Single Phase Amps

Purpose

Electrical services are usually sized by amperage, kilovolt-amps (kVA), or by horsepower (HP) requirements.  In order to size metering equipment properly, it is often necessary to calculate amps when the transformer size (kVA) is known.

The following formula may be used for calculating single-phase amps:

                             kVA  X  1,000

Single-phase amps   =  -------------------------

                                     Line to Line voltage

Example 1:
Assume you are using a 25 kVA 120/240 volt transformer.  The line to line voltage of the transformer is 240 volts.  Therefore, use 240 volts in the formula.

                            25 kVA  X  1,000             25,000 VA

Single-phase amps   =  -------------------------  =  -------------------  =  104 amps

                                                240 V                           240V

Rule of Thumb

Many utilities greatly simplify the above formula with the following rule of thumb.  “If the line to line voltage of the transformer secondary is 240 volts, multiply the kVA rating by 4”.  Or, “If the line to line voltage of the transformer secondary is 480 volts, multiply the kVA rating by 2”.

Example 2:
Assume you are using a 25 kVA 120/240 volt transformer with a line to line voltage of 240 volts.  Use the following formula.

Single-phase amps  =  4 X 25  =  100 amps

Although the actual value was previously calculated to be 104 amps, this answer is certainly close enough to size secondary conductors and metering equipment.

Example 3:
Assume you are using a 100 kVA 120/240 volt transformer with a line to line voltage of 240 volts.

Single-phase amps  =  4 X 100  =  400 amps

Example 4:
Assume you are using a 50 kVA 240/480 volt transformer with a line to line voltage of 480 volts.

Single-phase amps  =  2 X 50  =  100 amps

Never Add Amps!

Adding amps may be done under certain circumstances; however, it is also responsible for many incorrectly sized electrical installations.  Therefore, unless you are certain you thoroughly understand how circuits work, it is best not to get in the habit of adding amps.

Example 5:
In the following diagram, amperage has been measured on both phases.   Each phase has been measured at 100 amps.  This is a balanced 100 amp single phase load.  Not a 200 amp load!  The secondary conductors leaving the transformers and the rest of the main service must be capable of carrying 100 amps not 200 amps.  Do not get in the habit of adding amps!

Metering Equipment & Transformer Sizing

Example 6:
An electrician says a new single-phase 120/240 volt service will require up to 400 amps.  Use the previously stated rule of thumb in reverse.  Divide 400 amps by 4.  The answer is 100.  The service will require a 100 kVA transformer.  Typical self-contained meters can handle up to 200 amps.  Since this service requires up to 400 amps, an instrument rated meter with CT’s will be required.  Although a 400:5 CT could be used, a better selection would be a 200:5 CT with a RF rating of 2.0 or greater.  The lower ratio CT improves the accuracy of the metering when light load conditions exist.

Example 7:
An electrician says he needs to hook up a single-phase 10 HP 240 volt water well.  Most books list 1 HP = 746 watts, but when sizing equipment always assume 1 HP = 1 kVA.  Therefore, 10 HP = 10 kVA.  A 10 kVA transformer is required.  Since the line to line voltage will be 240 volts, 4 X 10 = 40 amps.  Obviously a class 100 or 200 self-contained meter can be used for this load.