All construction sites need power in order to be built, and how they get it is what electrical contractors provide,” said Todd Stafford, senior director, National Joint Apprenticeship Training Committee (NJATC). “Contractors have to provide safe power and to take into consideration grounding, shock hazards, short circuit considerations, vault currents and safety.”
There are two ways to provide electricity to a job site: permanent utility power or generator power. Power cable can be run to the main electrical panel of an existing facility or power can be provided by gas- or diesel-powered generators, which are generally stand-alone portable units.
Both options call for care. Unsafe equipment or work practices can cause electrical injuries on the job site. Problems with temporary electrical wiring on a construction site can cause burns, falls or heart damage. Mistakes that can lead to job injury are extensive and include overloading of normal-duty extension cords connected to temporary power, placement of extension cords near standing or flowing water, and failure to mark buried cables so that all trades are aware of their presence.
Management of temporary job site power starts with a plan. “When we do presentations, before we even get a job, we have to come up with a temporary power plan for a project,” said Jim Whiteaker, preconstruction supervisor, Dynalectric Co., San Diego. “So, we base a plan on our past experience, size of the project, owners requirement and what type of equipment that will need temporary power. We do layouts based on this information. Unknown loads of equipment from the owner and general contractor are also required, or we make our best estimate for these loads. Once we get the project, we do preplanning with the utility, San Diego Gas and Electric (SDG&E), and send them a temporary power load summary, and it goes into their planning stages. When they are complete, they send out a set of drawings. We have a preconstruction meeting with SDG&E and construction can start.”
Plans for temporary job site power for construction sites differ from those designed for a temporary event or situation.
“With the requirements we have to meet, temporary power is a lot more involved than in the old days,” said Gary Laidman, of ESI Inc., a company with offices in Cincinnati and other cities. ESI is in charge of design and implementation of the temporary power along with all power distribution for construction of the Cleveland Clinic Heart Center. Construction began on the $400-million,1-million-square-foot project in 2005 and is scheduled for completion in 2008.
The 10-story complex will ultimately include outpatient diagnostic facilities, more than 100 exam rooms and more than 150 physician offices, 16 cardiothoracic operating rooms, laboratories, intensive care units, cardiac radiology and nuclear medicine facilities, more than 200 patient rooms and a conference center. The ESI contract for the temporary job site power is valued at more than a million dollars.
“In the past, you would go into a building and run a bunch of lights around and you didn’t have to worry about foot-candles,” said Laidman. “Nowadays, especially with a project of this size, you can’t do it by the seat of your pants. You have to have a design. Things are constantly changing and we have to keep up with the rules and regulations defined by OSHA [the Occupational Safety and Health Administration].”
Laidman worked on the design with Whiting Turner Contracting Co., Cleveland, the project manager. He needed to address the power needs of all the trades. The design provided for OSHA-approved lighting, for power up and down the complete building, to run a power feed to the tower cranes, and to four double buck hoists. The design also included a power run to a de-watering system that circled the site.
Due to the site’s proximity to Lake Erie, power was required to run pumps to lower the water table so that excavation could be done for the basement and foundation of the building.
The design by ESI’s Laidman called for tying into the Cleveland Clinic’s high-voltage (11,400 volt) loop system, which runs around the hospital campus. To implement it, ESI set up two temporary high-voltage substations, 1,500 and 2,000 kilovolt-amperes, in temporary enclosures.
From those substations, they ran feeder conduits into the building where they set up temporary 480-volt distribution panels in the basement.
From the distribution panels, they ran conduits up the building with temporary wiring, providing 480 volts to every floor. Then, on each floor, they built mini power distribution centers consisting of a transformer with 480-volt/277 voltage panel on one side and a 120/208-volt panel on the other side.
“Once we transformed the power to 480/277, which we distributed throughout the building, we used transformers to step it down to a user voltage of 110 volts for drills and lighting or 120 to 208 volts,” said Laidman.
“We laid it out so as to not use anything longer than a 100-foot extension cord. Our rule of thumb is not to go much further than that because of possible voltage drops,” Laidman said. “And, we only use No. 12 heavy-duty cord for hand power tools. We couldn’t use open wire, so we ran the higher voltages in conduits or flexible conduits.”
ESI did the lighting with Romex wiring. Four 100-watt metal halide lamps were hung from the ceilings of the building slabs in the early stages of the project, on 65-foot centers strung with Romex wire. The company also installed lighting contactors.
“The reason for them,” said Laidman, “is that we take all the temporary lighting for the job and tie it into the contactors, which are then controlled by time clocks. Lights can then be turned off when crews aren’t working.”
Temporary power for events
Temporary power for events is usually done with generators.
“Typically, an engineer will go to a site and determine the amount of power needed and the size of the generators,” said Eric Johnston, senior vice president, Triton Generators. “Contractors use generators on sites where they don’t have any utility power or when they are working on a building in which the permanent utility power has not been installed. They also use them for temporary events like a golf tournament or a concert when the power at the site is not sufficient or is too far away or in a poor position to use utility power.”
When planning a job that calls for the use of generator power, electrical contractors use generators they own or rent. Some 26 generators ranging in size from 60 to 400 kilowatts were rented to provide temporary job site power for the Grand Prix of Cleveland, an annual event held in June at an airport facility. They were needed to distribute power to vendors, TV networks, motor homes, catering facilities, ticket booths, the pits and more. Laidman, again in charge of design and implementation, drew up a design based on the size of required distribution and subsequent size of needed generators.
“There’s a big difference between construction temporary and event temporary even though it’s the same theory,” said Laidman. “There are requirements we have to follow in regard to how temporary cable is placed and what type of temporary cable we’re using. For temporary events, we use a lot of Camlock cable to run between our generators and the distribution panels. Inside the distribution panels are branch breakers. Off of those we feed different amperages to the booths or vendors, motor coaches, catering trucks and so on depending on their needs.”
He also has to make sure the grounding is correct for the generators and the distribution equipment according to the National Electrical Code.
“For us, safety is number one,” he said. “All power cord and hand tools are GFI protected.”
Then there’s the issue of readiness.
“The key to having it go well is planning,” said Laidman. “Generators and other equipment have to be rented in advance. Then once the design is implemented, it is monitored. We check and double check. During the Grand Prix event, electricians, personnel from the generator rental company and mechanics are on-site to make sure the equipment is running properly and to address any problems. Whether it’s temporary power for a construction site or for an event, it’s got to be safe and dependable.” EC
CASEY, author of "Kids Inventing! A Handbook for Young Inventors" and "Women Invent! Two Centuries of Discoveries that have Changed Our World," can be reached at firstname.lastname@example.org or www.susancaseybooks.com.