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 is often less than 1.0 where code demand factors or actual non-coincident operation apply. For example, a building with 100kVA connected lighting but only 80kVA running simultaneously has a demand factor of 0.80.
In the CEC, Rule 8-200 is the method for single dwellings; other occupancies use Rule 8-210 with Table 14. The CEC also sets factors for specific load types:
| Load Type | CEC Rule | Typical Demand Factor |
|---|---|---|
| Electric heating | Rule 62-118 | 75-100% |
| Motors (largest) | Rule 28-108 | 125% |
| Motors (all others) | Rule 28-108 | 100% |
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 typically greater than or equal to 1.0 when individual peak demands do not occur at the same time. Do not assume 1.0 unless the loads are fully coincident.
Worked Example: Small Commercial Building
Illustrative estimate, not the CEC Rule 8-210 method: the unit loads and demand factors below are assumptions for illustration. 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.
From connected load to the transformer size
The article’s 5,000 sq ft office example, not a specific building. Equipment is drawn larger than scale.
Parts
General lighting: 5,000 sq ft × 15 VA/sq ft = 75,000 VA connected; first 10 kVA at 100 %, the remaining 65 kVA at 75 % → 58,750 VA demand.
Receptacles: 5,000 sq ft × 10 VA/sq ft = 50,000 VA; first 10 kVA at 100 %, the remaining 40 kVA at 70 % → 38,000 VA.
RTU #1, 25 kVA: the largest motor load, taken at 125 % → 31,250 VA. Rule 28-108
RTU #2, 15 kVA: the other motor loads at 100 % → 15,000 VA. Rule 28-108
Water heater, 5 kW: at 100 % → 5,000 VA.
Transformer: 148 kVA demand → the next standard size up, 150 kVA. With a 20–25 % future allowance, 148 × 1.25 = 185 kVA → 225 kVA (a cost decision with the client).
Switchboard and feeders are sized from the demand load, not the 170 kVA connected load.
Illustrative estimate from the article, not the CEC Rule 8-210 method: the unit loads and demand factors are assumptions. kVA = kW ÷ power factor.
- Feeders in conduit
- Liquid-tight flexible conduit
- Nameplate load
- Selected transformer
The kVA Formula
The fundamental relationships you need:
Single Phase:
kVA = (V × I) / 1,000
Three Phase:
kVA = (V × I × √3) / 1,000
Current from kVA (Three Phase):
I = kVA × 1,000 / (V × √3)
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 significant 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
Planning a factory expansion or adding major equipment?
See how ETEM assesses industrial loads, transformers, switchboards, MCCs, feeders and service capacity before construction. Read the detailed factory expansion and industrial power-upgrade guide.
Explore Industrial Electrical Capacity Studies →Frequently Asked Questions
What is the difference between connected load and demand load?
Connected load is the sum of all equipment nameplate ratings. Demand load applies CEC demand factors to reflect that not all equipment runs simultaneously. Always use demand load for service sizing.
How do I size a transformer for a commercial building?
Calculate total demand load using CEC demand factors, then select the next standard transformer size up (75, 112.5, 150, 225, 300, 500, 750, or 1000 kVA). Consider 20-25% future capacity.
What is the CEC 125% motor rule?
CEC Rule 28-108 requires conductors supplying two or more motors to have an ampacity of at least 125% of the full-load current of the largest motor plus 100% of the full-load current of all other motors.
