Calculator Instructions
The wire size calculator will give you a very simple and quick
solution to the problem of calculating the size of wires and cables for pump
applications in golf courses, landscape projects, and agriculture. The requirements of the
National Electrical Code (NEC) and local codes must always be followed.
Source Voltage
– The choices in the calculator are the most common and are nominal values.
Number of Phases
– This is usually singlephase or 3phase. For singlephase, three conductors
are required whereas four conductors are required for 3phase circuits. One of
these conductors is the equipment ground, which can be a reduced size as
governed by NEC article 250122. A Paige Electric representative can help you
determine your exact needs.
Amperes
– The number of amperes drawn by the load is to be obtained from the
manufacturer of the equipment. For motors, the current must be obtained from
tables 430148 (singlephase)
and 430150 (3phase) of the
NEC. Care must be taken to follow the guidelines of the NEC with regard to the
number plugged into the calculator.
The NEC requires that for single motor installation, the current used in the
calculation be 1.25 times the load current from the above mentioned tables. When
more than one motor is used, the current of the largest load must be multiplied
by 1.25, plus the sum of the rest.
Example:
Three pumps of a pump station (30, 10, and 1 ½ horsepower) are connected to the
power source of 460volts, 3phase. From table 430150, they draw 40 amps, 14
amps, and 3 amps respectively. The figure we would plug into the calculator
would be:
(40 x 1.25) + 14 + 3 = 50 + 14+ 3 = 67 amps.
Allowable Voltage Drop
– The calculator allows entry of a voltage drop, but caution should be used when
doing so to make sure you are calculating the wire sizes in accordance with NEC
article 21019 (FPN No. 4.) The NEC allows a maximum of a 3% voltage drop on the
main branch of a circuit at the farthest outlet of power and 5% total to both
feeders and branch circuits to the farthest outlet.
Calculations
– The calculator uses the following formulas for calculating wire size, which
are based on Ohm’s Law.
For singlephase circuits: 
Wire Circular Mils = 
(Conductor Resistivity)(2)(Amps)(One Way Distance in Feet) 

Allowable Voltage Drop 
For 3phase circuits: 
Wire Circular Mils = 
(Conductor Resistivity)(2)(Amps)(One Way Distance in Feet)(.866) 

Allowable Voltage Drop 
Note – Conductor resistivity is 11.2 for copper and 17.4 for
aluminum at a temperature of 127º F (53º C)