Designing a Circuit in a Commercial Building

Thick smoke wafts through the office building. The fire alarm is flashing and blaring. This is your first day on the job. Where is the stairwell? You look up and see a red exit sign above one of the doors. As you emerge from the building, coughing, you see your new boss and coworkers standing in the parking lot. Even though you were unfamiliar with the building, you found your way to safety. Thanks to a whole team of engineers and craftsmen, the exit signs led you in the right direction.

Most internally-illuminated exit signs today use LED lamps. One major advantage of LEDs is the life span of the light. LEDs last 50,000–200,000 hours, while incandescent bulbs last only about 1,000–2,000 hours. A longer life span means that the lamps must be replaced less often, saving in both material and labor cost. In addition, for the same brightness, LEDs consume 2–5 watts of power versus 30 watts for incandescent lamps. Lower wattage translates into less energy used, which is better for the environment and cheaper to operate over the life span of the building.

Most commercial buildings receive their electric supply from a utility company at 480 and 277 volts AC. Each electric circuit in the building is protected by 20 amp circuit breakers. Electrical codes limit the load on each circuit to 80% of the circuit breaker rating, which is 4,432 watts for each circuit. Since each LED exit sign draws around 5 watts of power, an engineer could design about 800 exit signs on each circuit. In practice, all the exit signs on one floor are usually connected to a single circuit. Sometimes hallway lights are also connected to the same circuit. Since these lights are switched on and off and exit signs are not, the exit signs are connected to a part of the circuit that is not switched.

What happens if the power goes out? Do exit signs stay illuminated if the power from the utility company to the building is interrupted? Absolutely yes! To ensure that exit signs remain illuminated in the absence of utility power, one of two design approaches is chosen. If the building has an emergency generator, exit signs are connected to an electric circuit served by the generator. Another option is to install exit signs that have batteries for backup power. The disadvantage of having each exit sign contain its own battery is that the batteries require testing, maintenance, and replacement at regular intervals. Since paying someone to test, maintain, and replace the batteries is expensive, exit lights are usually connected to the emergency generator when available.

Electrical engineers design circuits every day. They have to consider electrical theory, taking into account voltage, current, and size of the conductor and circuit breaker, as well as other more advanced factors. In addition, engineers must be aware of the practical and financial consequences of their design choices. They attempt to minimize construction and maintenance costs while meeting all applicable codes and regulations. For example, the exit signs above were connected directly to the 277 volt circuit. This practice eliminates the need for a transformer. Transformers take one voltage and change it to another. They can be expensive. Next time you look up in a room and see lights or walk down a corridor and see the exit sign above a door, remember that engineering design combined with an electrician's skill made it all possible!

Multiple Choice

Which of the following statements correctly describes an advantage of LEDs over incandescent light bulbs?

• LEDs have a longer lifespan.

• LEDs use more watts for the same illumination.

• LEDs can be replaced remotely.

• LEDs are harder to purchase.

Multiple Choice

Commercial buildings receive power from the utility company at -

• 120 and 208 volts AC.

• 120 and 480 volts AC.

• 208 and 277 volts AC.

• 277 and 480 volts AC.

Multiple Choice

Exit signs -

• are switched on and off every day.

• have to stay illuminated even when utility power is lost.

• are never connected to a circuit with other lights.

• always have their own batteries.

Multiple Choice

In paragraph 4, which two pieces of equipment serve the same function?

• Exit lights and emergency generators

• Exit lights and hallway lights

• Batteries and emergency generators

• Exit lights and batteries

Multiple Choice

Installing exit lights that run on batteries into a building means that -

• technicians have to test, maintain, and replace the batteries regularly.

• building occupants have to report burned out lights immediately.

• the building requires a larger emergency generator than usual.

• the exit lights have to be placed closer together because they are not as bright.

Multiple Choice

Electrical engineers are responsible for -

• running conduits and wires through the building.

• making sure building occupants have enough stairwells.

• designing the electric circuits to applicable codes.

• hiring utility companies.