A 3-phase 100 amp contactor with 3 sets of points would consume more
power than a 30-40 amp single-phase contactor with 2 sets of points.
A 3-phase contactor rated for 100 amp is much larger size with heavy
contact points that must be held firm by the magnetic coil ... plus the
3-phase contactor has 3 sets of points instead of 2 set for single
phase.
This would hold true even if both coils were 120 volt.
Contactors with Hot-to-Hot 208-240-480 volt coils would consume less
power than Hot-to-Neutral 120-208-277 volt coils. This is because any
Hot-to-Hot wiring delivers more power to the load than Hot-to-Neutral.
So a household contactor that controls water heater would be slightly more effiicient
with 240 volt coil vs the 120 volt coil.
A quick test on 2 different Siemens 45EG20AF 30-40 amp single-phase
contactor with 2 sets of points shows consistent 11 Watts of power
being consumed across the coil ... 11 watts is smaller than 25-40 watt
refrigerator light bulb
11 watt load left 'on' day-n-night for full year, would consume 9.5 Kwh
(11 watts x 24 hours x 365 = 9500 watt hours or 9.5 Kilowatthours) -or-
9.5 Kwh x 12 cents per Kwh = $1.14 + tax
... remember, the contactor is only 'on' when water heater is
on
By comparison, water heater elements are 3500, 4500 and 5500 (check
side of tank for rating) > so 11 watts consumed by contactor is
small fraction
Past 2 years, using a timer on our water heater, we cut the electric
bill by 11.7 Kwh per day with added insulation and hot water
conservation. Resources: How to install contactor to control water heater Contactor-life-expectancy.pdf
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