ELECTRICAL UNIT CONVERSION

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This site offers formulas for converting between electrical units.  It is dedicated to sharing information, formulas and other documentation to aide in the conversion of given electrical, force or power values into other electrical, force or power values. The formulae below are commonly known and used universally.  I use them here in examples to demonstrate the application of conversion formulae.  I have done my best to be accurate but I cannot guarantee that all of the following information is correct or appropriate for your purposes.

I welcome any comments or corrections. Thanks to all who have notified me of inaccuracies. This type of participation makes this site more valuable. Please click here for a link to email your comment, correction, suggestion or question.


How to convert Watts to Amps

Basics

You cannot convert Watts to Amps, since Watts are power (ultimately horsepower) and Amps are current (or flow if you like) unless you have the added element of Voltage to complete the equation. You must have at least least two of the following three: Amps, Volts and Watts, to be able to calculate the missing one. Since Watts are Amps multiplied by Volts, there is a clear relationship between them.

Click here to see my Ohm's Law Pie Chart that shows the relationships between power (P), Voltage (E), current (I) and resistance (R) .

Choose Your Topic Below

Three Phase Power

Computing Watts
Computing Volt-Amps
Computing Kilovolt-Amps
Computing KiloWatts
Converting Between kW and kVA
Computing kBTUs

Short and sweet formulae:

Convert kVA to Amps
Convert Watts to Volts
Convert Watts to Amps
Convert Amps to Watts
Convert Horsepower to Amps

Convert kW to Amps
Convert kW-Hours to kVA

HOW TO FIND AMPS
HOW TO FIND WATTS
HOW TO FIND KILOWATTS
HOW TO FIND KILOVOLT-AMPS
HOW TO FIND HORSEPOWER
HOW TO FIND KILOWATT-HOUR

Go to the Shotgun Section and browse

Go to the Shotgun Table of Formulae

A tiny bit of Mechanical and Motors   (See our NEW Mechanical and Motors page for more)

Basic Horsepower Calculations

Energy Measurement with Joules and Dynes


Introduction

You may need to convert Voltage, Amperage and electrical specifications from equipment into kW, kVA and BTU information that can be used to calculate overall power and HVAC requirements.  The following section addresses the process of taking basic electrical values and converting them into other types of electrical values.


Three Phase Power

You will notice that all of the equations that refer to three phase power contain the value 1.73 in the formula somewhere.  The value 1.73 is the square root of 3.  Intuitively, you can see how this value is applied in the formulae.  (3 phases therefore 1 phase = square root of 3)

Computing Watts When Volts and Amps are Known

        POWER (WATTS) = Volts x Amps

        POWER (WATTS) = 2.5Amps x 120Volts = 300 WATTS

   Generally: P=IE  

P= Power(WATTS)
I = Current(Amps)
E = Voltage(Volts)

   So: I = P/E and E = P/I
   Therefore: 1 Watt = 1 Ampere x 1 Volt

      Click here to see my Ohm's Law Pie Chart for complete relationships between power, current and voltage.

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Computing Volt-Amps (VA)

       Same as above.  Volt-Amps (VA) = Volts x Amps    =  300 VA

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Computing Kilovolt-Amps (kVA)

 kVA stands for "Thousand Volt-Amps".

A 2-Pole Single Phase 208-240 power source requires 2 hot wires from 2 different circuits (referred to as poles) from a power distribution panel.

       SINGLE PHASE

       KILOVOLT-Amps (kVA) = Volts x Amps / 1000
     
        Using the previous example:  120 x 2.5 = 300 VA       300 Va / 1000 = .3 kVA

       208-240 SINGLE-PHASE (2-POLE SINGLE-PHASE)

        kilovolt-Amps (kVA) = Volts x Amps /1000
  
        220 x 4.7 = 1034          1034 / 1000 = 1.034 kVA

       THREE-PHASE

        kilovolt-Amps (kVA) = Volts x Amps x 1.73 / 1000
 
        220 x 50 x 1.73 = 19,030       19,030 / 1000 =  19.030 kVA    This would be rounded to 19

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Computing KiloWatts

       SINGLE PHASE

        kiloWatt (kW) = Volts x Amps x Power Factor / 1000
  
        120 x 6.0 = 720 VA       720 VA x .85 = 612      612 / 1000 = .612 kW

      208-240 SINGLE-PHASE (2-POLE SINGLE-PHASE)

        kiloWatt (kW) = Volts x Amps x Power Factor x 2 / 1000
  
        220 x 4.7 x 2 = 2068          2068 x .85 = 1757.8      1757.8 / 1000 = 1.76 kW

       THREE-PHASE

        kiloWatt (kW) = Volts x Amps x Power Factor x 1.73
                                                                   1000

        220 x 50 x .85 x 1.73  = 16,175.50    16,175.50/1000 = 16.175 kW

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To Convert Between kW and kVA

        kW to kVA         kW / .95 =  kVA
        kVA TO kW       kVA x .95 =  kW

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To Convert Between kW-Hours and kVA

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Computing BTUs

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Shotgun Section

Here are conversions, short and sweet:

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Shotgun Table of Formulae

HOW TO FIND Amps (I)  
  Direct Current  
  When HP, E and EFF are known:  HP x 746 / E x EFF
   
  When kW and E are known:  kW x 1000 / E
     
  SINGLE PHASE  
  When P, E and PF are known: P / E x PF
   
  When HP, E, EFF and PF are known: HP x 746 / E x EFF x PF
   
When kW, E and PF are known: kW x 1000 / E x PF
   
  When kVA and E are known: kVA x 1000 / E
     
  THREE PHASE  
  When P, E and PF are known: P / E x PF x 1.73
   
  When HP, E, EFF and PF are known: HP x 746 / E x EFF x PF x 1.73
   
  When kW, E and PF are known: kW x 1000 / E x PF x 1.73
   
  When kVA and E are known: kVA x 1000 / E x 1.73
  (See abbreviations explained below)  
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HOW TO FIND WATTS (P)  
     
  When E and I are known: I x E
   
  When R and I are known: R x I2
   
  When E and R are known: E2  / R
  (See abbreviations explained below)  
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HOW TO FIND KILOWATTS (kW)  
  Direct Current  
  E and I must be known: E x I / 1000
     
  SINGLE PHASE  
  E, I and PF must be known: E x I x PF / 1000
     
  THREE PHASE  
  E, I and PF must be known: E x I x PF x 1.73 / 1000
  (See abbreviations explained below)  
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HOW TO FIND KILOVOLT-Amps (kVA)  
  SINGLE PHASE  
  E and I must be known: E x I / 1000
     
THREE PHASE
  E and I must be known: E x I x 1.73 / 1000
  (See abbreviations explained below)  
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HOW TO FIND HORSEPOWER (HP)  
  Direct Current  
E, I and EFF must be known: E x I x EFF / 746
     
  SINGLE PHASE  
E, I, PF and EFF must be known: E x I x PF x EFF / 746
     
  THREE PHASE  
  E, I, PF and EFF must be known: E x I x PF x EFF x 1.73 / 746
(See abbreviations explained below)
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HOW TO FIND KILOWATT-HOUR (KwH)
BTU * (2.9307 * 10 -4)
FtLb * (3.7661 * 10 -7)
Joule  * (2.7777 * 10 -7)
   
WHERE:
  E = VOLTS  
P = WATTS
  R = OHMS  
  I = AMPS  
  HP = HORSEPOWER  
  PF = POWER FACTOR  
  kW = KILOWATTS  
  kVA = KILOVOLT-AMPS  
  EFF = EFFICIENCY (decimal)  

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Basic Horsepower Calculations

Horsepower is work done per unit of time. One HP equals 33,000 ft-lb of work per minute. When work is done by a source of torque (T) to produce (M) rotations about an axis, the work done is:

    radius x 2pi x rpm x lb. or 2pi TM

When rotation is at the rate N rpm, the HP delivered is:

    HP = radius x 2pi x rpm x lb. / 33,000 = TN / 5,250

For vertical or hoisting motion:

    HP = W x S / 33,000 x E

Where:

W = total weight in lbs. to be raised by motor
S = hoisting speed in feet per minute
E = overall mechanical efficiency of hoist and gearing. For purposes of estimating
E = .65 for eff. of hoist and "connected gear."

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Energy measurement with Joules and Dynes

Energy is measured in joules (Watt-seconds) or kiloWatt-hours. A power level of one Watt that continues for one second equals one joule.  The integrated energy from a 100-Watt light that runs for 60 seconds equals 6000 joules.

4.18 joules equal 1 calorie, which is enough energy to raise the temperature of one gram of water by one degree Celsius (or Centigrade).  

When it comes to energy density (Watts per liter or Watts per kilogram) it is difficult to beat gasoline. A lead-acid battery is good for about 125 thousand joules per kilogram. Lithium batteries can provide as much as 1.5 million joules per kilogram.  Gasoline tends to run about 45 million joules per kilogram. 

Joules:

1 joule is exactly 107 ergs.

1 joule is approximately equal to:

Units defined in terms of the joule include:

Useful to remember:

Dynes:

In physics, the dyne (symbol "dyn", from Greek δύναμις (dynamis) meaning power, force) is a unit of force specified in the centimeter-gram-second (CGS) system of units, a predecessor of the modern SI. One dyne is equal to exactly 10 micronewtons. Equivalently, the dyne is defined as "the force required to accelerate a mass of one gram at a rate of one centimeter per second squared":

1 dyn = 1 g x cm/s2 = 10 - 5 kg x m/s2 = 10 µN

The dyne per centimeter is the unit usually associated with measuring surface tension. For example, the surface tension of distilled water is 72 dyn/cm at 25°C (77°F).

Units of force
  newton
(SI unit)
dyne kilogram-force,
kilopond
pound-force poundal
1 N = 1 kg x m/s2 = 105 dyn ≈ 0.10197 kp ≈ 0.22481 lbf ≈ 7.2330 pdl
1 dyn = 10-5 N = 1 g x cm/s2 ≈ 1.0197 x 10-6 kp ≈ 2.2481 x 10-6 lbf ≈ 7.2330 x 10-5 pdl
1 kp = 9.80665 N = 980665 dyn = gn x (1 kg) ≈ 2.2046 lbf ≈ 70.932 pdl
1 lbf ≈ 4.448222 N ≈ 444822 dyn ≈ 0.45359 kp = gn x (1 lb) ≈ 32.174 pdl
1 pdl ≈ 0.138255 N ≈ 13825 dyn ≈ 0.014098 kp ≈ 0.031081 lbf = 1 lb x ft/s2
The value of gn as used in the official definition of the kilogram-force is used here for all gravitational units.

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Mechanical

General Approximations - RULES OF THUMB

Use these in the field for fast approximations:

At 3600 rpm, a motor develops a 1.5 lb-ft of torque per HP at rated HP output
At 1800 rpm, a motor develops a 3 lb-ft of torque per HP at rated HP output
At 1200 rpm, a motor develops a 4.5 lb-ft of torque per HP at rated HP output
At 900 rpm, a motor develops a 6 lb-ft of torque per HP at rated HP output

At 575 volts, a 3-phase motor draws 1 AMP per HP at rated HP output
At 460 volts, a 3-phase motor draws 1.25 AMP per HP at rated HP output
At 230 volts a 3-phase motor draws 2.5 AMP per HP at rated HP output

At 230 volts, a single-phase motor draws 5 AMP per HP at rated HP output
At 115 volts, a single-phase motor draws 10 AMP per HP at rated HP output

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