Electrical Distribution, Circuits, and Controls
Domain 5 - Electrical Distribution, Circuits, and Controls
This domain tests circuit design and control logic: voltage drop, conductor sizing, grounding, overcurrent protection, and how photocells, contactors, and network controls switch roadway lighting.
Core rules
- Roadway lighting is usually parallel multiwire circuits; series circuits are legacy and require special isolation procedures.
- Voltage drop is calculated over the round-trip conductor length for single-phase loads: VD = 2 × I × R × L. Keep within the project limit, often 3–5%.
- Equipment grounding conductors and bonding jumpers must connect metal poles, luminaire housings, and accessible metal parts; neutral and ground are separate downstream of the service.
- Overcurrent protection, disconnects, and fusing must match conductor and component ratings. Photocells switch contactors for large loads; time clocks and network controls provide scheduling and dimming.
- Confirm line/load terminals, voltage, and control protocol before replacing a photocell or driver.
Worked example
A 10 A single-phase load at 240 V is 200 ft from the source. 12 AWG copper has about 1.93 Ω per 1,000 ft. R = 1.93 × 0.2 = 0.386 Ω. VD = 2 × 10 × 0.386 = 7.72 V, or 3.2% of 240 V. That passes a 5% limit but not a 3% limit; increasing conductor size reduces VD.
Traps that cost marks
Using one-way length in the voltage-drop formula, mixing line and load terminals on a photocell, bonding neutral to ground at a pole, ignoring inrush from LED drivers, and assuming a control signal is compatible because the connector fits.