Five free practice questions on wiring, lighting and heating: block C of the Red Seal 309A Electrician exam, about 30 of its 100 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.
01The questions
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A metal cable tray system includes an expansion joint and several field-cut sections joined with splice plates. What must be done to keep the tray properly bonded?
- Jumper the joint and unlisted splices, and bond the tray.
- Bond only the section nearest the panel.
- Nothing; the tray is only a support and does not need bonding.
- Ground each section separately to the nearest structural steel.
Show the answer
A. Correct. A metal tray can be energized by a cable fault, so Rule 12-2208 requires it to be bonded at intervals not exceeding 15 m, with the bonding conductor sized from Table 16 on the largest ungrounded conductor in the tray; continuity across expansion joints and field splices is maintained with the manufacturer's bonding jumpers. The tray is exempt only where every cable in it is armoured or metal-sheathed and a warning notice is posted (12-2208(3),(4)).
Why not the others
B. A fault at the far end would then have no path; the whole tray must be continuous.
C. Any exposed metal that can become energized must be bonded; a tray carrying power cables is a prime example.
D. Separate grounds to random steel do not create a continuous fault path back to the source; the tray is made continuous and bonded to the system.
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A photocell rated for 5 A of switching current must control 40 A of parking-lot lighting. How is the circuit arranged?
- Eight photocells are paralleled to share the 40 A.
- The photocell controls a 40 A breaker's shunt trip.
- The photocell drives a lighting contactor's coil; the contactor carries the load.
- The photocell is wired in series with the load and derated to 40 A.
Show the answer
C. Correct. Control devices with small contact ratings operate a contactor or relay; the contactor's contacts, rated for the lighting load's current and inrush, do the heavy switching. The RSOS lists relays and contactors among control components and expects the electrician to match control requirements to loads controlled.
Why not the others
A. Paralleled contacts do not share current evenly and never switch at the same instant; one would carry the whole load and fail.
B. A shunt trip opens a breaker; it cannot close it again at dusk. Breakers are not switching devices for daily control.
D. A 5 A contact cannot be 'derated' upward; it would weld or burn open at 40 A.
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An electric duct heater is installed in a supply duct. How is its control circuit arranged so the heater cannot operate without airflow?
- An airflow (sail or pressure) switch is wired in series with the heater's contactor coil.
- The fan and heater are wired to the same breaker so they always run together.
- The heater is wired through the fan motor's overload contacts.
- A thermostat is set to turn the heater off if the duct gets too hot.
Show the answer
A. Correct. The airflow-proving switch closes only when the fan is moving air; placed in series with the contactor coil it is an interlock that prevents the elements from energizing in still air, where they would overheat and trip the limits or start a fire. The RSOS lists electrical interlocks, pressure switches and flow switches among HVAC control components.
Why not the others
B. A shared breaker does not prove airflow; a failed belt or closed damper would leave the heater running without air.
C. Overload contacts open only on motor overcurrent; a fan can be stopped by a broken belt or closed damper with its motor running normally.
D. High-limit protection is a backup; the interlock prevents the condition in the first place.
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A wall thermostat rated 16 A resistive must control 5000 W of baseboard heating at 240 V (about 21 A). How is the control arranged?
- The thermostat is used as is, because heaters cycle and the average current is lower.
- The heaters are re-wired for 120 V to halve the current.
- Two 16 A thermostats in parallel share the load.
- The thermostat switches the coil of a relay or contactor whose contacts carry the heater load.
Show the answer
D. Correct. A control device is never loaded beyond its contact rating; the heaters are fed through a contactor rated for the full resistive load, and the thermostat, which may then be a low-voltage type, controls only the coil. The RSOS lists relays and contactors among heating control components.
Why not the others
A. Contact ratings are for the current switched, not the average; 21 A through a 16 A thermostat welds or burns the contacts.
B. A 240 V heater on 120 V produces a quarter of its heat, and the current per heater is halved but the wiring becomes impractical.
C. Parallel contacts do not share current reliably; one carries the load and burns.
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A classroom's lights are controlled by a wall switch, but some of those luminaires are designated as emergency lighting on the generator. What is the purpose of the emergency lighting transfer (bypass) relay installed for that room?
- To dim the luminaires when the generator is running.
- To force the designated luminaires on from the emergency source, regardless of the switch.
- To let the fire alarm switch the room lights.
- To turn all the room's lights off during a power failure to conserve the generator.
Show the answer
B. Correct. The relay senses loss of the normal supply; when it drops out, its contacts connect the designated luminaires to the emergency circuit and bypass the local switch, so a dark, switched-off room still gets its emergency light. When normal power returns, the relay restores switch control. The RSOS lists automatic emergency power transfer control relays among emergency lighting components.
Why not the others
A. Dimming is not a function of the transfer relay.
C. Fire alarm interfaces are separate; the transfer relay responds to the loss of normal power.
D. The relay's job is to turn designated lights on, not off.
02Answer key
| Question | Answer | Sub-task |
|---|---|---|
| Q1 | A | Installs raceways |
| Q2 | C | Installs lighting controls |
| Q3 | A | Installs HVAC controls |
| Q4 | D | Installs electric heating system controls |
| Q5 | B | Installs exit and emergency lighting |
03What block C covers
This block covers the wiring that carries power to every load. It includes cables, conductor ampacity and voltage drop, conduit bending and fill, cable tray and boxes, luminaires, receptacles and switching, lighting controls and parking-lot standards, airport and traffic signal systems, HVAC connections and controls, electric heat and heat tracing, exit and emergency lighting, and cathodic protection.
The Red Seal Occupational Standard names this block “Installs, services and maintains wiring systems”. It carries 30% of the exam, about 30 of the 100 questions, split across six tasks. The share column is each task's weight within the block, as the standard publishes it.
| Task and its sub-tasks | Share of block |
|---|---|
| Installs, services and maintains raceways, conductors, cables and enclosures Installs conductors and cables; Installs conduit and fittings; Installs raceways; Installs boxes and enclosures; Performs servicing and maintenance of raceways, conductors, cables and enclosures | 30% |
| Installs, services and maintains branch circuitry and devices Installs luminaires; Installs wiring devices; Installs lighting controls; Installs lighting standards; Performs servicing of branch circuitry; Installs, services and maintains airport visual aid systems; Installs, services and maintains traffic signal lights and controls | 28% |
| Installs, services and maintains heating, ventilating and air-conditioning (HVAC) systems Connects HVAC systems and associated equipment; Installs HVAC controls; Performs servicing and maintenance of HVAC systems and controls | 14% |
| Installs, services and maintains electric heating systems Installs electric heating systems; Installs electric heating system controls; Performs servicing and maintenance of electric heating systems and controls | 14% |
| Installs, services and maintains exit and emergency lighting systems Installs exit and emergency lighting; Performs servicing and maintenance of exit and emergency lighting systems | 10% |
| Installs, services and maintains cathodic protection systems Installs cathodic protection systems; Performs servicing and maintenance of cathodic protection systems | 4% |
04What the questions turn on
Derating and termination limits
Four to six current-carrying conductors in one raceway take 80 % of the table ampacity. Derating may start from the 90 °C column, but the final ampacity cannot exceed the column for the lowest-rated termination, such as a 75 °C breaker terminal.
Voltage drop on long runs
Drop is current times resistance times twice the one-way length. A 16 A load 40 m out on No. 12 copper loses about 8.3 V, more than double the 3 % branch-circuit guideline at 120 V, so the conductor size goes up.
Box fill and conduit bends
Start box fill from the table count, then deduct one conductor for a fixture stud and one for every pair of wire connectors. Between pull points, the bends in a conduit run may total no more than 360 degrees, offsets included.
Electric heat is continuous
Heating loads are held to 80 % of the breaker rating. A 20 A, 240 V circuit allows 3840 W, so three 1000 W baseboards fit and a fourth does not. A good 2000 W, 240 V element reads 28.8 ohms.
05The other blocks
The 309A exam has five blocks. Each has its own page of free questions:
The 309A Electrician practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.