Building Electrical Load Schedule and Demand Calculator

What is a load schedule?

A load schedule lists every electrical load in a building or on a panel together with its power. It is the first step in settling the supply capacity, transformer and generator rating, main breaker and feeder cable. This tool totals the list, spreads it over the phases and gives the phase currents.

Installed power and demand

• Installed (connected) power: the sum of the nameplate ratings. Unit power × quantity.
• Demand: the power actually expected to be drawn at one time. Installed power × demand factor.

Not every load runs at once or at full power. Sockets are not all in use and motors rarely run fully loaded. The demand factor expresses that share and depends on the type of load. The tool does not suggest a value: you enter the one from the applicable regulations or your project. You can also apply a second factor to the total for diversity between load groups.

Active, reactive and apparent power

The reactive power of each line follows from its power factor: Q = P × tan φ. Total apparent power comes from adding the active and reactive powers separately: S = √(P² + Q²). Adding the kVA of loads with different power factors directly overstates the result.

Phase distribution and unbalance

Single phase loads are assigned to their phase (L1, L2, L3) and three phase loads are split equally. The current in each phase is I = S_phase / V_phase. Unbalance shows how far the most heavily loaded phase is above the average. Moving loads between phases brings it down. Breakers and cables are chosen for the most heavily loaded phase.

Example

Take 20 luminaires of 100 W on L1, 8 sockets (500 W, 50 percent demand) on L2, 2 air conditioners (1500 W) on L3 and a 6 kW three phase pump. The installed power is 15 kW and the demand is 13 kW. With a power factor of 0.8 on the reactive loads, the reactive power is 8.25 kvar and the apparent power 15.4 kVA. The phase currents are 18.6 A, 21.7 A and 27.2 A, an unbalance of 20.8 percent.

Limits

This is a preliminary calculation. Demand factors come from the applicable regulations and the conditions of the project. Cable size, protective devices, short circuit and voltage drop are calculated separately. Neutral current, harmonics and motor starting currents are not included. Loads are taken as inductive. For the situation after power factor correction, enter the corrected power factor.