NEC 2026 Art. 230.67) |
Hazardous Locations — Classification Summary (Art. 500)
| Class | Hazard | Division 1 | Division 2 |
|---|
| Class I | Flammable gases and vapors | Normally present | Present only during abnormal conditions |
| Class II | Combustible dust | Ignitable quantities present | Present only during abnormal conditions |
| Class III | Ignitable fibers and flyings | Produced, stored, or handled | Stored or handled |
Class I, Division 1 = explosion-proof equipment required. Class I, Division 2 = less stringent; listed equipment required.
Generator and Emergency Systems (Art. 445, 700)
| Requirement | Rule |
|---|
| Emergency system transfer time | Automatic transfer within 10 seconds of failure (Art. 700.12) |
| Generator disconnecting means | Required — within sight of generator or lockable in open position |
| Feeder wiring for emergency systems | Must be kept entirely independent of other wiring (Art. 700.10) |
| Generator output OCPD | 125% of nameplate current rating |
| Legally required standby | Transfer within 60 seconds (Art. 701) |
NEC 2026 New Requirements — Master Level
| Article | New Requirement | Impact |
|---|
| 230.67 | SPD required at service equipment | Must include in every new service design |
| 625 | EV charging fully rewritten — V2G (bidirectional) addressed | Affects load calcs for EV infrastructure |
| 706 | Energy storage systems expanded | Battery systems, solar+storage, disconnecting means, ventilation |
| 210.17 | EV-ready outlet required in new single-family garages | Design impact for all new residential projects |
Master Exam Tips
- The master exam tests design — not just code lookups. Practice complete system problems: service → panel → branch circuits.
- On transformer questions, always determine if it is single-phase or three-phase first — formulas are different.
- Load calculation questions: identify the method (standard vs. optional) before starting to calculate.
- Demand factors reduce the load — do not forget to apply them or you will oversize and get the wrong service amps.
- For motor calculations: write out each step separately. Conductor, OCPD, overload, and disconnect are all sized differently.