Load Calculations & Transformer Sizing: A Practical Guide

An oversized transformer wastes capital and increases standby losses. An undersized one overheats, trips, and halts operations. Here's how to calculate electrical loads and size transformers correctly — with worked examples.

The Fundamentals

Connected Load vs. Demand Load

The connected load is the sum of all nameplate ratings of every piece of electrical equipment in a building. But not all equipment runs at the same time or at full capacity. The demand load accounts for this reality using demand factors.

Connected Load × Demand Factor = Demand Load
This is the actual load the electrical system must be designed to carry.

Demand Factor

The demand factor is the ratio of the maximum demand to the total connected load. It's always ≤ 1.0. For example, a building with 100kVA connected lighting but only 80kVA running simultaneously has a demand factor of 0.80.

The CEC provides specific demand factors for different load types:

Load Type	CEC Rule	Typical Demand Factor
General lighting (first 10kVA)	Rule 8-200	100%
General lighting (over 10kVA)	Rule 8-200	70–90%
Receptacle loads (first 10kVA)	Rule 8-200	100%
Receptacle loads (over 10kVA)	Rule 8-200	70%
Electric heating	Rule 62	75–100%
Motors (largest)	Rule 8-104	125%
Motors (all others)	Rule 8-104	100%
Kitchen equipment	Rule 8-200	65–80%

Diversity Factor

The diversity factor accounts for the probability that not all circuits will be at peak simultaneously. It applies when combining multiple feeders or sub-panels:

Diversity Factor = Sum of individual maximum demands ÷ Maximum demand of the whole system
Diversity factor is always ≥ 1.0 (the inverse of how demand factor works)

Worked Example: Small Commercial Building

Let's size the transformer for a 5,000 sq ft office building:

Step 1: Calculate Connected Loads

Load	Calculation	VA
General lighting	5,000 sqft × 15 VA/sqft	75,000
Receptacles	5,000 sqft × 10 VA/sqft	50,000
HVAC (RTU #1)	25 kVA nameplate	25,000
HVAC (RTU #2)	15 kVA nameplate	15,000
Water heater	5 kW	5,000
Total connected		170,000

Step 2: Apply Demand Factors

Load	Connected	Demand Factor	Demand VA
Lighting (first 10kVA)	10,000	100%	10,000
Lighting (remaining)	65,000	75%	48,750
Receptacles (first 10kVA)	10,000	100%	10,000
Receptacles (remaining)	40,000	70%	28,000
HVAC (largest motor × 125%)	25,000	125%	31,250
HVAC (other motors)	15,000	100%	15,000
Water heater	5,000	100%	5,000
Total demand			148,000

Step 3: Size the Transformer

Total demand = 148 kVA. Standard transformer sizes are: 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA.

Select the next standard size up: 150 kVA transformer.

Engineering Judgment: Some engineers apply an additional 20–25% future capacity allowance. In this case, 148 × 1.25 = 185 kVA, which would push the selection to a 225 kVA transformer. This is a cost-benefit decision the client should be involved in.

The kVA Formula

The fundamental relationships you need:

Quick Reference: Transformer Full Load Amps

Transformer kVA	FLA @ 208V	FLA @ 480V	FLA @ 600V
75	208A	90A	72A
112.5	312A	135A	108A
150	416A	180A	144A
225	625A	271A	217A
300	833A	361A	289A
500	1,388A	601A	481A
750	2,082A	902A	722A
1000	2,776A	1,203A	962A

Common Sizing Mistakes

• Using connected load without demand factors — results in 30–50% oversizing
• Forgetting the 125% motor factor — CEC requires it for the largest motor
• Ignoring future capacity — adding EV charging, solar, or tenant loads later
• Mixing kW and kVA — kVA = kW ÷ power factor (PF typically 0.85–0.90)
• Not coordinating with the utility — transformer availability and lead times vary