In short. Connected load is the sum of all ratings. Demand load is what is actually drawn at a given time, and maximum demand is the highest value over a period. Generator and transformer sizing should rest on demand and maximum demand, with proper allowance for starting and growth, not on connected load alone.
Definitions
| Term | Meaning |
|---|---|
| Connected load | Sum of the ratings of all installed loads |
| Demand load | Load drawn over a defined interval |
| Maximum demand | Highest demand over the study period (for example a billing month) |
| Demand factor | Maximum demand divided by connected load for a group |
| Diversity factor | Sum of individual maximum demands divided by the group’s combined maximum demand |
| Load factor | Average load divided by maximum demand |
Demand factor = Maximum demand ÷ Connected load
Why not use connected load
Not all loads run together. Standby pumps, seasonal loads and intermittent equipment never contribute at the same time. Sizing on connected load oversizes the generator, which then runs lightly loaded, increasing fuel use and wet-stacking risk. Sizing on average load undersizes it. Maximum demand with sensible margins is the target.
Sources of demand data
- Utility bills and meters: maximum demand in kVA or kW.
- Interval logging: load profile with time stamps, best for sizing.
- Clamp-on measurements on feeders over a representative period.
- Estimated demand factors for new plants, from similar facilities, to be refined.
Applying factors
- Group loads by board and process.
- Apply demand factors by group, justified by operations.
- Apply diversity between groups where the groups peak at different times.
- Add the largest motor starting effect separately. See motor starting.
- Add growth and design margin.
Backup power view
For a generator that supports the whole site, use the demand during an outage, which may be lower or higher than normal, since some loads are shed and others, such as cooling, may rise in hot weather. For critical-load backup, use the critical subtotal. See generator sizing.
Illustrative example
A site has 1,200 kW of connected load. Interval data shows a maximum demand of 640 kW and an average of 380 kW. The demand factor is about 0.53, load factor about 0.59. A generator sized on connected load would be about twice what is needed, while sizing on the average would miss the peaks. The design uses maximum demand, a starting allowance for the largest motor, and growth.
Three loads, three meanings
| Term | Meaning | Use |
|---|---|---|
| Connected load | Sum of ratings of everything installed | Upper bound, rarely the design figure |
| Demand load | Load actually drawn over a defined interval | Basis for sizing |
| Maximum demand | Highest demand over a period, often a 15 or 30 minute average | Utility billing, generator sizing |
Demand factor = maximum demand ÷ connected load; Diversity factor = sum of individual maxima ÷ coincident maximum
Worked example (illustrative)
A building has 1,200 kVA connected. Metered maximum demand is 720 kVA. Demand factor is 0.60. If a future expansion of 15 percent is expected, design demand is 720 × 1.15 = 828 kVA, not 1,200 kVA. Sizing the generator on connected load would buy roughly 45 percent more capacity than the demand needs, and a lightly loaded diesel runs poorly.
Sources of demand data
- Utility bills and maximum demand records.
- Logged data from a power analyser over at least a representative week.
- Load list with demand factors for new projects.
Generator view
The generator must carry the standby-backed demand, plus the largest starting step. A site may have high maximum demand from air conditioning in summer but critical demand far lower. Define which loads the generator carries. See critical-load classification.
See generator sizing.
Common mistakes
- Using connected load as the design load.
- Applying diversity without operational evidence.
- Using a one-day recording that missed the peak.
What to send us
Send bills, interval data or load lists. Send your application details. See also load schedule.
Questions engineers ask
What is a typical demand factor?
It varies by process. Use measured or process-specific values rather than generic numbers.
How long should I record?
Long enough to include the highest-load operating conditions, often several weeks or a full cycle of production.
Is maximum demand the same as peak current?
Maximum demand is averaged over an interval. Short peaks, such as starting, are separate.
Can bills show true demand?
They show billed demand, which may use an averaging interval. Interval data gives more detail.
This guide gives general principles. Use measured data wherever possible.