Why average? A motor starting up might draw 600A for 2 seconds, but that doesn't melt the cables. The 15-minute average is what heats the transformer. Why does it matter? Utilities size their transformers for your MD. If you have a 5-minute spike once a month, you are paying for infrastructure you use 0.01% of the time. You cannot simply add up the nameplate ratings of all breakers. That is Connected Load , not Maximum Demand. Diversity saves you.
In the world of electrical engineering and facility management, Maximum Demand (MD) is a critical metric. It isn't just about keeping the lights on; it directly impacts your capital expenditure (cable/transformer sizing) and operational expenditure (electricity bills). maximum demand calculator
Grab your last 12 utility bills. Find the highest "Demand" value. Compare it to the sum of your connected load. You will likely find you are paying for capacity you rarely use. Why average
[ MD = (Load_1 \times DF_1) + (Load_2 \times DF_2) + ... + (Load_n \times DF_n) ] Why does it matter
In most commercial tariffs, the electricity bill is split into two parts: and Demand Charges (kVA/kW) . The Demand Charge is often the larger shock—it is based on the highest average load recorded during a billing window.
Disclaimer: This guide provides estimation methods. Always consult a licensed electrical engineer for final design and safety compliance.