Chapter 13: Power Supply and Distribution

13.1 What electrical systems serve residences vs small commercial (~100 kVA)?

Residences: 120/240 V single-phase, 3-wire; Small commercial: 120/208 V three-phase, 4-wire (handles larger and mixed loads).;

13.2 What systems serve mixed single-phase and three-phase loads?

120/208 V, 3-phase, 4-wire system (provides both 120 V and 208 V).;

13.3 What is voltage spread? Voltage drop?

Voltage spread = difference between highest and lowest system voltages; Voltage drop = reduction in voltage along conductors due to resistance.;

13.4 Max motor size on 100-kVA single-phase system? Why lower?

Typically limited to ~20–30 kVA; large motors cause voltage drop, imbalance, and starting issues → must be much smaller than system rating.;

13.5 Voltage to ground (120/208 V, 3φ, 4-wire)?

120 V (line-to-neutral voltage).;

13.6 Voltage to ground (480 V, 3φ, 3-wire)?

Typically ~277 V if grounded system (480/277 system); if ungrounded, no defined voltage to ground.;

13.7 Why should systems be grounded?

Safety, fault protection, and stable voltage reference.;

13.8 Must all electrical systems be grounded?

No, but most building systems are grounded for safety and code compliance.;

13.9 Should switchboard IC be higher or lower than short-circuit current?

Higher; must safely interrupt maximum possible fault current.;

13.10 Examples of NEC wire types and temp ratings?

THHN (90°C), THWN (75–90°C), XHHW (90°C); insulation type determines rating.;

13.11 Best on-floor power system for high-rise office?

Raised floor or modular underfloor distribution → flexible for computers and layout changes.;

13.12 Switches for control from two locations?

Two 3-way switches (no 4-way needed unless more than two locations).;

13.13 Advantages of fuses vs circuit breakers?

Fuses: fast, reliable, cheap; Breakers: reusable, convenient, adjustable.;

13.14 Currents in 120/240 V system (1500 W & 1000 W loads)?

I1 = 1500/120 = 12.5 A; I2 = 1000/120 ≈ 8.3 A; Neutral = difference ≈ 4.2 A (loads on opposite legs).;

13.15 Short-circuit current (5% impedance, 500 A full load)?

Isc ≈ 500/0.05 = 10,000 A (fault current increases as impedance decreases).;

13.16 NEC ampacity for #3/0 conductor?

≈ 200 A (single); reduced with multiple conductors (derating); conduit size depends on fill (use NEC tables).;

13.17 Best distribution system for 20,000 ft² school?

Use 480/277 V 3-phase for mechanical loads and lighting; step down to 120/208 V for receptacles.;

13.18 Standard Europe frequency and voltage?

50 Hz; ~230 V single-phase (utilization).;

13.19 Line-to-line voltage from line-to-neutral?

V_LL = √3 × V_LN → 220→~381 V; 2400→~4157 V.;

13.20 Max short-circuit current (300 kVA, 480 V, 3φ, 3% impedance)?

Isc ≈ (300,000)/(1.732×480×0.03) ≈ 12,000 A.;

13.21 If voltage is 240 V instead of 480 V?

Isc doubles → ≈ 24,000 A (lower voltage → higher current for same power).;

13.22 Required interrupting capacity?

Must exceed calculated Isc → ≥12 kA and ≥24 kA respectively (choose higher safety margin).;

13.23 Three alternative power sources?

Utility power, on-site generators, renewable sources (e.g., solar).;

13.24 Difference between K-4 and K-13 transformers?

K-13 handles higher harmonic loads (e.g., electronics) than K-4.;

13.25 Why are large cables stranded?

More flexible, easier installation, reduces mechanical stress.;

13.26 Standby vs uninterruptible power?

Standby: backup with delay; UPS: instant power with no interruption.;

13.27 Functions of K-type transformers, UPS, SPD?

K-type: handle harmonics; UPS: provide continuous power; SPD: protect against voltage surges.;

13.28 Generator indoors vs outdoors (pros/cons)?

Indoors: protected, quieter; needs ventilation; Outdoors: easier exhaust, less noise inside, but exposed to weather.