📊 Demand Factors in Transformer Sizing: Why Connected Load Isn't Design Load

Why Summing Nameplate Loads Would Oversize Every Transformer

If a transformer were sized to the simple sum of every connected load's full nameplate rating, the result would almost always be dramatically oversized, because not every piece of connected equipment operates at its full rated capacity simultaneously. Lighting circuits, HVAC equipment, receptacles, and process loads each have their own realistic pattern of actual usage — some run continuously near full load, others cycle on and off, and many rarely if ever reach their full nameplate rating at the same moment as everything else in the building. Demand factors exist specifically to convert this unrealistic "sum of everything at 100%" figure into a defensible estimate of actual coincident peak demand.

What a Demand Factor Actually Represents

A demand factor is a percentage (or decimal fraction) applied to a load category's connected kVA to estimate what fraction of that load is realistically expected to be drawing power at the same time as the building's overall peak demand. A demand factor of 100% means the full connected load is assumed to contribute at peak (appropriate for loads like continuous lighting that genuinely tend to run near full capacity together); a demand factor of 70-80% for office equipment reflects the reality that not every workstation, printer, and monitor draws its full rated power simultaneously, even during business hours.

Where Demand Factors Come From

NEC Article 220 provides standardized demand factor tables and methods for common occupancy types (dwelling units, commercial, and various specific occupancy categories) — these aren't arbitrary estimates but codified values derived from decades of accumulated load-survey data across many buildings of each type. For loads or occupancy types not explicitly covered by a standard NEC table, engineering judgment based on the specific equipment's actual duty cycle and the facility's operating pattern is used instead, ideally informed by actual metered data from similar existing facilities where available.

Why Different Load Categories Get Different Factors

The demand factor for a given load category reflects that category's actual diversity — how much variation exists in when different pieces of equipment within that category are actually drawing power. Lighting in a commercial building often gets 100% demand factor because most fixtures in an occupied space tend to be on together during operating hours, offering little diversity to credit. HVAC equipment often gets a lower factor (70-90% is common) because not every unit runs at full capacity simultaneously — staging, part-load operation, and non-simultaneous peak cooling/heating demand across different zones all reduce the realistic coincident peak below the sum of every unit's full nameplate rating.

A Common Mistake: Applying Demand Factors Too Aggressively

While demand factors are a legitimate and code-sanctioned way to right-size equipment, applying an overly optimistic demand factor — especially to a load category without solid supporting data or code authorization — risks undersizing the transformer for actual operating conditions. This is particularly a risk for loads with genuinely low diversity in practice (a facility that actually does tend to run most of its equipment simultaneously, despite belonging to a load category where standard tables assume more diversity) or in facilities expected to grow or change occupancy type over their service life. Conservative, code-based demand factors — rather than aggressively assumed low factors — are the more defensible approach for permit-level design.

How This Feeds Into the Transformer Sizing Calculation

The workflow this site's Transformer Sizing Calculator follows — sum each load category's connected kVA times its demand factor, add a design margin for growth, then round up to the nearest standard ANSI/IEEE size — mirrors the standard NEC 220-based commercial load calculation process. Getting the demand factor inputs right for each load category is what separates a defensibly sized transformer from one that's either wastefully oversized (unnecessarily high capital cost) or genuinely undersized (a real overheating and premature-failure risk covered in the companion loading and thermal-aging article).