Renting a generator without first calculating the actual load requirement is one of the most common and avoidable mistakes on industrial and construction projects in Alberta. The consequences range from a unit that trips repeatedly under peak demand to a significantly oversized generator burning excess fuel at a day rate that wasn’t budgeted. Neither outcome is acceptable on a project with real schedule and cost pressure. Getting the load calculation right before the equipment is ordered is the work that makes everything downstream go smoothly.
Start With a Full Inventory of Connected Loads
The starting point is a complete list of every electrical load the generator will serve — not an approximation, and not just the largest loads. Lighting circuits, instrumentation panels, heat tracing, HVAC in site trailers, battery chargers, and small tools all contribute to the total demand. Each load has a running wattage and, for motor-driven equipment, a starting current that exceeds the running draw significantly.
Document each load with its voltage, phase, running kW, and power factor where known. Single-phase and three-phase loads need to be tracked separately because the generator’s phase balance affects voltage stability. A load inventory that misses even a few moderate-sized loads can produce a generator spec that’s inadequate before the first full operating day.
Account for Motor Starting Currents
Motor starting current is where most load calculations go wrong. An induction motor draws three to six times its full-load running current during startup, for a duration that can range from one to several seconds depending on the motor size and load type. A compressor rated at 15 kW running may draw 60 kW or more on starting — and if the generator isn’t sized to handle that inrush without excessive voltage drop, nuisance trips become a daily operational problem.
The correct approach is to identify the largest motor on the site, calculate its starting kVA requirement, and ensure the generator can handle that starting event without dropping voltage below the threshold that causes other connected equipment to fault. NexSource Power’s team works through starting current calculations as part of the rental specification process — confirming that the unit deployed can handle peak demand, not just steady-state running load.
Apply a Demand Factor and Growth Margin
A generator sized to exactly match the calculated running load has no headroom for load additions, temporary overloads, or the reality that site loads rarely match the initial estimate perfectly. Standard practice is to apply a demand factor — typically 70-80% of connected load as the expected simultaneous demand — and then size the generator so that expected demand sits at no more than 70-75% of the unit’s rated output.That headroom absorbs starting surges, accommodates load additions during the project, and keeps the generator operating in the efficiency range where fuel consumption is optimized. A generator running continuously at 40% load is a fuel and maintenance cost problem. NexSource Power helps clients arrive at the right sizing before equipment is dispatched — not after the unit is on site and proving inadequate.
Sign up