Question

The service to a multistory building is 277/480-volt, 3-phase, 4-wire. there are 125 120-volt, duplex, general-purpose receptacles to be installed on each floor. a 3-phase, 480-volt primary, 120/208-volt secondary transformer is to be installed on each floor to supply the receptacles. calculate the transformer secondary kilovolt-ampere (kva) devoted to only the general-purpose receptacle load.

Answer 1

The transformer on each floor dedicated to powering the 125 general-purpose 120-volt duplex receptacles requires a 45kVA secondary rating.

Step-by-step explanation:

The question asks for the calculation of the secondary kilovolt-ampere (kVA) rating for a transformer that will supply only the general-purpose receptacles in a multistory building. Since each floor requires 125 120-volt duplex receptacles, and we assume each receptacle will use the standard 180VA (as per NEC standard for general-purpose receptacles), the total VA for one floor is 125 receptacles × 2 (duplex) × 180VA = 45,000VA or 45kVA. With a transformer having a 120/208V secondary, this results in a secondary of 45kVA devoted to the receptacles on each floor. It's important to note that power calculations for transformers typically involve considerations for power factor and efficiency which are not provided here, so this calculation is based on purely resistive loads.

Answer 2

To calculate the transformer secondary kilovolt-ampere (kVA) devoted to only the general-purpose receptacle load, calculate the power consumed by each receptacle load and sum them up for all the receptacles on each floor.

Step-by-step explanation:

To calculate the transformer secondary kilovolt-ampere (kVA) devoted to only the general-purpose receptacle load, we need to first calculate the power (in kilowatts) consumed by each receptacle load and then sum them up for all the receptacles on each floor.

Each duplex general-purpose receptacle has a voltage rating of 120V. To calculate the power, we'll use the formula:

Power (kW) = (Voltage (V) * Current (A)) / 1000

Given that we have 125 duplex receptacles on each floor, we can calculate the total power consumed by multiplying this number by the power consumed by each receptacle:

Total Power (kW) = Power (kW) * Number of Receptacles