Web searches have led me nowhere, and a Man hates to ASK for
using a 3 ph. Size 1 magnetic motor starter, to run a 1 phase 220v. 5
hp motor rated at 25 Full Load Amps, using 2 heaters instead of three.
( Q. is each heater the 25 FLA value?? or only half that value?? )
I have Square D and GE starters available.
THANKS for your expert input!
The rule is "one heater for each current leg". You're right in that
score. Pick a heater value the next size larger than your full load
rating for starters "no pun intended". The ambient temperature of the
controller housing can affect the trip point on overload switches, so
there is room for adjustment if you have problems with tripping. There
is nothing wrong with using a 3ph starter for single phase. Gives you a
spare contact for running auxiliary equipment, like a dust collector or
Yes, use the 25A current value to size your overload heaters. The FLA
values for any motor type indicate the current which flows in each
phase. I.E. - You would still use 25A to size heaters if the motor
That being said, the actual heaters you need may not be labeled with
"25A" - the current they're labeled with should be slightly greater
than the full load current of the motor to which they're applied.
Most heaters are manufactured for a 'range' of FLA motor currents.
You'll also need to consider which starter you're using (more
precisely, which overload relay is used in the starter), as each
manufacturer makes heaters for use with their specific equipment.
Look up the manufacturer on the web, you can usually find heater table
references. You need to use heaters specific to the actual overload
relay model in your starter. You can also find overload heater tables
referenced by equipment distributors, such as Graybar and Grainger.
If you really get detailed about this (some might say this is a bit AR
for non-industrial applications), you can size heaters for a
difference in ambient temperature between the motor and the starter
locations. But, that's usually not significant unless it's a large
For a single-phase motor, you can use just two heaters, and two poles
of a three-phase starter/contactor. A more protective setup would use
all three phases of the starter - just run one leg of the motor
circuit through two poles (in series connection) of the contactor and
Hope this helps,
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