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Solenoids current and wire sizing

(October 2001)


Paul and I have been having a very interesting discussion off-list concerning proper wire sizing for electric solenoid valves.  I would like your input:


(1) I have always sized the wiring based on the inrush current of the solenoid.  I try to maintain a minimum voltage of 20 volts at the solenoid (sometimes that may be as low as 18.5 or 19 volts).

(2) Most turf irrigation manuals have wire sizing charts that appear to be based on the solenoid holding current rather than inrush current.  (These charts also reflect system pressure.)  One manufacturer's chart seems to provide wire sizes that allow voltages as low as 17.6 volts based on the solenoid holding current.  Other charts seem to offer similar wire sizing guidelines.

(3) There is usually a "power spike" from the controller when a station is initially turned on.  I have no idea how much the voltage increases during this spike (some sources seem to indicate 2x or more) nor how long it lasts.

 OK, my questions - I am guessing that the turf irrigation companies are depending on the controller "power spike" to provide enough voltage to push the required inrush current through the wire and initially energize the solenoid.  This would allow sizing of wiring based on holding current rather than inrush current.

(a) Do you size wiring based on these charts?  If so have you had any marginal installations that did not work reliably?

(b) If you calculate voltage drop and so forth to size wiring, do you use inrush or holding current?  And what do you use as an allowable voltage at the solenoid to ensure reliable operation?

(c) Does anyone have a formula for the additional voltage or current required to engage a solenoid with increasing water pressure (before I start developing one from scratch!)?

Bryan Smith

 I have had discussions off list with a number of folks over the past few days.  There is no firm consensus, but I thought you may like a brief summary of the various methods we discussed.

(1) Almost everyone agrees that few solenoids will work below 20 volts AC.  Most indicated some solenoids require more voltage than this to operate properly.

(2)   Several folks do not use solenoid "inrush" current when calculating voltage drop in the wiring.  This inrush current is seen as a "transient" current.   They instead design for the solenoid holding current voltage drop.  These same designers are determining wire sizes based on a 10% voltage drop using holding current. That usually means they design for a 2.4 to 2.6 volt drop.

 Other designers are determining wire sizes based on solenoid inrush current and a minimum of 20 volts at the solenoid (or more depending on the solneoid's requirement).

These two methods may not be as far apart as they first seem. One uses a smaller current and a smaller allowable voltage drop than the other, so the difference in wire size may be small.

(3) A solenoid rated at 60 hz will require 20% more current when used on a 50Hz circuit (something I had never considered since all of my experience has been with 60 hz power).

Bryan Smith