D Electrical Conversions

This appendix describes how to calculate and convert common electrical values.

Units

The units used in most basic electrical conversions are in the following table.

Table D-1 Basic Electrical Units

A =Amperes (Amps)	PF =Power Factor
Btu =British Thermal Unit	V =Volts
kVA =Kilovolt-amperes	VA =Volt-amperes
kW =Kilowatts	W =Watts

Volts, Watts, and Amperes

Equation: Watts [W] =Amperes [A] x Volts [V]

Example

Given the following values you can use any two values to calculate a third: Watts =2400 (2.4 kW), Volts =120, Amps =20.

2.4 kW ÷ 20 A =120 V

120 V x 20 A =2,400 W (or 2.4 kW)

2,400 W ÷ 120 V =20 A

Volt-Amperes (VA)

Equation: Volts [VAC] x Amps [A-rms] =Volt-amperes [VA]

Example: A small server is plugged into a 120 VAC 60 Hz power source and is rated at 2.5 amps.

120 VAC x 2.5 amps =300 VA

Kilovolt-Amperes (kVA)

Equation: Volts [VAC] x Amps [A-rms] ÷ 1000 =kilo-volt-amperes [kVA]

Three Phase Equation: kVA (phase 1) +kVA (phase 2) +kVA (phase 3) =kVA (total)

Examples:

Single Phase:

120 VAC x 2.5 amps ÷ 1000 =0.3 kVA
Three Phase:

Phase 1: 120 VAC x 2.5 amps ÷ 1000 =0.3 kVA

Phase 2: 120 VAC x 2.5 amps ÷ 1000 =0.3 kVA

Phase 3: 120 VAC x 2.5 amps ÷ 1000 =0.3 kVA

Total kVA: 0.3 kVA +0.3 kVA +0.3 kVA =0.9 kVA

Kilowatts

The formula requires a value for the "power factor." The power factor (PF) is not a precise value (unless specifically known for the equipment). This factor can vary from 60–95% and is rarely published on the equipment.

Equation: Volts [V] x Amps [A] x Power Factor [PF] ÷ 1000 =Kilowatts [kW]

Examples: For these calculations a power factor of 0.85 is used.

Single Phase:

A medium-sized server is plugged into a 120 VAC 60 Hz power source and is rated at 6.0 amps.

120 VAC x 6 amps x 0.85 (PF) =612 ÷ 1,000 is 0.612 kW
208–240 Two Phase:

A server using a 220 VAC power source is rated at 4.7 amps.

220 VAC x 4.7 amps x 2 (phases) x 0.85 (PF) =1757.8 ÷ 1,000 =1.76 kW
Three Phase:

For 3-Phase power configurations you also need to multiply by 1.73.

A large disk subsystem has 192 physical volumes, a power source of 208 VAC, and is rated at 22 amps.

208 VAC x 22 amps x 1.73 x 0.85 (PF) ÷ 1000 =6.729 kW

Convert kW and kVA

The power factor is used to convert between kW and kVA. The power factor is an approximation (unless known for the equipment).

Equation: Kilowatts [kW] x Power Factor [PF] =kilo-volt-amperes [kVA]

British Thermal Units

Equation: 1 kW =3,413 Btu

Thus: 3.41214 x Watts [W] =Btu/hr

Many manufacturers publish kW, kVA and Btu in their equipment specifications. Often, dividing the Btu value by 3413 does not equal the published kW value.

Use the value provided by the manufacturer, otherwise use the formula.