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Heater Strips, sometimes referred to as space heaters, winding heaters, or silicone heaters, are used to maintain internal air temperature above the dew point during motor shutdown periods. This style of heater prevents any water accumulation caused by moisture condensation inside the motor. EIS recommends winding heaters for installation in damp locations and potential for quick temperature changes. These heaters should be activated when the motor is de-energized.


Space heaters can operate at -56°C (-70°F) up to 200°C (394°F) continuously!


Q: Where do these heaters go? 

A: These heaters are typically secured with tape or tie cord on end turns of the winding stator and conform to the shape of the coil end surface. This space is the most exposed area for electric motor windings.

Q: Are there other types of heaters customers can use to prevent water accumulation? 

A: The only winding heater currently in use for NEMA frame motors is a flexible silicone type. The flexible space heater consists of a heating element enclosed within the silicone rubber jacket.


There are two commonly used equations to determine the correct watts for a winding heater. The first H=DL/35 where H is heat, D is the diameter of the end bell in feet, and L is the machined length between the bell centers in feet. 

The second W=2DL where W is watts, D is the diameter of stator laminations in inches, and L for the length of the stator core in inches. The first shows the units in kilo-watts and feet and the second in watts and inches.

Each manufacturer will have different measurements, so your calculations could be very close or very different depending on this. These formulas are used to get you close but are not necessarily accurate. It is a responsible practice to test the heaters before installation to confirm the temperature rise meets your needs. 

Note from EASA: Small motors need the correct or slightly higher wattage than calculated, where larger motors generally use wattage between the two calculated values.


  1. Heater leads are brought out through the existing motor terminal box or an alternative terminal box connected to available single-phase voltage.

  2. Connect heater leads to a motor control circuit relay contact, which energizes the heater when the motor's power is disconnected. 

  3. Do not connect heaters to run while the motor is connected and running.


NEMA Cheat Sheet by Frame Size


In many cases, EIS recommends installing thermostats to control the temperature or to use temperature and process control modules such as the simple Watlow EHG SL10 controller that provides a power and safety limit controller in one package that features ambient operating range from 0 up to 70°C (32 to 128°F). For a more sophisticated option, the F4t provides a fully integrated process controller with best-in-class ease of use. It features up to 14 different alarms, an ambient operating range of -18 up to 50C°(0 to 122F°), 4/4 control and limit loops, as many as 24 monitor channels, a remote limit reset, and also provides a 4.3" PCAP color graphical touch screen.

Simply the heaters need to be wired separately to an auxiliary contactor. In many cases, the contactor has a normally open function that energizes the heaters when the motor voltage is shut off. This will close the circuit and energize the heaters to heat the windings to maintain low moisture content. 

Note: Heaters are bases on a 70°F ambient. Please contact EIS to verify you're using the correct heater if your ambient temperature is significantly different.


Winding heaters supplied by the manufacturer are engineered and designed to the motor's exact square inch space. Therefore it is best to utilize the size provided by the manufacturer. In the case of adding heater strips to an existing motor or a rewind job, EASA recommends: "Small motors need the correct or slightly higher wattage than calculated, where larger motors generally use a wattage in between the two calculated values. "

Winding Heater Diagram